Calculate
Battery power vs capacity is the difference between supplying an appliance and sustaining it for a period. A large energy capacity does not by itself guarantee enough continuous output or starting power. This guide separates the power check from the runtime calculation, and also separates outage backup from financial savings. It is a planning explanation, not a wiring or installation guide.
Record the inputs before calculating
Collect the battery’s usable energy definition, current state of charge, reserve or shutdown floor, continuous output, permitted surge, inverter limits and appliance loads. Do not apply a depth-of-discharge reduction again if the supplied usable figure already includes it. Check whether manufacturer capacity is an AC-delivered figure before applying another efficiency factor. Confirm limits for the whole system, not just one component.
Worked example and formula
Illustrative numbers only. Replace every assumed measurement, price, rate and performance factor with evidence for your situation. A 2026 review date does not turn an example into a current market quotation.
5 kWh × (1 − 0.20) × 0.90 = 3.6 kWh
3.6 kWh / 0.6 kW = 6 h
3 kW simultaneous load > 2 kW assumed continuous output
Assume 5 kWh usable DC energy, a 20% energy amount reserved for this scenario, and 90% delivery efficiency not already included. That leaves 3.6 kWh for the stated planning window, or six hours at a constant 0.6 kW. But an assumed 2 kW continuous-output system cannot supply a simultaneous 3 kW load simply because the energy calculation looks adequate. Reserve behaviour changes by operating mode and model.
Interpret the result correctly
Appliances may cycle or have starting loads; a constant-watt estimate is only a simplification. State whether solar charging is excluded. Holding energy for outages can reduce the amount available for normal bill savings. During an outage a manufacturer may use energy below a grid-connected reserve setting; verify the actual mode instead of assuming one reserve policy applies everywhere. Consult the installer for circuit and compatibility decisions.
Keep the original measurements, source date, units and assumptions beside the result. Change one assumption at a time to see its effect. A comparison is meaningful only when both options use the same scope. Do not silently combine different test procedures, products, dates or fee bases. Round a purchasing quantity upward only after completing the calculation; retain enough precision for intermediate steps.
Frequently asked questions
Which measurements should I use?
Use matched battery/inverter documentation and a measured appliance schedule, distinguishing power from energy.
Is the example a guaranteed result?
No. Six hours assumes the stated constant load, energy window and efficiency, with no recharge or other loads.
What should I check before acting?
Check continuous and starting limits, state of charge, reserve policy and whether losses or discharge limits are already included.
Use the result and related tools
Run a conservative runtime scenario in the off-grid planning tool, then compare financial savings separately. Never treat a backup-hours result as a payback calculation.
Sources and review scope
Reviewed 22 October 2026. The examples and arithmetic are original editorial illustrations, not field tests, certified designs, current price surveys or promised savings. Sources support the stated concepts; they do not certify these example inputs. The linked calculators have their own assumptions. These articles address specific decisions and complement the existing calculator guides. No Google search-volume or ranking claim is made.