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Home Battery Storage Duration Estimator

This estimator calculates how long a home battery can support a steady electrical load after allowing for a protected energy reserve and discharge losses. It starts with usable stored energy rather than the battery's nominal cell rating, then converts the energy available to the load into hours and minutes. Homeowners, backup-power planners, and small renewable-system designers can use the result to test essential-load scenarios and compare load reduction with added storage. The estimate assumes the entered average load remains constant. Real loads cycle, motors can have high starting demand, battery management systems impose power limits, and usable capacity changes with temperature, age, and discharge rate. The duration result therefore describes an energy balance, not a guarantee that the inverter can start or continuously operate every connected device.

Calculation inputs

Result
—
Calculated result
Deliverable energy—
Reserved energy—
Duration in minutes—

1. Enter usable stored energy

Use the kWh available within the battery system's permitted operating window.

2. Estimate average load

Add the average power of devices expected to run together, in kW.

3. Protect a reserve

Choose the share of usable energy you do not plan to discharge.

4. Apply discharge efficiency

Use a battery-to-load efficiency that includes relevant conversion losses.

5. Check energy and runtime

Confirm the load is also within the battery and inverter power ratings before relying on the duration.

Deliverable energy = Usable stored energy × (1 − Reserve ÷ 100) × (Discharge efficiency ÷ 100)

Duration (hours) = Deliverable energy ÷ Average load

Energy is in kWh and load in kW. The calculation assumes constant load and does not model power surges or capacity fade.

What the result means

The displayed hours are the energy-limited runtime at the entered average load after reserve and efficiency deductions.

Results are planning estimates based on the values entered. Confirm equipment limits, site conditions, and project assumptions before making a purchase or investment decision.

Given

13.5 kWh, a 2.4 kW load, 15% reserve, and 94% efficiency.

Calculation

Deliverable energy = 13.5 × 0.85 × 0.94 = 10.79 kWh. Duration = 10.79 ÷ 2.4 = 4.49 hours, or about 270 minutes.

Result

The modeled steady-load duration is 4.49 hours. Actual runtime will change as the load varies.

Should I enter nominal or usable battery capacity?

Enter usable capacity within the permitted state-of-charge window. If only nominal capacity is known, first deduct the limits imposed by the battery management system.

Why use average load instead of daily energy consumption?

Runtime depends on power while the battery is discharging. Convert the appliances running during the event into an average kW load.

Does the estimate account for startup surge?

No. Verify that inverter and battery power ratings can handle both continuous demand and short startup peaks.

Can reserve be set to zero?

Yes for a mathematical scenario, but operating guidance and warranty limits may require a reserve. Follow the manufacturer's allowed discharge window.

Why might actual runtime be shorter?

Cold temperature, battery age, high discharge rate, conversion losses, and unexpected loads can reduce delivered energy. Treat the result as a planning estimate.