Blackout Battery Runtime Calculator

Estimate your runtime

Your runtime estimate

The runtime formula

The estimate is battery watt-hours multiplied by the usable-capacity percentage, multiplied again by conversion efficiency, then divided by the load in watts. A 1,000 Wh battery at 90% usable capacity and 85% conversion efficiency provides about 765 Wh of estimated usable AC energy. At a steady 100-watt load, that is roughly 7.65 hours.

Real batteries do not behave like perfect tanks. Temperature, battery age, inverter overhead, discharge rate, cable losses and device cycling all change the result. Some manufacturers also reserve part of the battery capacity automatically. Treat the figure as a planning estimate and validate it by testing the actual equipment.

Why the load number matters more than the headline capacity

People often ask how long a “1 kWh power station” lasts as though there were one answer. There is not. At a 20-watt load, it may last well over a day. At 500 watts, only a small number of hours. The exact same battery can therefore be either a communications lifeline or a very short-lived attempt to preserve normal household behaviour.

Measure the combined load of the devices you intend to run at the same time. If a fridge cycles between low draw and a higher compressor draw, a plug-in energy meter over several hours can give a better average than the label alone. For electronics such as phones, the charging load usually changes as the battery fills.

AI doomsday scenario: battery percentage becomes time

In the fictional collapse, a household stops thinking in percentages. Twenty per cent becomes “two phone charges and four hours of light.” The laptop is no longer entertainment; it is an expensive way to read information that might not even be authentic. The router gets switched off between scheduled communication windows. Every device now competes for the same finite reserve.

This is one of the most useful mental shifts in preparedness. Convert energy into tasks. How many hours of lighting? How many radio updates? How many phone recharges? How long can essential equipment run? Once the battery has a job list, waste becomes visible.

Do not confuse battery runtime with medical resilience

Powered medical devices need device-specific continuity plans. Generic watt-hour arithmetic cannot account for clinical consequences, alarm behaviour, battery-management systems or the need for uninterrupted operation. Anyone who relies on medical equipment should follow the manufacturer, equipment supplier and clinical team guidance and use the Priority Services arrangements available through energy networks where eligible.

For ordinary household loads, test the calculation. Fully charge the battery, connect the intended devices and record actual runtime in safe conditions. If reality differs, use the real result as your planning figure. Preparedness gets stronger when estimates are replaced by tested numbers.

Use runtime as a rationing schedule

Once you have an hours figure, split it into communication windows rather than running equipment continuously by habit. If a battery can support a 100-watt load for roughly eight hours, that could mean eight continuous hours or sixteen half-hour operating windows spread across a longer outage. The second plan may preserve information access and lighting far longer if the devices do not need to stay on.

Write the schedule beside the battery. Decide which loads are essential, which are optional and which should never be connected during an extended outage. If the situation deteriorates, reduce the average load and recalculate. A runtime estimate becomes much more useful when it changes behaviour before the battery reaches its final ten per cent.