A 10 kWh battery with a reserve
10 × 1 × 0.8 × 0.95 = 7.6 kWh AC. The total load is 2.1 kW, so runtime is about 3.619 hours.
Estimated runtime 3.619 h Open in the calculator
Battery runtime and usable energy
See how much energy is available above your battery reserve, then check whether it can supply the load for the time you need.
Runtime
Power limit exceeded
10 kWh nominal DC capacity at 100% retained capacity.
Check the values to see a result.
- AC energy above reserve
- 7.6 kWh
- Load including auxiliaries
- 6.1 kW
- State-of-charge window
- 80%
The combined AC load exceeds the entered discharge limit. This battery cannot supply that load under these assumptions.
How it’s calculated
- Available DC = nominal × retained capacity × SOC window=10 × 1 × 0.8=8 kWh
- Available AC = available DC × discharge efficiency=8 × 0.95=7.6 kWh
Available AC kWh = nominal DC kWh × retained capacity × max(0, present SOC − reserve SOC) × discharge efficiencyruntime = available AC kWh ÷ combined AC kW
The combined AC load exceeds the entered discharge limit. This battery cannot supply that load under these assumptions.
Tip: A battery can have enough kWh and still lack the kW to start or run a load. Check available discharge power as well as stored energy.
From battery capacity to runtime
Battery runtime depends on energy above reserve and the power drawn from it. A 10 kWh nominal DC battery at 90% SOC with a 10% reserve has 8 kWh before discharge losses. At 95% one-way efficiency it delivers about 7.6 kWh AC. A 2 kW load plus 0.1 kW of auxiliaries uses that energy in about 3.62 hours.
That answer is useful only if the system can deliver 2.1 kW in its current operating mode. The battery management system, inverter and backup output can each impose a lower limit. The calculator checks your entered AC limit before showing a runtime.
Use the correct capacity boundary
DC energy above reserve = nominal DC kWh × retained fraction × SOC window
Start from nominal DC capacity. Multiply by retained capacity if ageing or a specified derating has reduced it, then by present SOC minus reserve. If your manufacturer already gives usable capacity over an allowed SOC window, applying that window again would understate the energy.
Allow for discharge losses once
Available AC kWh = available DC kWh × one-way discharge efficiency
Use the efficiency for the discharge path at the relevant load. Round-trip efficiency compares energy out with energy put in across a full charge/discharge cycle. It includes charging losses and cannot be used directly as one-way discharge efficiency. Do not add the same auxiliary loss both here and as an AC load.
Check power before duration
Combined AC load = supplied load + additional AC auxiliaries
Use the lowest applicable current discharge limit expressed at the AC output. Energy in kWh cannot overcome a power limit in kW. A refrigerator compressor may also need a short starting surge above its normal running power, which this steady-load calculation does not model.
Divide by the load
Runtime (hours) = available AC kWh ÷ combined AC kW
For varying loads, use a representative average only for an energy estimate. Separately check peaks against the power limit. A time-series simulation is more useful when PV charges the battery during the same period, or when load and efficiency change substantially.
Battery runtime examples
Enough energy, insufficient power
The battery still has 7.6 kWh available, but a 6.1 kW combined load exceeds the 5 kW discharge limit. The result is “Power limit exceeded”, with no valid runtime for that load under these assumptions.
Runtime Power limit exceeded Open in the calculator
Questions about Battery runtime
Is a 10 kWh battery able to supply 10 kW?
Not necessarily. kWh states energy; kW states power. The battery or inverter datasheet gives a continuous discharge power and may give a separate short-duration peak. Current temperature and SOC can reduce the available power.
Can I enter the usable capacity from a datasheet?
This calculator expects nominal DC capacity before reserve and conversion losses. A usable capacity may already exclude reserve, and an AC energy specification may already include discharge losses. Check the definition before entering it; subtracting the same losses twice gives the wrong answer.
Why does runtime stop at the reserve?
The reserve is an operating limit, not necessarily an empty battery. Energy below that level may be unavailable to the selected operating mode. If present SOC is at or below reserve, this calculator shows no energy available for that discharge.
Does this prove my battery can run the house in a power cut?
No. Backup operation also depends on the inverter, transfer arrangement, permitted circuits, neutral and earthing arrangements, and starting loads. Check the system’s supported backup mode and installation design.
Limits of this result
- Example inputs are an illustrative system, not a specific battery specification. Use manufacturer energy definitions and current BMS/PCS limits.
- The estimate treats the SOC window as the same fraction of retained energy; actual usable energy depends on the battery and its SOC definition.
- The estimate uses constant power and one-way DC-to-AC efficiency. Round-trip efficiency includes charging losses and is not a substitute.
- Starting surges, temperature, self-discharge, changing load, cell voltage and protective shutdown are outside this model. An energy result does not prove that a system can provide backup power.
- Enter nominal DC energy. If the datasheet gives usable energy or delivered AC energy, do not apply its reserve and conversion losses again.