Solar Sizing Reference: kWh, Panel Wattage, and Sun Hours

Every honest solar decision reduces to three numbers and the arithmetic that links them: how much electricity you use in a year, measured in kilowatt-hours; how much sun your location delivers, measured in peak sun hours; and how much power each panel produces, measured in watts. Costs, incentives, and financing all move around from month to month, but the physics that turns those three numbers into a system size, a panel count, and a daily production figure does not. This reference collects that stable math in one place, as lookup tables you can cite rather than a walkthrough you have to follow.

Think of this page as the cheat-sheet that sits behind the step-by-step method in our solar sizing field guide and the count-focused panel-count briefing. Where those articles teach you how to size a system, this one tabulates the answers: kilowatt-hours to system size, panel counts at common wattages, per-panel and per-kilowatt production by region, and the conversions between them. Every table is illustrative and built from the same formulas, and you can drop your own numbers into the sizing calculator at any point to get your specific figure instead of the nearest row.

The three numbers that drive every solar system

Before any table, fix the three inputs, because everything downstream is arithmetic on them. The first is your annual electricity use in kilowatt-hours, printed on your utility bills and best taken from a full twelve months rather than a single statement. A typical single-family home lands somewhere between 8,000 and 12,000 kilowatt-hours a year, a careful apartment near 5,000, and an all-electric home with a heat pump and an electric vehicle well past 18,000. This is the numerator of almost every sizing question.

The second is peak sun hours, the annual daily average number of full-strength sunlight hours your location receives. It sits in the denominator of the sizing formula, so it moves your system size as much as your usage does. The third is panel wattage, the rated output of a single panel under laboratory conditions, commonly 350 to 450 watts today. A fourth quiet input, the derate factor near 0.8, converts ideal ratings into real-world output.

Hold those four in mind and the rest of this reference is substitution. Usage tells you how much energy the array must replace, sun hours tell you how hard each kilowatt works, wattage tells you how many physical panels a given capacity takes, and the derate keeps every figure honest against a laboratory that flatters the hardware. None of the four is hard to find, and none of them shifts with the price of equipment, which is exactly why the tables built on them stay useful year to year.

The core sizing formulas in one place

Here are the four relationships this entire reference rests on, written once so you can carry them anywhere. Read them as plain arithmetic, not algebra, and the worked examples later will substitute real numbers into each.

Notice how the derate near 0.8 appears in every production and sizing line but not in the panel-count line, because counting panels is pure hardware division while production depends on real-world losses. Notice too that peak sun hours drive both sizing and production but pull in opposite directions: more sun means a smaller system for the same usage, and more production from the same system. That single input is the biggest reason two identical homes in different regions end up with different arrays.

One more note on the derate. This reference uses 0.8 as its standing figure because it is the conservative, commonly cited value and it leaves a little honest margin. Some installers and tools use 0.85, which produces a slightly smaller system and slightly higher production for the same inputs. Wherever a table below would shift materially between the two, the text flags it, so you can read the numbers against whichever assumption your own quote uses. The sizing calculator lets you switch the derate and watch every figure move.

What a kilowatt-hour measures

A kilowatt-hour is the unit your whole solar decision is denominated in, so it earns a plain definition. A kilowatt-hour is the energy used by a one-kilowatt load running for one hour. A 100 watt lamp left on for ten hours uses one kilowatt-hour; a 1,500 watt space heater running for forty minutes uses one kilowatt-hour. Your utility meters and bills you in these units, which is why annual kilowatt-hours, not dollars, are the honest anchor for sizing: rates change, but the energy your array has to replace is a physical quantity.

The reason to insist on annual kilowatt-hours rather than a monthly snapshot is seasonality. Summer air conditioning or winter electric heat can make one month double another, so multiplying a single bill by twelve is the most common way people misjudge their own usage. Many utility statements print a rolling twelve-month total or a small bar chart of the past year; either gives you the figure the formulas need without adding up twelve separate bills. Our note on reading your electric bill walks through where that number hides on a typical statement.

Keep the distinction between energy and power clear from the start. Kilowatt-hours measure energy, the total you consume over time, while kilowatts measure power, the rate at which energy flows. Your bill is in kilowatt-hours; your solar system is quoted in kilowatts. This reference is careful to keep usage and production in kilowatt-hours and system size in kilowatts, because sliding between the two units unnoticed is the root of most sizing confusion, a point the unit section below returns to in detail.

What peak sun hours mean

Peak sun hours are the input people most often misunderstand, so define the term precisely. A peak sun hour is one hour during which sunlight strikes the panel at 1,000 watts per square meter, the standard full-strength intensity that panels are rated against. The daily peak sun hours for a location is the number of those full-strength hours the location averages per day across a year, once you compress the weaker morning and evening sun and the cloudy stretches into an equivalent count of full-strength hours. A place can have twelve hours of daylight and only four and a half peak sun hours.

This is not a figure anyone measures with a stopwatch; it is derived from long-run solar-resource records. The National Renewable Energy Laboratory, NREL, is the standard reference for it in the United States, publishing the solar-resource data that installers and sizing tools draw on. When a proposal cites your location’s sun figure, it is almost always tracing back, directly or indirectly, to that kind of long-run average rather than to last week’s weather. That is what makes it usable for sizing: it is a stable, climatological number, not a forecast.

Two cautions keep the input honest. First, peak sun hours are an annual daily average, so a system sized to a bright July figure would fall short every winter; always use the year-round average unless you are deliberately sizing for a single season. Second, the figure already assumes reasonably clear exposure, so heavy shading from trees or a neighbor’s roofline is a separate loss on top of it, not something the sun-hours number captures. Our coverage of whether panels work in winter shows how the seasonal spread around that annual average behaves in cold, dark months.

Peak sun hours by region

Here is the input that swings the most between homes, tabled as illustrative annual daily averages by broad region. Treat these as bands to place yourself in rather than precise coordinates, and confirm your own figure against a solar-resource map or a reputable tool. The point of the table is the spread: the sunniest common regions deliver well over half again the daily sun of the cloudiest, which is why geography reshapes every downstream number.

Read the table and the sizing consequence is immediate. Because peak sun hours sit in the denominator of the size formula, a home in the Pacific Northwest at 3.5 hours needs a substantially larger array than the same home in Arizona at 5.75 hours to cover identical usage, roughly 64 percent more capacity. The same physical panel also earns less in the cloudier region, so both the count and the per-panel output move together. When you carry your usage into the tables that follow, use your own region’s band, not a national average, because that single substitution is where most generic online estimates go wrong.

Panel wattage today: 350 to 450 watts

The third input, panel wattage, has drifted upward for years, so pin it to the current range. A modern residential panel commonly falls between roughly 350 and 450 watts, and this reference uses 400 watts as a clean middle figure throughout. Wattage is the panel’s rating under standard test conditions, essentially full, clean, cool midday sun, which is exactly the ideal the derate later discounts. A decade ago a typical residential panel sat closer to 250 watts, so the same roof holds far more capacity today, and any rule of thumb built on older panels undercounts a modern array.

Because residential panels come in a fairly tight physical size, higher wattage mostly reflects higher efficiency, meaning more output pulled from the same rectangle of glass. That has a direct consequence for counts: a higher-wattage panel lowers the number you need for a given system size, which matters most when roof space is tight. The table below shows how the same illustrative 8.2 kilowatt system, the size our worked example lands on, converts to different panel counts across the common wattage range.

The pattern is worth internalizing: stepping from 350 to 450 watt panels drops the count for the same capacity by about a fifth and shrinks the roof footprint to match. On a large, unobstructed roof the cheaper standard-wattage panels often win on total cost, while on a small or cut-up roof paying up for higher-wattage panels buys the same production in fewer slots. Either way, the system size in kilowatts stays fixed; only the number of rectangles changes.

The derate factor and real-world losses

The derate factor is the least intuitive part of solar math, so give it a plain account. It is a single multiplier, commonly near 0.8, that shrinks a panel’s ideal laboratory output down to what it actually delivers on a roof across a year. Panels are rated under standard test conditions that are bright, cool, clean, and perfectly aimed. A real roof is almost none of those things for most of the day, so the derate gathers every ordinary loss into one honest haircut rather than pretending the rating is the reality.

What goes into that roughly 20 percent haircut? The inverter loses a little converting the panels’ direct current into the alternating current your house uses. Panels run hotter than the test temperature and lose output when hot. Wiring has resistance, so some energy is lost between the roof and the panel. Dust, pollen, and the occasional dusting of snow shade the glass. And panels are rarely aimed at the theoretically perfect angle. None of these is large alone, but together they typically pull real production 15 to 20 percent below the lab number.

That is why this reference multiplies by 0.8, and why some tools use 0.85 for a milder assumption. The choice matters: at 0.8 our 10,800 kilowatt-hour example sizes to about 8.2 kilowatts, while at 0.85 it trims to about 7.7 kilowatts, the figure our sizing field guide uses. Neither is wrong; they encode slightly different assumptions about losses. The one genuine error is dropping the derate entirely, which undersizes a system by about a fifth and produces a confident number that a real roof will never meet.

System size from your annual kWh

Now the first lookup table proper: annual usage to system size. These figures assume an average 4.5 peak sun hours and a 0.8 derate, so each kilowatt makes about 4.5 times 365 times 0.8, which is roughly 1,314 kilowatt-hours a year. To size a system, divide your annual kilowatt-hours by that 1,314 figure. The table does the division for a range of common usage levels; find the row nearest your own annual kilowatt-hours and read across.

Read the table and the shape is nearly linear: system size grows almost in step with usage, because usage is the numerator of the formula. Cutting your consumption before you size, by sealing ducts, swapping to efficient appliances, or trimming phantom loads, buys a smaller and cheaper array for the same comfort, which is why efficiency first is almost always the cheaper path to the same bill. The panel columns also restate the wattage lesson: the 450 watt column runs a few panels shorter than the 400 watt column at every usage level.

Two adjustments make the table your own. If your region is cloudier than average, your system size climbs; if sunnier, it falls, and the regional table below quantifies the swing. And if you only want to cover part of your bill rather than all of it, scale the usage figure by your offset goal before you look it up, since covering 80 percent of a 12,000 kilowatt-hour home is the same as sizing a 9,600 kilowatt-hour full offset. The sizing calculator folds both adjustments in automatically.

Daily production from system size and sun hours

Flip the question around and ask what a given system makes each day. Daily production is system kilowatts times peak sun hours times the 0.8 derate, so a 10 kilowatt system at 4.5 sun hours makes about 10 times 4.5 times 0.8, or 36 kilowatt-hours a day. The table below crosses common system sizes against the three sun bands so you can read production straight off, whether you are checking a quote’s estimate or picturing a real day of output.

Daily production is an average, not a promise about any single day, which is the crucial caveat. A clear midsummer day can produce well above the row figure and a heavy overcast winter day far below it; the number in the table is the year-round daily mean the annual math implies. That is why comparing your monitoring app’s reading to this table only makes sense across a full month of similar weather, not day to day, a point the maintenance and monitoring habit in our electric bill reading coverage reinforces.

The same table also frames battery and appliance questions. If you know an appliance’s daily draw in kilowatt-hours, you can read which system size covers it against your region’s column: a central air conditioner pulling 20 kilowatt-hours on a hot day, for instance, is well within an 8 kilowatt system’s average output in most regions, though timing and storage complicate the picture, as our note on running an air conditioner on solar works through.

Panel count at common wattages

Panels needed is the purest conversion in solar: system watts divided by panel watts, rounded up, with no derate involved because you are counting hardware, not estimating output. The table below converts a spread of system sizes into panel counts at the four common wattages, so you can move between the kilowatt figure installers quote and the panel count you will actually see on the roof. Round-ups mean the counts step in whole panels, since you cannot install a fraction of one.

The reverse direction is just as useful and worth practicing, because it lets you audit a quote. Multiply a proposal’s panel count by its panel wattage and divide by 1,000 to recover the system size in kilowatts, then check it against the size the usage table implied. Twenty 400 watt panels recover to an 8 kilowatt system, since 20 times 400 is 8,000 watts. If a quote’s recovered size lands far from what your usage justifies, either the panel wattage, the count, or the sizing assumption is off, and that is exactly the discrepancy worth questioning before you sign.

Keep both numbers in view when you compare installers. System size in kilowatts lets you compare proposals built from different panel wattages on equal footing, while the panel count is the physical reality on the roof. Prices are quoted per watt precisely to normalize across panel sizes, a habit our solar cost breakdown leans on, so carry the kilowatt figure between quotes and translate to panels only when you want to picture the array.

Annual production per panel by sun hours

Sometimes you want the smallest unit: what one panel makes in a year. Per-panel annual production is the panel’s kilowatts times peak sun hours times 365 times the 0.8 derate. A 400 watt panel at 4.5 sun hours makes about 0.4 times 4.5 times 365 times 0.8, which is roughly 525 kilowatt-hours a year. The table crosses the common wattages against the three sun bands, so you can estimate how many panels replace a known chunk of usage.

The table is the quickest way to see why the same house needs different counts in different places. A 400 watt panel earns about 409 kilowatt-hours a year in a cloudy 3.5 hour region but past 670 in the sunny southwest, a difference of more than 60 percent from geography alone. Divide your annual usage by the cell that matches your panel and region, round up, and you have a panel count without touching system size at all. It is the same answer the other tables give, arrived at from the per-panel direction.

One honest caveat belongs here: these are first-year, well-oriented figures. Panels fade slowly with age, commonly losing a small fraction of a percent each year, so a panel might produce around 88 to 90 percent of its original output after 25 years. The derate handles first-year losses, and this gentle degradation is the multi-decade version of the same honesty. For sizing, it is a reason not to build a razor-thin full offset in year one, not a reason to add many panels, as our note on how long solar panels last explains.

The master kWh to system size table

If you keep only one table from this reference, keep this one, because it folds usage and region together into a single lookup. It crosses common annual usage levels against the three sun bands, at 400 watt panels and a 0.8 derate, and gives both the system size and the panel count in each cell. Find your usage row, slide to your region’s column, and read your illustrative system size and count in one move.

Read across any single row and the regional swing is stark: the same 10,800 kilowatt-hour home needs about 27 panels in a cloudy northern region but only 16 in the desert southwest, an 11 panel gap driven entirely by sun. Read down any column and the near-linear scaling with usage reappears. Between the two axes, this one table answers the most common sizing question, what size system do I need, for a wide range of homes without any arithmetic on your part.

The cells are illustrative starting points, not quotes, and three refinements make them yours. Adjust the usage figure by your offset goal if you want partial coverage. Nudge toward the neighboring column if your local sun sits between the bands. And if you install higher-wattage panels, drop the count using the wattage table above while keeping the system size fixed. When you want your exact number rather than the nearest cell, the sizing calculator runs the same formulas on your own inputs.

A worked example in longhand

Numbers land better worked in full, so take one home all the way through, writing the arithmetic longhand rather than as symbols. The Delgado household pulls a full year of bills and adds them to 10,800 kilowatt-hours, close to a typical single-family figure. They live in an average region at 4.5 peak sun hours, they plan to use standard 400 watt panels, and they will size with a conservative 0.8 derate. Those are the four inputs; everything else is arithmetic.

Start with how much one kilowatt makes in a year. Multiply 4.5 peak sun hours by 365 days to get 1,642.5, then multiply by the 0.8 derate to get about 1,314. So each kilowatt of their array produces roughly 1,314 kilowatt-hours a year. Now size the system: divide their 10,800 kilowatt-hours of usage by that 1,314, which comes to about 8.2. The Delgados need an 8.2 kilowatt system. Had they used a milder 0.85 derate, the same division would land near 7.7 kilowatts instead, which shows how much the derate assumption alone moves the answer.

Convert that size to panels. An 8.2 kilowatt system is 8,200 watts, and 8,200 divided by 400 watts per panel is 20.5, which rounds up to 21 panels. Check the production to confirm the size is right: 8.2 kilowatts times 4.5 sun hours times the 0.8 derate is about 29.5 kilowatt-hours a day, and 29.5 times 365 days is roughly 10,800 kilowatt-hours a year, which matches their usage as intended. The circle closes: the system sized to their bill produces, on average, what their bill consumes. Run your own four inputs through the sizing calculator to trace the same longhand on your numbers.

Converting a monthly bill into annual kWh

Many readers know their monthly dollar bill better than their annual kilowatt-hours, so bridge the two. The direct route is to read kilowatt-hours off the bill itself, since most statements print the energy used that period, and to add up twelve months or use the rolling annual total many utilities show. That is always the most accurate path, because it uses your real energy rather than an inference from dollars.

When you only have a dollar figure, you can estimate kilowatt-hours by dividing your total bill by your all-in rate per kilowatt-hour. If a home pays 180 dollars a month at an all-in rate near 0.17 dollars per kilowatt-hour, that is about 1,059 kilowatt-hours a month, or roughly 12,700 a year. The all-in rate is your total bill divided by the kilowatt-hours it covered, not the supply rate a plan advertises, because fixed charges and delivery fees mean the true cost per kilowatt-hour is usually higher than the headline number. Our electric bill reference shows how to compute that all-in rate from a single statement.

Two cautions keep the conversion honest. Rates vary widely by region and by season, so a single national rate is only a rough placeholder; use your own all-in figure for anything you act on. And because rates drift over time while your energy use is comparatively stable, prefer sizing from kilowatt-hours whenever you can, treating the dollar-to-energy conversion as a fallback for when a full year of usage data is not in front of you. Once you have annual kilowatt-hours, every table in this reference is a straight lookup.

Reading system size and panel count both ways

System size and panel count describe one array two ways, and fluently converting between them is what lets you audit a quote instead of trusting it. System size in kilowatts is the total wattage of every panel added up and divided by 1,000. Panel count is how many physical panels sit on the roof. Twenty 400 watt panels make an 8 kilowatt system, because 20 times 400 is 8,000 watts. Our worked 8.2 kilowatt figure rounds up to 21 panels, nominally an 8.4 kilowatt array as installed, the small gap being the round-up from a fractional panel.

Installers usually lead with the kilowatt figure because it normalizes across panel sizes, while homeowners tend to think in panels. A 7.6 kilowatt system might be 19 panels at 400 watts or 18 at 425 watts, nearly the same kilowatts, different counts. Prices are quoted per watt for the same reason, so the kilowatt number is the one to carry between quotes. Think in kilowatts when you compare systems and prices, and translate to panels only when you want to picture the roof.

The audit move is quick. Take any proposal, multiply its panel count by its stated panel wattage, divide by 1,000, and you have recovered the system size in kilowatts. Compare that recovered size against the size your usage justified from the master table, and against the price per watt in our cost breakdown. A recovered size far above your usage may mean an oversized system you will export at a low rate; far below, an undersized one that leaves a residual bill. Either way, the two-way conversion turns a glossy proposal into numbers you can check.

Kilowatts vs kilowatt-hours: the unit that trips people up

The single most common confusion in all of solar is treating kilowatts and kilowatt-hours as interchangeable, so separate them cleanly. A kilowatt is power, a rate, the size of the tap. A kilowatt-hour is energy, an amount, the water that flows through the tap over time. Your solar system is sized in kilowatts because that is its instantaneous capacity in full sun. Your utility bills you in kilowatt-hours because that is the energy you actually consumed over the month.

The bridge between them is time. One kilowatt running flat out for one hour delivers exactly one kilowatt-hour. So an 8 kilowatt array does not make 8 kilowatt-hours; it makes 8 kilowatt-hours only during each hour it runs at full output, and across a real day of varying sun it produces the daily figure the production table gives, which is where peak sun hours come in. Peak sun hours are, in effect, the number of full-capacity hours per day, which is exactly why multiplying kilowatts by peak sun hours converts a power rating into a daily energy figure.

Watch for the same confusion in batteries and appliances, where it recurs. A battery’s capacity is in kilowatt-hours, the energy it stores, while its power rating in kilowatts sets how much it can deliver at once; the two are different specifications, as our battery-count briefing details. An appliance’s wattage is power, and multiplying it by the hours it runs gives the kilowatt-hours it consumes. Keep the units straight and every figure in this reference stays legible; blur them and a system size and a daily production figure start to look like the same number, which they never are.

Sanity-checking a quoted production estimate

A proposal usually states an expected annual production in kilowatt-hours, and this reference gives you three quick ways to sanity-check it before you accept it. The first is the per-kilowatt rule. Divide the quoted annual production by the system size in kilowatts, and confirm the result falls in the 1,000 to 1,700 kilowatt-hours per kilowatt band that your region supports. A quote claiming 1,900 kilowatt-hours per kilowatt in a cloudy region is optimistic; one showing 900 in the desert southwest is oddly conservative or reflects heavy shading.

The second check works from the panels. Multiply the panel count by the per-panel annual figure from the production table for your region and wattage, and compare it to the quoted total. If a proposal lists 21 panels of 400 watts in an average region, the table’s roughly 526 kilowatt-hours per panel implies about 11,000 kilowatt-hours a year; a quoted figure far from that deserves a question. The third check is the derate. Ask what derate or performance ratio the estimate assumes, and confirm it is somewhere near 0.8, not an unstated 1.0 that quietly inflates the number by a fifth.

These checks do not require the installer’s software, only the tables here and a few divisions, which is the point of keeping a reference. A production estimate is the figure your whole payback rests on, so a proposal that cannot survive a per-kilowatt and a per-panel cross-check is one to slow down on. Carry the checked production figure into the payback math in our break-even briefing, because an inflated production estimate shortens payback on paper in a way the roof will never deliver.

How regional sun changes the same system

To make the regional lever concrete, hold one home fixed and move it across the country. The Okafor family uses 10,800 kilowatt-hours a year and wants a full offset with 400 watt panels at a 0.8 derate. In the desert southwest at 5.75 peak sun hours, each kilowatt makes about 1,679 kilowatt-hours a year, so they need about 6.4 kilowatts, roughly 16 panels. Move the identical household to an average 4.5 hour region and each kilowatt makes about 1,314 kilowatt-hours, pushing them to about 8.2 kilowatts and 21 panels.

Move them again to a cloudy 3.5 hour region in the Pacific Northwest, where each kilowatt makes only about 1,022 kilowatt-hours a year, and the same 10,800 kilowatt-hours now needs about 10.6 kilowatts and 27 panels. Same family, same appliances, same offset goal, and the array swings from 16 to 27 panels purely on geography. That 11 panel spread is the single clearest illustration of why a national average is only a starting point and why your own region’s sun band belongs in every lookup.

The regional lever cuts the other way on production too. That desert-southwest 6.4 kilowatt system and the Pacific Northwest 10.6 kilowatt system both produce about the same 10,800 kilowatt-hours a year, because each was sized to the same usage, but the northern system had to be far larger to get there. So a cloudier region pays more for the same energy, in panels and in dollars, which is worth weighing against local electricity rates when you price the project against our cost breakdown. Run your own region and usage through the sizing calculator to see where your array lands.

Where the real-world losses go

The derate near 0.8 represents roughly a fifth of a panel’s lab output lost before it reaches your meter, and it helps to see where that loss goes. The breakdown below is illustrative, dividing the total real-world loss into its usual contributors as shares that sum to 100. The exact split shifts with climate, hardware, and site, but the rough ordering is stable: conversion and heat dominate, with soiling and wiring filling out most of the rest.

Reading the split explains why the derate is not a single universal number. A cool, breezy, coastal roof with a high-efficiency inverter loses less to heat and conversion and may justify a 0.85 factor, while a hot inland roof caked in seasonal dust loses more and sits nearer 0.8 or below. Two of the largest slices, soiling and heat, also vary by season, which is part of why production swings across the year around the annual average the sun-hours figure encodes.

The practical upshot is modest but useful. You cannot eliminate the derate, since inverter conversion and heat are physics, but you can avoid making it worse: keep the array reasonably clean, ensure good airflow behind the panels, and choose a quality inverter. None of that changes the tables in this reference, which already assume a realistic 0.8, but it does mean a well-built system lands at the optimistic end of the band rather than the pessimistic one.

A note on costs and incentives

This reference deliberately keeps costs and incentives out of the core tables, because unlike the physics they change constantly and would date the page. Equipment prices, installed cost per watt, tax credits, rebates, and net-metering rules all move from year to year and vary by state and utility, so any specific figure here would risk being stale by the time you read it. Treat every dollar amount you encounter elsewhere as something to confirm current before you rely on it.

What the stable math does give you is the denominator those volatile figures attach to. Once this reference hands you a system size in kilowatts, an installer’s cost per watt turns it into a price, and the current tax credit turns that into a net cost, both of which you should verify against today’s rules rather than a number from a prior year. The relationship, size times cost per watt, is stable; the cost per watt itself is not. That separation is exactly why sizing belongs in a reference and pricing belongs in a page you refresh.

For the money side, lean on the articles built to be updated. Our solar cost breakdown carries the current price-per-watt ranges and the tax-credit mechanics, and the payback briefing turns a size, a price, and a savings rate into a break-even you can test. Size your system from the stable tables here, then price it there, and confirm the credit and net-metering rules with a current source, because those are the figures most likely to have shifted since anything was written.

The one-page sizing cheat sheet

Here is the whole reference compressed into a single scannable list, the part worth saving. Work top to bottom with your own three inputs and you will have a defensible system size, panel count, and production figure before any quote arrives.

• Find your annual usage in kilowatt-hours from a full twelve months of bills, not a single month.
• Find your region's annual average peak sun hours: about 3.5 in the Pacific Northwest and Northeast, 4.5 average, 5.75 in the desert southwest.
• Pick a panel wattage in the 350 to 450 watt range; use 400 as a clean middle figure.
• Compute per-kilowatt annual output: sun hours times 365 times a 0.8 derate, about 1,314 kilowatt-hours at average sun.
• Size the system: annual kilowatt-hours divided by that per-kilowatt figure gives kilowatts.
• Count panels: system kilowatts times 1,000, divided by panel watts, rounded up.
• Estimate daily production: system kilowatts times sun hours times 0.8.
• Estimate annual production: daily production times 365, and confirm it lands near your usage.
• Sanity-check any quote: annual production divided by system size should sit in the 1,000 to 1,700 per-kilowatt band.
• Confirm costs, tax credits, and net-metering rules against a current source; the math here is stable, those figures are not.

Keep this list beside your bills and any proposal, and the reference becomes a working tool rather than a read. Every line traces back to one of the four core formulas, so if a number ever looks off, you can rebuild it from scratch rather than trusting a black box. Drop your own three inputs into the sizing calculator to replace each illustrative figure with your specific one.

The bottom line

Solar sizing is not a mystery that a quote reveals; it is arithmetic on three stable numbers. Your annual kilowatt-hours, your local peak sun hours, and your panel wattage, tied together by a derate near 0.8, give you a system size, a panel count, and a daily production figure that hold up regardless of what equipment prices or tax credits happen to be doing. This reference tables those relationships so you can look up the answer rather than rederive it, and so you can check any installer’s numbers against a source you understand.

Use the tables as illustrative starting points, refine them with your own bills and region, and keep the volatile money figures where they belong, in the pages built to be refreshed. Size your system from the stable math here, then price it against our cost breakdown, time the break-even with the payback briefing, work the method step by step in the sizing field guide, and cross-check the count from the other direction in the panel-count briefing. When you want your own figure instead of the nearest table row, the sizing calculator runs every formula in this reference live on your numbers.

WattBarn publishes this reference to help you look up and check solar math, not to replace a licensed installer’s site-specific work. The usage levels, sun hours, wattages, system sizes, panel counts, and production figures in every table are illustrative examples built from the stable formulas to show the relationships, not measurements of your home, and your real usage, roof orientation, shading, hardware, and local conditions will produce different numbers. Costs, tax credits, and net-metering rules are intentionally left out of the core tables because they change often; confirm any such figure against a current source. Let an installer’s on-site assessment and written production estimate, not these worked sketches, decide the system you actually buy.