Solar Battery Backup Runtime Calculator

The hardest part of an outage can be reaching the next solar window

A daily solar estimate can look adequate while the battery still runs out overnight. The calculator therefore checks the energy available at the actual starting state of charge against the critical-load energy needed before meaningful solar production can return. It also separates available PV power from the battery's own charging limit, because a large array does not guarantee that storage can absorb all of that power at once.

How to use this calculator

Start with a real battery preset or enter usable capacity per unit and battery count manually. Set the actual starting state of charge, backup reserve and average critical load. Use the load planner to select representative circuits, adjust the preset wattage assumptions when you know measured or manufacturer values, and add a custom load when needed.

Then set a realistic simultaneous running load. Runtime is an energy question measured in kWh, while the ability to run several appliances at once is a continuous-power question measured in kW. If the backup plan includes compressors, pumps or other motors, use the separate surge screen with the startup kW and required duration from the equipment documentation.

For solar-assisted backup, use the recharge-window screen to compare average available solar, the backed-up load that must be served first, the effective solar window, charging efficiency and the battery system's maximum charge power. Then use the 72-hour simulator to stress-test three consecutive solar days and compare whether the scenario is better solved with more storage, more modeled outage solar or a smaller average critical-load budget.

Battery-only runtime vs solar-assisted backup

The primary runtime results remain battery-only. Optional outage-solar inputs are evaluated separately: average solar power available to backed-up loads or charging, effective solar hours, conversion efficiency and the number of hours until solar production returns. The weather sensitivity table gives a quick static comparison, while the 72-hour simulator follows battery state of charge through repeated charge and discharge periods.

The recharge and multi-day screens add another constraint: solar must first support the modeled household load, and the remaining power may still be capped by the battery or system charging limit. This separation matters because a grid-tied solar array does not automatically keep operating during a blackout. Solar-assisted backup requires compatible islanding, inverter, transfer and battery architecture.

What the results can help you compare

Use the tool to compare critical-load strategies, battery models, battery counts, reserve settings, starting-SOC scenarios, short-duration startup requirements, recharge targets and multi-day weather sequences. The 72-hour sizing screens add minimum full battery count, minimum modeled base solar and maximum sustainable average critical load for the same energy scenario.

Important screening limits

This is a planning calculator, not an electrical design or an outage forecast. The 72-hour tool uses 15-minute calculation steps but represents solar as a flat average power level during each effective solar window rather than a true irradiance curve or forecast. It does not model generator interaction, battery degradation, cold-weather derating, dynamic appliance cycling, manufacturer-specific reserve logic, high-SOC charge taper or every multi-battery power-sharing rule. The short-surge screen only compares catalog ratings that are published in directly comparable kW and duration terms; ratings expressed in motor current such as LRA remain a data gap. Verify critical-circuit loads, motor-start requirements, islanding capability, charging limits and final equipment sizing with current product documentation and a qualified installer.