Solar & battery
Solar Battery Runtime Calculator
Works how long a battery runs a load, after the losses.
Result
Depth of discharge is how much of the nameplate you may actually use. Lithium iron phosphate tolerates 80% to 90%; lead acid should not go past 50% if you want it to last. Round-trip efficiency then takes another 5% to 15% on the way in and out. Real loads are not constant, and a fridge cycling on and off averages far less than its nameplate. Runtime also falls in the cold, and lithium batteries may refuse to charge below freezing without a heater.
Get a Free Business AuditHow the calculation works
Usable energy divided by the load, where usable is nameplate times depth of discharge and the load is adjusted for inverter conversion losses. The result is shorter than a nameplate calculation suggests by both the depth of discharge margin and the conversion efficiency.
What moves the number
- Usable capacity after depth of discharge.
- Load in watts, including standby loads.
- Inverter efficiency at the actual load level.
- Temperature, which reduces available capacity.
Where this one goes wrong
Inverter efficiency is poorer at very low loads than at moderate ones, so a large inverter running a small load wastes a meaningful share of the battery on conversion, which matters in a long outage where the load is mostly a refrigerator. Standby and parasitic loads are the other omission: devices drawing a few watts continuously consume a surprising amount over a multi-day outage, and they are rarely in the load list.
Before you quote
- Include standby and parasitic loads.
- Use inverter efficiency at the actual load, not peak.
- Apply depth of discharge rather than nameplate capacity.
Questions
About this calculator
Why does a battery run out sooner than calculated?
Depth of discharge limits the usable share, inverter conversion loses more, and standby loads consume continuously. Each is modest and together they shorten runtime meaningfully compared with a nameplate calculation.
Does inverter size affect runtime?
Yes, because efficiency is poorer at very low loads. A large inverter running only a refrigerator wastes a meaningful share on conversion, which matters most in exactly the long outage the battery was bought for.
What loads get forgotten?
Standby and parasitic draws: network equipment, controls, clocks and devices in standby. Each is a few watts and over a multi-day outage they add up to a noticeable share of a battery, and they are almost never on the list.
The formula
runtime = nameplate x depth x (1 - reserve) x efficiency x (1 - inverter loss) / load
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How to use this calculator
- Battery nameplate (kWh). Enter your own figure; the page starts at 13.5.
- Battery type. Enter your own figure; the page starts at Lithium iron phosphate (80% usable, 95% efficient).
- Load (watts). Enter your own figure; the page starts at 750.
- Reserve you will not use (%). Enter your own figure; the page starts at 10%.
- Inverter loss (%). Enter your own figure; the page starts at 6%.
The result updates as you type, so you can try a figure, see what it does to runtime, 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 nameplate (kWh) | 13.5 |
| Battery type | Lithium iron phosphate (80% usable, 95% efficient) |
| Load (watts) | 750 |
| Reserve you will not use | 10% |
| Inverter loss | 6% |
What comes out
| Result | Value |
|---|---|
| Runtime main result | 11.6 |
| Energy available | 8.7 |
| If used that heavily each day | 0.5 |
| Load it runs for 24 hours | 362 |
A 13.5 kWh battery delivers 8.7 kWh in practice, which runs 750 W for 11.6 hours. Held for a full day it would support 362 W continuously.
How sensitive the answer is
Holding everything else steady and changing only battery nameplate (kwh), this is what happens to runtime. It is worth knowing which of your inputs the answer actually hinges on, and which barely move it.
| Battery nameplate (kWh) | Runtime |
|---|---|
| 6.75 | 5.8 |
| 10.13 | 8.7 |
| 13.5 example above | 11.6 |
| 20.25 | 17.4 |
| 27 | 23.1 |
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 nameplate (kWh)
- A number, between 0 and 5000, starting at 13.5.
- Battery type
- Choose from Lithium iron phosphate (80% usable, 95% efficient), Lithium NMC (90% usable, 94% efficient), AGM lead acid (50% usable, 85% efficient), Flooded lead acid (50% usable, 80% efficient).
- Load (watts)
- A number, between 0 and 500000, starting at 750.
- Reserve you will not use (%)
- A percentage, between 0 and 50, starting at 10%.
- Inverter loss (%)
- A percentage, between 0 and 25, starting at 6%.
What the results tell you
| Result | What it is |
|---|---|
| Runtime | The headline result. A decimal figure. |
| Energy available | A decimal figure. |
| If used that heavily each day | A decimal figure. |
| Load it runs for 24 hours | 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.
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