Set up a useful comparison
Voltage multiplied by amp-hours gives nominal watt-hours. Usable-capacity and efficiency factors reduce that energy before it reaches the load. For batteries in series, add voltage but do not add amp-hours; for a correctly designed parallel bank, add amp-hours at the same voltage. Follow the manufacturer’s limits.
Worked example
A nominal 12 V, 100 Ah battery contains 1,200 Wh. With 80% usable capacity and 90% conversion efficiency, 864 Wh reaches the load. A constant 200 W load therefore has an energy-based runtime estimate of 4.32 hours. Actual equipment may shut down sooner.
Make the result your own
Replace the example figures, then change one assumption at a time. Read the breakdown as well as the headline answer. The calculator shows its formula and lets you copy or share the result. Shared scenario links include the inputs you choose.
No default is a current market price or an official rate. Calculations do not account for costs or physical effects that are not represented by an input. Check the underlying bill, contract or equipment specification before making a consequential decision.