Solar & battery
Solar Production Calculator
Estimates annual production, where location and orientation matter more than the hardware.
Result
Peak sun hours are not daylight hours. They are the day's total solar energy expressed as hours at 1,000 W per square metre, so a long overcast day can be worth fewer peak sun hours than a short bright one. Use a figure for your own location and month rather than a national average. System derate covers inverter losses, wiring, heat, soiling and panel mismatch. Around 0.80 is the figure most design tools use; a hot climate, a dusty site or a shaded roof pushes it lower. It is why a system never produces the sum of its panel ratings. Production is seasonal: a system in a northern climate can make three or four times as much in June as in December, so an annual average says nothing about any particular month.
Get a Free Business AuditHow the calculation works
Array size times peak sun hours times days times the system derate gives annual energy. Peak sun hours is the daily solar resource expressed as equivalent hours at full test irradiance, which makes the arithmetic simple, and it varies by location, orientation and tilt rather than being a property of the equipment.
What moves the number
- Peak sun hours for the location.
- Azimuth, meaning which way the array faces.
- Tilt, which matters less than azimuth in most cases.
- Derate for temperature, soiling, inverter and wiring.
Where this one goes wrong
Production is seasonal and the annual figure conceals it: a system meeting annual consumption still exports heavily in summer and draws from the grid in winter, which matters enormously under a tariff that does not credit exports at retail. Model the monthly shape where the economics depend on it. Degradation also reduces output gradually over the system's life, which a first-year figure does not show and a payback calculation should include.
Before you quote
- Model monthly production, not only annual.
- Use the site's own azimuth and tilt rather than an ideal.
- Include annual degradation in any long projection.
Where the rules come from
Questions
About this calculator
What are peak sun hours?
The daily solar resource expressed as equivalent hours at full test irradiance, which makes production arithmetic straightforward. It varies by location, orientation and tilt, and published data for a site is more reliable than any rule of thumb.
Why does the monthly shape matter?
Because a system that matches annual consumption still exports heavily in summer and imports in winter. Under a tariff that credits exports below retail, that mismatch changes the economics substantially, which an annual figure hides completely.
Does production decline over time?
Gradually, through module degradation, which is why a payback calculation using first-year output overstates the later years. Manufacturers publish a degradation rate and warranty, and including it is the difference between a projection and a sales figure.
The formula
annual kWh = kW x peak sun hours x derate x (1 - shading) x 365
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How to use this calculator
- System size (kW DC). Enter your own figure; the page starts at 7.2.
- Peak sun hours per day. Enter your own figure; the page starts at Mid-Atlantic, Ohio Valley (4.5).
- System derate (%). Enter your own figure; the page starts at 80%.
- Shading loss (%). Enter your own figure; the page starts at 3%.
- Electricity rate per kWh ($). Enter your own figure; the page starts at $0.16.
The result updates as you type, so you can try a figure, see what it does to annual production, and change it back. Nothing you enter is sent anywhere or saved.
A worked example
These are the figures the calculator opens with. Every number in the results column below is produced by the same calculation the page runs, so you can follow it through before putting your own figures in.
What goes in
| Input | Value |
|---|---|
| System size (kW DC) | 7.2 |
| Peak sun hours per day | Mid-Atlantic, Ohio Valley (4.5) |
| System derate | 80% |
| Shading loss | 3% |
| Electricity rate per kWh | $0.16 |
What comes out
| Result | Value |
|---|---|
| Annual production main result | 9,177 |
| Daily average | 25.1 |
| Monthly average | 765 |
| Value at your rate | $1,468 |
| Yield per kW installed | 1,275 |
About 9,177 kWh a year, worth $1,468 at 0.16 per kWh. That is 1,275 kWh for every kW installed, which is the number to compare systems on.
How sensitive the answer is
Holding everything else steady and changing only electricity rate per kwh, this is what happens to annual production. It is worth knowing which of your inputs the answer actually hinges on, and which barely move it.
| Electricity rate per kWh ($) | Annual production |
|---|---|
| $0.08 | 9,177 |
| $0.12 | 9,177 |
| $0.16 example above | 9,177 |
| $0.24 | 9,177 |
| $0.32 | 9,177 |
Every other input is held at the example value while this one changes, which is not how a real job behaves. Treat it as a guide to which number matters most, not as five separate quotes.
What each figure means
- System size (kW DC)
- A number, between 0 and 5000, starting at 7.2.
- Peak sun hours per day
- Choose from Pacific Northwest, northern UK (3.5), Northeast, Midwest (4.0), Mid-Atlantic, Ohio Valley (4.5), Southeast, coastal California (5.0), Texas, inland California (5.5), Arizona, Nevada, New Mexico (6.0).
- System derate (%)
- A percentage, between 40 and 100, starting at 80%.
- Shading loss (%)
- A percentage, between 0 and 60, starting at 3%.
- Electricity rate per kWh ($)
- A money amount, starting at $0.16.
What the results tell you
| Result | What it is |
|---|---|
| Annual production | The headline result. A number. |
| Daily average | A decimal figure. |
| Monthly average | A number. |
| Value at your rate | A money figure. |
| Yield per kW installed | A number. |
A design still needs a site visit
Output and sizing figures use the assumptions you enter, not measured irradiance, shading or roof orientation at your address. Real production depends on all three, plus inverter behaviour and local interconnection rules.
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