A solar panel calculator answers a sizing question: given how much electricity a building uses and how much sunlight the site receives, how many panels would it take to generate a comparable amount over a year? That is a well-defined piece of arithmetic, and this page does it explicitly, showing every intermediate figure rather than producing a single number from hidden assumptions.
The most important design decision here is that the solar resource is an input rather than a default. Peak sun hours vary by more than a factor of two across inhabited latitudes, and a calculator that quietly assumes a value produces answers that are wrong everywhere except the one place the assumption came from. Arb Digital publishes a free tools library, and this page belongs to a new ecology section alongside the electricity bill calculator, which prices the consumption side and does not size generation, and the water usage calculator, which covers an unrelated resource.
What This Solar Panel Calculator Does
It converts an annual electricity figure into an array. The headline result is the number of panels needed to generate your target share of that consumption over a year, together with the resulting system size in kilowatts. The supporting figures give the annual generation that array would produce, the roof area it occupies, the value of the electricity it displaces at your price, and a simple payback period from your installed cost.
The two bars compare annual use with annual generation directly, because panel counts round upward and the result is rarely an exact match. Seeing a 2% overshoot or a 15% shortfall on the bar is more informative than reading two numbers in different boxes.
What this page does not do is tell you whether to install anything. It reports arithmetic. Incentives, tariffs, export payments, financing, roof condition, planning constraints and the value of self-consumption all bear on whether a system makes sense, and none of them are modelled here. Nothing on this page is financial advice, and the payback figure is a simple arithmetic ratio rather than an investment appraisal.
How to Use It
- Enter twelve months of measured electricity use. Add up a year of bills. Using one month multiplied by twelve is the single most common way this calculation goes wrong, because consumption is strongly seasonal.
- Look up your peak sun hours. This is the daily average solar energy per square metre at your location. Use a published solar resource dataset for your latitude and climate rather than the placeholder value.
- Set the panel rating and area. Both are on the panel datasheet. Common residential modules sit between roughly 350 W and 500 W, with a physical area around 1.7 to 2.2 square metres.
- Set a realistic performance ratio. 80% is a reasonable starting figure for a well-oriented, unshaded roof. Reduce it substantially for shading, a poor azimuth, or a hot climate.
- Read the array size before the payback. The panel count and roof area are physical constraints; the payback figure depends entirely on prices you entered and will change whenever they do.
The Formula / How It's Calculated
The chain runs from one panel to the whole array.
Annual generation per panel (kWh) = panel watts ÷ 1,000 × peak sun hours × 365 × performance ratio. Target generation = annual use × target share. Panels = target generation ÷ generation per panel, rounded up to a whole panel. System size (kW) = panels × panel watts ÷ 1,000. Roof area = panels × panel area. Annual bill offset = the lesser of generation and use, × price per kWh. Simple payback = installed cost ÷ annual bill offset.
Worked example, matching the values the page loads with. A 420 W panel at 4.5 peak sun hours generates 0.42 × 4.5 = 1.89 kWh on an average day before losses; across 365 days that is 689.85 kWh, and at an 80% performance ratio it is 551.88 kWh per panel per year. Covering 10,800 kWh needs 10,800 ÷ 551.88 = 19.57 panels, rounded up to 20 panels. That is 20 × 420 W = 8.4 kW, generating 20 × 551.88 = 11,038 kWh a year and occupying 20 × 1.95 = 39.0 m². At 0.16 per kWh the offset is capped at the 10,800 kWh actually used, giving 1,728 a year. At 2.80 per watt the installed cost is 8,400 × 2.80 = 23,520, so simple payback is 23,520 ÷ 1,728 = 13.6 years.
Note the cap in the offset step. Generating more than you use does not automatically save more money, because the surplus is exported rather than consumed, and export is usually paid at a different rate. Capping the offset at consumption keeps the payback figure conservative rather than optimistic.
Peak Sun Hours: What the Number Actually Means
Peak sun hours is a compression device. Real irradiance rises and falls through the day, and rather than integrate that curve, the convention expresses the day's total energy as the number of hours it would have taken at a standard reference intensity of 1,000 watts per square metre. A site receiving 4.5 kWh per square metre in a day has 4.5 peak sun hours, regardless of whether the sun was above the horizon for eight hours or sixteen.
That is why the unit matters. The US Department of Energy's Solar Radiation Basics notes that solar resource for photovoltaic purposes is expressed in kilowatt-hours per square metre or watts per square metre, and it is the kWh/m²/day figure that converts one-for-one into peak sun hours. If a dataset gives you an annual kWh/m² total instead, divide by 365 first.
Two practical warnings. First, the figure depends on the plane of measurement: horizontal irradiance, tilted-plane irradiance and direct normal irradiance are different numbers for the same location, and a fixed roof array should be sized on the tilted-plane figure for its actual orientation. Second, an annual average hides enormous seasonal variation — a site averaging 4.5 might see 6.5 in June and 1.8 in December — which is why annual generation and winter self-sufficiency are entirely different questions.
Why the Performance Ratio Is Not Optional
A panel rated 420 W is rated under standard test conditions: 1,000 W/m² of irradiance, a cell temperature of 25 °C and a defined light spectrum. Roofs do not provide those conditions. The performance ratio is the single number that carries every deviation between the laboratory and the roof.
Temperature is usually the largest component. Photovoltaic output falls as cell temperature rises, and cells in full sun run well above air temperature, so a hot summer afternoon produces less than the rating suggests even at peak irradiance. The DOE's overview of Solar Photovoltaic Performance and Efficiency Basics sets out the underlying mechanisms — wavelength, recombination, temperature and reflection — that determine how much incident light becomes electricity in the first place.
The rest of the ratio is made up of inverter conversion losses, direct-current and alternating-current wiring resistance, soiling from dust and pollen, module mismatch, and any shading. Shading deserves separate mention because it is not proportional: a small shadow across one module can disproportionately affect a string, depending on how the system is wired and whether module-level electronics are fitted. If any part of the roof is shaded for part of the day, an 80% performance ratio is optimistic and the figure should come from a site-specific assessment rather than a rule of thumb.
Annual Matching Versus Actually Using the Power
Sizing an array to match annual consumption is the standard approach, and it hides a timing problem worth understanding. Generation peaks around midday and in summer. Household demand peaks in the early evening and in winter. A system that generates exactly as much as a home uses over a year may still export half of what it makes and import half of what it needs.
How much that matters depends entirely on the tariff. Where exported units are credited at the same rate as imported ones, timing barely affects the economics. Where export is paid at a fraction of the import rate — which is increasingly common — the value of a kilowatt-hour depends on when it is generated, and the simple payback figure on this page will be optimistic because it prices every displaced unit at the full import rate.
The practical consequence for sizing is that a larger array does not scale value linearly once generation exceeds daytime demand. If you want to model this properly, split your consumption into daytime and non-daytime portions, size against the daytime portion, and treat the rest as a separate export calculation. The electricity bill calculator handles the tariff arithmetic, and the payback period calculator and break-even calculator cover more careful versions of the return question than a single division can.
What This Calculation Deliberately Leaves Out
Several real effects are outside the model, and knowing which ones they are is what keeps the output honest.
Degradation. Panels lose a small fraction of their output each year, so generation in year twenty is below generation in year one. A simple payback based on first-year generation slightly understates the payback period.
Inflation and price movement. The offset is priced at today's electricity rate held constant. Real electricity prices move, and the direction of that movement changes the answer materially over a twenty-year horizon.
Incentives, tariffs and taxes. Grants, rebates, tax treatment and export payments vary by country and change frequently. None are modelled, so the installed cost you enter should already be net of anything you are certain of.
Roof geometry. The area figure is the panels' own area, with no allowance for spacing, setbacks, obstructions, walkways or the fact that a roof plane may not accept a rectangular array. Treat it as a lower bound on usable roof, not a layout.
Batteries and inverter sizing. Storage changes the self-consumption picture entirely and is not represented, and inverters are commonly sized below the array's rated output, which clips a small amount of peak generation. For the material side of a roof project, the roof pitch calculator and square footage calculator are more directly useful than this page.
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Browse Free Tools Talk to Arb DigitalCommon Mistakes to Avoid
- Estimating annual use from one month — consumption is seasonal, and a summer month multiplied by twelve can be a third out in either direction.
- Accepting a default sun-hours figure — the solar resource varies by more than a factor of two between locations, and it is the input that most changes the answer.
- Using the horizontal irradiance figure for a tilted roof — the plane of measurement matters, and the two numbers are not interchangeable.
- Leaving the performance ratio at a default on a shaded roof — shading is not proportional, and 80% is optimistic wherever any part of the array is shaded during the day.
- Reading simple payback as a return — it ignores degradation, price movement, export rates and financing, and it is a ratio rather than an appraisal.
Related Free Tools From Arb Digital
Use the electricity bill calculator to price consumption and tariffs, the payback period calculator for a more careful version of the return question, and the energy converter to move between kilowatt-hours, joules and BTU. The square footage calculator and roof pitch calculator help with the roof itself, and the water usage calculator covers household water. Everything else is in the free online tools hub.
Frequently Asked Questions
Peak sun hours express a day's total solar energy as the number of hours it would take at a reference intensity of 1,000 watts per square metre. A site receiving 4.5 kWh per square metre a day has 4.5 peak sun hours. Use a published solar resource dataset for your location and for the plane your panels will sit on.
Because a default sun-hours value is wrong almost everywhere. The solar resource varies by more than a factor of two between inhabited latitudes and climates, so the tool takes it as an input and asks you to look up the figure that applies to your site.
Around 80% is a reasonable starting point for a well-oriented, unshaded roof. It covers inverter and wiring losses, soiling, temperature effects and orientation. Reduce it substantially for any shading, a poor azimuth or a consistently hot climate, and get a site-specific figure if shading is present.
No. Generation peaks at midday and in summer while household demand peaks in the evening and in winter, so annual totals can match while the timing does not. Without storage, a matched system typically exports a large share of what it makes and imports a large share of what it needs.
Because surplus generation is exported rather than used, and export is usually paid at a different rate from the price of imported electricity. Capping the offset at consumption keeps the payback figure conservative instead of pricing every exported unit at the full retail rate.
No, it is the combined area of the panels themselves. Real layouts need spacing, edge setbacks, access routes and clearance around vents and chimneys, and a roof plane may not accept a rectangular array. Treat the figure as a lower bound.
No. It uses first-year generation throughout. Panels lose a small fraction of output each year, so a payback period based on year-one generation is slightly shorter than one that models declining output over the system's life.
This tool performs sizing and simple arithmetic only. It is not financial advice, not an engineering assessment and not a recommendation to purchase anything. Any real installation should be sized and specified by a qualified installer using site-specific measurements.
