THE FIELD NOTES / GUIDE

How long will a portable battery last?

Start with energy, then check power.

Watt-hours describe stored energy. Watts describe how quickly a device consumes it. A large energy capacity does not mean an inverter can start every appliance.

Build a daily energy budget

Record each device’s quantity and measured Wh per day. Alternatively, multiply its average watts by daily operating hours. A 10 W light used for 5 hours consumes 50 Wh. Two lights consume 100 Wh. These are arithmetic examples, not product measurements.

Account for the energy you can use

Starting below full charge, keeping a reserve and conversion losses all reduce the energy delivered. At 100% starting charge, a 20% reserve and 85% efficiency, a rated 1,000 Wh battery delivers an estimated 680 Wh. A 300 Wh/day load therefore runs about 2.27 days.

Check compatibility separately

Check continuous output, startup surge, voltage and connector requirements against the manufacturer’s specifications. Aging, low temperature and inverter standby draw can change the result. Our planner does not validate these conditions.

Sources and next steps

Energy Saving Trust: battery storage — background reference, not an endorsement.