howwatt.
Batteries & backup power

Energy. Made clear.

Find out how long your battery will last and how much capacity your devices need.

Capacity unit
Wh

From the battery label or power station specifications.

W

Average power used by your devices, not the maximum rating of your inverter.

Losses & available energy
%

90% is a starting assumption. Efficiency depends on the inverter and its load.

%

The fraction of nominal energy you plan to use. Allow for starting charge, reserve and battery limits.

W

Separate inverter losses. If already included in your efficiency figure, leave this at 0 W.

Estimated result

Battery runtime

—h

Available energy
—
Battery power draw
—

An estimate for your inputs. Actual performance depends on temperature, battery condition and changing loads.

Capacity alone does not establish compatibility. Check battery voltage, battery and BMS current limits, startup loads and inverter specifications.

All calculators

01

Battery runtime

Estimate how long your battery can power your devices.

02

Battery capacity

Find the battery size for your power needs and runtime.

03

My devices

Build a list of devices and estimate your home backup needs.

How we calculate

First, we convert capacity to watt-hours if needed. Then we account for available energy and the power drawn from the battery.

Runtime (h) = capacity (Wh) × usable fraction ÷ (load (W) ÷ efficiency + inverter idle power (W))

Example: a 2048 Wh battery, 200 W load, 90% usable capacity, 90% efficiency and 10 W idle power. Estimate: 2048 × 0.9 ÷ (200 ÷ 0.9 + 10) ≈ 7.94 hours.

Percentages in the formulas are decimal fractions: 90% = 0.9. Energy (Wh) = capacity (Ah) × nominal voltage (V).

Sources & assumptions

These formulas estimate energy. Efficiency, usable capacity and power consumption are user inputs, not specifications of a particular model.

Methodology updated October 6, 2026

Common questions

Why do I need voltage when entering amp-hours?

Energy depends on voltage: Wh = Ah × V. A 100 Ah battery at 12.8 V stores 1280 Wh, while a 100 Ah battery at 51.2 V stores 5120 Wh of nominal energy.

How do I calculate power station runtime?

Select Wh or kWh and enter the rated battery energy. Allow for conversion losses when using AC outlets. If entering measured usable AC output energy, use 100% efficiency, 100% usable capacity and 0 W idle consumption.

Why can actual runtime differ?

Loads vary, efficiency changes with load, and battery performance depends on temperature, age and discharge current. This simple model may overestimate lead-acid runtime at high discharge rates.