Solar & battery
Solar Inverter Size Calculator
Sizes an inverter, where a ratio slightly above the array is usually deliberate.
Result
Oversizing the array relative to the inverter is deliberate. Panels rarely reach their rating, so a ratio of 1.15 to 1.3 keeps the inverter working near its efficient range for more of the day and loses only a fraction of a percent of annual energy to clipping. For an off-grid or hybrid inverter, the starting surge of motors usually sets the size rather than the array.
Get a Free Business AuditHow the calculation works
The inverter's rating against the array's rating, expressed as a DC to AC ratio, with the clipping that results at peak production. Arrays are commonly sized somewhat above the inverter rating because the array rarely produces its nameplate output, which makes a modest amount of clipping at the peak cheaper than an oversized inverter.
What moves the number
- Array rating against inverter rating.
- Expected peak production, which is below nameplate.
- How much energy is lost to clipping.
- Input voltage limits at the coldest expected temperature.
Where this one goes wrong
String voltage at the coldest expected temperature is the limit that must not be exceeded, because module open circuit voltage rises as temperature falls and a string that is within limits in summer can exceed the inverter's maximum input on a cold clear morning, which is both a code and an equipment issue. Calculate it at the record low for the site. Clipping losses are modest at reasonable ratios and worth modelling rather than guessing, since they depend on the site's production profile.
Before you quote
- Calculate string voltage at the coldest expected temperature.
- Model clipping losses rather than assuming them negligible.
- Confirm the configuration against the adopted code edition.
Where the rules come from
Questions
About this calculator
Why is the array larger than the inverter?
Because the array rarely produces its nameplate output, so a modest amount of clipping at the very peak is cheaper than paying for inverter capacity that is almost never used. The ratio is a deliberate design choice rather than an error.
What limits string size?
The inverter's maximum input voltage, checked at the coldest expected temperature, because module open circuit voltage rises as temperature falls. A string within limits in summer can exceed the maximum on a cold clear morning.
Is clipping a problem?
Not at reasonable ratios, where the energy lost is small relative to the inverter cost saved. It is worth modelling against the site's production profile rather than guessing, since a very high ratio does begin to cost real energy.
The formula
inverter kW = array kW DC / DC-to-AC ratio
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How to use this calculator
- Array size (kW DC). Enter your own figure; the page starts at 7.2.
- DC to AC ratio. Above 1.15 is normal and deliberate.
- Largest simultaneous load (W). Enter your own figure; the page starts at 6,000.
- Motor starting surge (W). Enter your own figure; the page starts at 9,000.
- Inverter efficiency (%). Enter your own figure; the page starts at 96%.
The result updates as you type, so you can try a figure, see what it does to inverter size, 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 |
|---|---|
| Array size (kW DC) | 7.2 |
| DC to AC ratio | 1.2 |
| Largest simultaneous load (W) | 6,000 |
| Motor starting surge (W) | 9,000 |
| Inverter efficiency | 96% |
What comes out
| Result | Value |
|---|---|
| Inverter size main result | 6.0 |
| Peak AC output | 5.9 |
| Production clipped at peak | 1.96% |
| Surge rating needed | 9.0 |
A 7.2 kW array at a 1.2 ratio wants a 6.0 kW inverter. On the brightest days that clips about 1.96% of production, which is the trade accepted for a cheaper inverter.
How sensitive the answer is
Holding everything else steady and changing only array size (kw dc), this is what happens to inverter size. It is worth knowing which of your inputs the answer actually hinges on, and which barely move it.
| Array size (kW DC) | Inverter size |
|---|---|
| 3.6 | 3.0 |
| 5.4 | 4.5 |
| 7.2 example above | 6.0 |
| 10.8 | 9.0 |
| 14.4 | 12.0 |
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
- Array size (kW DC)
- A number, between 0 and 5000, starting at 7.2.
- DC to AC ratio
- Above 1.15 is normal and deliberate.
- Largest simultaneous load (W)
- A number, between 0 and 500000, starting at 6,000.
- Motor starting surge (W)
- A number, between 0 and 500000, starting at 9,000.
- Inverter efficiency (%)
- A percentage, between 80 and 100, starting at 96%.
What the results tell you
| Result | What it is |
|---|---|
| Inverter size | The headline result. A decimal figure. |
| Peak AC output | A decimal figure. |
| Production clipped at peak | A percentage. |
| Surge rating needed | A decimal figure. |
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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