Solar & battery
Solar Panel Output Calculator
Works module output, where the temperature coefficient explains the summer shortfall.
Result
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. Panel ratings are measured under standard test conditions at 25 degrees C. Real cells run far hotter than that on a roof and produce measurably less, which is part of what the derate accounts for.
Get a Free Business AuditHow the calculation works
The module's rated output adjusted for actual irradiance and for cell temperature using the manufacturer's temperature coefficient. Silicon modules lose output as cell temperature rises above the 25 degree test condition, and a module in full sun runs well above ambient, which is why the hottest days are not the most productive.
What moves the number
- Irradiance at the time, which scales output directly.
- Cell temperature against the 25 degree test condition.
- The module's temperature coefficient from its datasheet.
- Soiling and any shading.
Where this one goes wrong
Cell temperature is well above ambient in full sun, by a margin that depends on mounting and airflow, so a flush roof mount with little air gap runs hotter and produces less than the same module on an open rack. That is a design consequence rather than a measurement curiosity. The counterpart is that cold bright days produce above the nameplate rating, which is why string voltage has to be checked at the coldest expected temperature rather than at test conditions.
Before you quote
- Use the datasheet temperature coefficient, not a generic figure.
- Remember cell temperature exceeds ambient in full sun.
- Check string voltage at the coldest expected temperature.
Where the rules come from
Questions
About this calculator
Why do panels produce less on the hottest days?
Because silicon loses output as cell temperature rises above the 25 degree test condition, and a module in full sun runs well above ambient. The irradiance is high and the temperature penalty offsets part of it.
Does mounting affect output?
Yes. A flush roof mount with little air gap runs hotter than the same module on an open rack with airflow behind it, and the hotter module produces less. It is a real design consequence rather than a rounding detail.
Can a module exceed its rating?
On cold bright days, yes, which is why string voltage must be checked at the coldest expected temperature rather than at test conditions. An array that is fine in summer can exceed an inverter's input limit on a cold clear morning.
The formula
daily kWh = panel watts / 1000 x peak sun hours x derate
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How to use this calculator
- Panel rating (watts). Enter your own figure; the page starts at 400.
- 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%.
- Number of panels. Enter your own figure; the page starts at 1.
- Years to project. Enter your own figure; the page starts at 25.
- Annual degradation (%). Enter your own figure; the page starts at 0.5%.
The result updates as you type, so you can try a figure, see what it does to per panel per day, 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 |
|---|---|
| Panel rating (watts) | 400 |
| Peak sun hours per day | Mid-Atlantic, Ohio Valley (4.5) |
| System derate | 80% |
| Number of panels | 1 |
| Years to project | 25 |
| Annual degradation | 0.5% |
What comes out
| Result | Value |
|---|---|
| Per panel per day main result | 1.4 |
| Per panel per year | 526 |
| Whole array per year | 526 |
| Array over the period | 12,381 |
| Output in the final year | 88.7% |
A 400 W panel makes 1.4 kWh a day here, 526 kWh a year. After 25 years it is still producing 88.7% of what it does new.
How sensitive the answer is
Holding everything else steady and changing only panel rating (watts), this is what happens to per panel per day. It is worth knowing which of your inputs the answer actually hinges on, and which barely move it.
| Panel rating (watts) | Per panel per day |
|---|---|
| 200 | 0.7 |
| 300 | 1.1 |
| 400 example above | 1.4 |
| 600 | 2.2 |
| 800 | 2.9 |
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
- Panel rating (watts)
- A number, between 0 and 1000, starting at 400.
- 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%.
- Number of panels
- A number, between 0 and 10000, starting at 1.
- Years to project
- A number, between 0 and 40, starting at 25.
- Annual degradation (%)
- A percentage, between 0 and 5, starting at 0.5%.
What the results tell you
| Result | What it is |
|---|---|
| Per panel per day | The headline result. A decimal figure. |
| Per panel per year | A number. |
| Whole array per year | A number. |
| Array over the period | A number. |
| Output in the final year | A percentage. |
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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