Solar & battery
Solar Inverter Load Calculator
Works inverter loading, where continuous and surge capability are different numbers.
Result
Only one motor is assumed to start at a time, which is the normal design case. Adding every surge together sizes an inverter far larger than reality needs. Surge ratings are also time-limited, typically a few seconds, so a load that surges repeatedly needs a soft starter rather than a bigger inverter.
Get a Free Business AuditHow the calculation works
The continuous load the inverter must supply, and separately the surge it must tolerate, against the inverter's continuous and surge ratings. Inverters are specified with both, and a load list that fits the continuous rating can still exceed the surge rating when a motor starts.
What moves the number
- Continuous simultaneous load.
- Surge from motor starting.
- Inverter continuous and surge ratings and their durations.
- Ambient temperature, which can derate the inverter.
Where this one goes wrong
Surge ratings are specified for a duration, and a brief rating does not help with a load whose starting current persists longer, which is why the motor's actual starting characteristic matters rather than a generic multiplier. Ambient temperature also derates many inverters, so one installed in a hot garage or in direct sun delivers less than its rating, and placing it in a cooler location is free capacity.
Before you quote
- Check surge rating duration against the motor's starting characteristic.
- Account for temperature derating at the installed location.
- Add the continuous loads that genuinely coincide, not every load.
Questions
About this calculator
What is the difference between continuous and surge rating?
Continuous is what the inverter supplies indefinitely; surge is a higher figure it tolerates briefly, specified for a duration. A load list fitting the continuous rating can still exceed the surge rating when a motor starts.
Does a surge rating guarantee a motor will start?
Not by itself, because the rating is for a stated duration and some motors draw high current for longer than that. The motor's actual starting characteristic is what matters rather than a generic multiplier of running current.
Does location affect inverter capacity?
Yes, since many inverters derate at high ambient temperature. One installed in a hot garage or in direct sun delivers less than its rating, which makes a cooler, shaded, ventilated location effectively free capacity.
The formula
worst case = continuous load + the largest single starting surge
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How to use this calculator
- Fridge (running W). Enter your own figure; the page starts at 150.
- Fridge surge (W). Enter your own figure; the page starts at 900.
- Pump (running W). Enter your own figure; the page starts at 750.
- Pump surge (W). Enter your own figure; the page starts at 2,200.
- Everything else running (W). Enter your own figure; the page starts at 600.
- Inverter continuous rating (kW). Enter your own figure; the page starts at 5.
The result updates as you type, so you can try a figure, see what it does to continuous load, 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 |
|---|---|
| Fridge (running W) | 150 |
| Fridge surge (W) | 900 |
| Pump (running W) | 750 |
| Pump surge (W) | 2,200 |
| Everything else running (W) | 600 |
| Inverter continuous rating (kW) | 5 |
| Inverter surge multiple | 2 |
What comes out
| Result | Value |
|---|---|
| Continuous load main result | 1,500 |
| Worst-case surge | 2,950 |
| Inverter surge capability | 10,000 |
| Continuous headroom | 70.0% |
| Surge margin | 70.5% |
Continuous load is 1,500 W with 70.0% headroom, and the worst surge of 2,950 W sits inside the inverter's 10,000 W surge capability.
How sensitive the answer is
Holding everything else steady and changing only fridge (running w), this is what happens to continuous load. It is worth knowing which of your inputs the answer actually hinges on, and which barely move it.
| Fridge (running W) | Continuous load |
|---|---|
| 75 | 1,425 |
| 113 | 1,463 |
| 150 example above | 1,500 |
| 225 | 1,575 |
| 300 | 1,650 |
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
- Fridge (running W)
- A number, between 0 and 5000, starting at 150.
- Fridge surge (W)
- A number, between 0 and 20000, starting at 900.
- Pump (running W)
- A number, between 0 and 20000, starting at 750.
- Pump surge (W)
- A number, between 0 and 60000, starting at 2,200.
- Everything else running (W)
- A number, between 0 and 50000, starting at 600.
- Inverter continuous rating (kW)
- A number, between 0 and 500, starting at 5.
- Inverter surge multiple
- A number, between 1 and 5, starting at 2.
What the results tell you
| Result | What it is |
|---|---|
| Continuous load | The headline result. A number. |
| Worst-case surge | A number. |
| Inverter surge capability | A number. |
| Continuous headroom | A percentage. |
| Surge margin | 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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