Solar & battery
Solar Array Size Calculator
Sizes an array against a target, where the derate separates nameplate from energy.
Result
String length is limited by the inverter's maximum input voltage at the coldest expected temperature, because panel voltage rises as it gets colder. A string sized on a summer figure can exceed the inverter's limit on the first frosty morning. Array dimensions here exclude module gaps, rail overhang and the fire setbacks code requires.
Get a Free Business AuditHow the calculation works
The energy target divided by the annual energy per kilowatt of array at the site, which is peak sun hours times days times the derate. The resulting array size is a nameplate figure, and the whole point of the derate is that nameplate and delivered energy are different quantities.
What moves the number
- Energy target in kilowatt hours per year.
- Site peak sun hours on the array plane.
- Derate for temperature, soiling, conversion and wiring.
- Shading losses, which are site specific.
Where this one goes wrong
Shading is the loss that cannot be fixed with a derate factor, because it affects specific strings at specific times and its impact depends on the module and inverter architecture. A site with meaningful shading needs an assessment rather than an assumption, and module-level electronics change the answer. The other check is whether the roof can physically take the array the target requires, since the energy answer and the area answer frequently disagree.
Before you quote
- Assess shading on site rather than applying a generic loss.
- Check the array fits the usable roof area.
- Use site-specific peak sun hours on the array plane.
Where the rules come from
Questions
About this calculator
Can shading be handled with a derate factor?
Not well, because it affects specific strings at specific times and its impact depends on the module and inverter architecture. A site with meaningful shading needs an actual assessment, and module-level electronics change the result.
What if the array does not fit?
Then the area answer governs rather than the energy answer, which happens frequently. Either higher efficiency modules, additional roof planes, or a smaller target closes the gap, and the decision belongs in the design rather than being discovered on the roof.
How much does the derate matter?
Enough to change the array size materially, since it accounts for temperature, soiling, inverter conversion, wiring and mismatch together. Sizing from nameplate without it produces a system that falls short of its target by that whole margin.
The formula
panels = system watts / panel watts; rows = panels / panels per row
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How to use this calculator
- System size (kW DC). Enter your own figure; the page starts at 7.2.
- Panel rating (watts). Enter your own figure; the page starts at 400.
- Panel length (in) (in). Enter your own figure; the page starts at 74 in.
- Panel width (in) (in). Enter your own figure; the page starts at 41 in.
- Orientation. Enter your own figure; the page starts at Portrait.
- Panels per row. Enter your own figure; the page starts at 8.
The result updates as you type, so you can try a figure, see what it does to panels, 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 |
| Panel rating (watts) | 400 |
| Panel length (in) | 74 in |
| Panel width (in) | 41 in |
| Orientation | Portrait |
| Panels per row | 8 |
| Panels per string | 12 |
What comes out
| Result | Value |
|---|---|
| Panels main result | 18 |
| Rows | 3 |
| Strings | 2 |
| Array width | 27.3 |
| Array depth | 18.5 |
| Array area | 506 |
18 panels in 3 rows and 2 strings, occupying roughly 27.3 by 18.5 feet, 506 square feet of roof plane.
How sensitive the answer is
Holding everything else steady and changing only system size (kw dc), this is what happens to panels. It is worth knowing which of your inputs the answer actually hinges on, and which barely move it.
| System size (kW DC) | Panels |
|---|---|
| 3.6 | 9 |
| 5.4 | 14 |
| 7.2 example above | 18 |
| 10.8 | 27 |
| 14.4 | 36 |
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.
- Panel rating (watts)
- A number, between 0 and 1000, starting at 400.
- Panel length (in) (in)
- Measured in in, between 0 and 200, starting at 74 in.
- Panel width (in) (in)
- Measured in in, between 0 and 120, starting at 41 in.
- Orientation
- Choose from Portrait, Landscape.
- Panels per row
- A number, between 0 and 200, starting at 8.
- Panels per string
- Set by the inverter's voltage window and the coldest expected temperature.
What the results tell you
| Result | What it is |
|---|---|
| Panels | The headline result. A number. |
| Rows | A number. |
| Strings | A number. |
| Array width | A decimal figure. |
| Array depth | A decimal figure. |
| Array area | 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.
Keep going
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