Solar & battery
Solar Inverter Battery Calculator
Matches inverter and battery, where both power and energy have to agree.
Result
Three separate limits have to be satisfied and they are often confused: the inverter's continuous rating carries the load, its surge rating starts motors, and the battery's charge rate controls how fast it refills. A large battery on a small inverter still trips when the well pump starts.
Get a Free Business AuditHow the calculation works
The inverter's power capability against what the loads require, and the battery's energy against how long they must run, with the inverter's own efficiency and the battery's discharge rate limits checked against each other. A mismatched pair fails in one direction or the other.
What moves the number
- Inverter continuous and surge capability.
- Battery maximum discharge rate.
- Energy required over the intended duration.
- Voltage compatibility between the two.
Where this one goes wrong
A battery has a maximum discharge rate, and an inverter capable of more power than the battery can supply will trip or damage the battery rather than delivering it, which is the mismatch that catches people pairing components from different sources. Check the battery's rate limit against the inverter's capability. Voltage compatibility is the other requirement, since the two have to operate on the same nominal bus and within each other's windows.
Before you quote
- Check the battery's maximum discharge rate against the inverter's draw.
- Confirm voltage compatibility between inverter and battery.
- Size energy and power separately, then verify they match.
Questions
About this calculator
Can an inverter be too large for a battery?
Yes. A battery has a maximum discharge rate, and an inverter capable of drawing more will trip or stress the battery rather than delivering the power. That mismatch is common when components are chosen from different sources.
What has to match between the two?
Nominal voltage, the operating voltage windows, and the power the battery can deliver against what the inverter will draw. Energy and power are separate requirements and both pairs of figures have to be compatible.
Are integrated systems easier?
Generally, because the manufacturer has matched the components and warranted them together. Assembling from separate sources gives more flexibility and puts the compatibility verification on the designer, which is work rather than an afterthought.
The formula
runtime = usable kWh / continuous load; recharge = usable kWh / the lower of charge rate and array output
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How to use this calculator
- Battery usable capacity (kWh). Enter your own figure; the page starts at 10.8.
- Continuous backup load (kW). Enter your own figure; the page starts at 3.5.
- Largest motor surge (kW). Enter your own figure; the page starts at 7.
- Hybrid inverter rating (kW). Enter your own figure; the page starts at 5.
- Maximum battery charge rate (kW). Enter your own figure; the page starts at 5.
- Array size (kW DC). Enter your own figure; the page starts at 7.2.
The result updates as you type, so you can try a figure, see what it does to runtime on battery, 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 |
|---|---|
| Battery usable capacity (kWh) | 10.8 |
| Continuous backup load (kW) | 3.5 |
| Largest motor surge (kW) | 7 |
| Hybrid inverter rating (kW) | 5 |
| Maximum battery charge rate (kW) | 5 |
| Array size (kW DC) | 7.2 |
What comes out
| Result | Value |
|---|---|
| Runtime on battery main result | 3.1 |
| Continuous headroom | 30.0% |
| Surge margin at 2x rating | 30.0% |
| Hours to recharge from empty | 2.2 |
| Charge rate as a fraction of capacity | 0.5 |
The battery runs the load for 3.1 hours and recharges from empty in 2.2 hours of good sun. Continuous headroom is 30.0%.
How sensitive the answer is
Holding everything else steady and changing only battery usable capacity (kwh), this is what happens to runtime on battery. It is worth knowing which of your inputs the answer actually hinges on, and which barely move it.
| Battery usable capacity (kWh) | Runtime on battery |
|---|---|
| 5.4 | 1.5 |
| 8.1 | 2.3 |
| 10.8 example above | 3.1 |
| 16.2 | 4.6 |
| 21.6 | 6.2 |
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
- Battery usable capacity (kWh)
- A number, between 0 and 5000, starting at 10.8.
- Continuous backup load (kW)
- A number, between 0 and 500, starting at 3.5.
- Largest motor surge (kW)
- A number, between 0 and 500, starting at 7.
- Hybrid inverter rating (kW)
- A number, between 0 and 500, starting at 5.
- Maximum battery charge rate (kW)
- A number, between 0 and 500, starting at 5.
- Array size (kW DC)
- A number, between 0 and 5000, starting at 7.2.
What the results tell you
| Result | What it is |
|---|---|
| Runtime on battery | The headline result. A decimal figure. |
| Continuous headroom | A percentage. |
| Surge margin at 2x rating | A percentage. |
| Hours to recharge from empty | A decimal figure. |
| Charge rate as a fraction of capacity | 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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