Battery runtime is approximately the energy available to your loads, in kilowatt-hours, divided by their average power demand, in kilowatts. A battery system with 5 kWh available at its AC outlets would supply a steady 500 W load for about 10 hours. The important input is usable energy for that discharge—not simply the battery’s nameplate capacity.
Battery Runtime Reference Table
Find your average AC load in the left column, then read across to the energy available at the AC output. Every entry is calculated using the formula below; these are illustrative energy budgets, not manufacturer runtime ratings.
| Average AC load | 1 kWh available | 2 kWh available | 5 kWh available | 10 kWh available | 20 kWh available |
|---|---|---|---|---|---|
| 100 W | 10 h | 20 h | 50 h | 100 h | 200 h |
| 250 W | 4 h | 8 h | 20 h | 40 h | 80 h |
| 500 W | 2 h | 4 h | 10 h | 20 h | 40 h |
| 1,000 W | 1 h | 2 h | 5 h | 10 h | 20 h |
| 2,000 W | 0.5 h | 1 h | 2.5 h | 5 h | 10 h |
| 5,000 W | 0.2 h | 0.4 h | 1 h | 2 h | 4 h |
The table assumes no charging during the discharge period. Each energy column represents energy remaining for the connected loads after the applicable reserve, conversion losses, and system consumption. It does not assume that every battery-and-inverter combination can supply every listed load.
How Battery Runtime Is Calculated
[
t \approx \frac{E_{\mathrm{AC,available}}}{P_{\mathrm{AC,average}}}
]
Where:
- (t) = estimated runtime, in hours.
- (E_{\mathrm{AC,available}}) = energy deliverable to the AC loads from the starting charge level to the selected stopping point, in kWh.
- (P_{\mathrm{AC,average}}) = average real power drawn by those loads, in kW. Divide watts by 1,000 to obtain kW.
Use watts or kW for the energy calculation, not VA or kVA. Check the inverter’s apparent-power and starting-load capabilities separately.
For cycling equipment such as refrigerators and pumps, use energy consumption measured over a representative operating period to establish average demand. The running wattage alone does not describe how often the equipment operates, and the average does not establish whether its motor will start.
Choose the Right Energy Input
Nameplate capacity is not necessarily energy available to the load
Check whether the manufacturer’s capacity figure describes nominal DC energy, usable DC energy, or energy delivered at the AC output. Those values have different measurement boundaries.
When starting with nominal DC energy, an approximate planning relationship is:
[
E_{\mathrm{AC,available}}
\approx E_{\mathrm{DC,full}}
\times (s_{\mathrm{start}}-s_{\mathrm{stop}})
\times \eta_{\mathrm{effective}}
]
Where:
- (E_{\mathrm{DC,full}}) = estimated full-charge DC energy capacity under the relevant battery condition, temperature, and discharge rate, in kWh.
- (s_{\mathrm{start}}) and (s_{\mathrm{stop}}) = starting and stopping state of charge, expressed as fractions on the same scale.
- (\eta_{\mathrm{effective}}) = effective fraction of that DC discharge energy reaching the AC loads, including conversion and system-consumption losses over the operating period.
This approximation treats the selected state-of-charge window as a proportional energy window. Use manufacturer discharge data or measured delivered energy when available.
Do not subtract an inaccessible internal reserve twice. Likewise, if a capacity figure already represents energy delivered at the AC output under applicable conditions, do not apply inverter losses to it again.
Account for inverter consumption at the expected load
Inverter maximum efficiency is not an all-load efficiency guarantee. Victron’s Inverter Smart 1600–5000 VA datasheet lists maximum efficiency and zero-load consumption separately. Use data appropriate to the actual model, operating mode, and load; at light loads, the inverter’s own consumption can materially affect runtime.
The example below uses one effective efficiency allowance that includes system consumption. If you instead model auxiliary consumption separately, include it once—not both as an additional load and within the same loss allowance.
Worked Example: A 5 kWh Battery at 500 W
Assume an illustrative battery system has:
- 5.0 kWh of full-charge DC energy available under the assumed operating conditions.
- A starting state of charge of 100% and a selected stopping point of 20%.
- An assumed effective DC-to-AC delivery efficiency of 90%, including system consumption at this load.
- A constant 500 W AC load and no solar, generator, or grid charging.
The energy available to the load is:
[
E_{\mathrm{AC,available}}
=5.0\times(1.00-0.20)\times0.90
=3.6\ \mathrm{kWh}
]
Estimated runtime is:
[
t=\frac{3.6}{0.500}=7.2\ \mathrm{hours}
]
That is approximately 7 hours 12 minutes. Using the table’s 5 kWh column would overstate this example’s runtime because the battery’s 5 kWh DC capacity is not its available AC energy.
The 20% stopping point and 90% effective efficiency are example assumptions, not universal battery settings or manufacturer specifications.
What Can Change the Result?
Discharge rate and battery chemistry
A fixed energy budget does not capture every battery’s response to load. Lead-acid capacity is particularly sensitive to discharge rate: an Ah rating measured over a long discharge period should not be assumed available at a much higher current. Victron’s SmartShunt manual, “Battery capacity and Peukert exponent”, explains this relationship and notes that lead-acid batteries are more affected than lithium batteries. For a specific lead-acid bank, use the manufacturer’s discharge tables at the relevant current, temperature, and cutoff voltage.
Temperature, condition, and protection limits
Use capacity data that match the battery’s temperature and condition rather than applying one generic reduction factor. A protection shutdown can also stop discharge before the planned energy budget is delivered. For example, the Victron Lithium Smart Battery Manual, revision 21, January 2026, describes model-specific cell-voltage and temperature monitoring and the associated battery-management-system actions. Check your own battery’s limits and approved settings.
Continuous output and motor starting
Enough kWh does not prove that the system can supply the required kW. Check the inverter’s continuous W and VA ratings at the expected temperature, its short-duration overload capability, and the battery/BMS discharge limits. Victron’s Inverter Smart datasheet, for example, lists separate continuous-power ratings by temperature and a separate peak-power rating; those limits cannot be inferred from stored energy alone.
Use this table to estimate how long a supported load may run. Confirm whether the actual system can carry that load—including motor starting—from the battery, inverter, and appliance documentation. A runtime estimate does not establish installation compliance; any fixed connection to building wiring also requires the applicable local requirements, equipment instructions, and a qualified installer’s assessment.