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Battery questions
Estimate battery runtime under changing loads
A runtime estimate can fall sharply when a pump starts, then rise when it stops. The battery did not suddenly gain energy; the assumed future load changed.
Define what “remaining” means
Choose whether the calculation ends at a planned reserve, a manufacturer-defined operating limit or another documented threshold. A display’s time-to-reserve is different from time-to-empty. Treat the battery’s protective shutdown as a protection function, not a normal scheduling target.
Establish an energy basis you can support: current charge estimate, capacity and the allowed planning interval. The Ah-to-energy worksheet explains the distinction between nominal energy and a usable-energy assumption. An exact-looking runtime cannot repair an uncertain starting value.
Break the evening into blocks
An illustrative evening plan
Assume 3.6 kWh is available above the chosen reserve after whatever losses your planning model includes. From 6–8 p.m., expected load averages 0.9 kW: 1.8 kWh. From 8–11 p.m., it averages 0.3 kW: another 0.9 kWh. The scenario has 0.9 kWh left at 11 p.m.
At a later constant 0.2 kW, that remainder corresponds to 4.5 hours. A single calculation using the initial 0.9 kW would have predicted four hours for the whole 3.6 kWh, missing the planned reduction.
These figures are made up to show the method. Use measured intervals from your own normal routine, and avoid counting the same conversion loss in both the available-energy estimate and each load block.
Add uncertainty where it actually lives
Mark loads that are likely, optional or weather-dependent. Give a pump or heater a plausible operating-time range. Create a second scenario with higher demand or no expected solar contribution rather than applying an unexplained “safety factor” to every number.
Future sunlight is uncertain, and the displayed solar power now is not a forecast for the next six hours. The DOE storage overview describes storage as a way to bridge mismatches between generation and demand. Your runtime plan must still specify the mismatch being assumed.
Keep energy and peak power separate
A plan can fit within the energy budget while exceeding a device’s power capability at one moment. Starting loads and simultaneous appliances require a separate equipment-capability check against the installed system’s documentation. This guide does not size an inverter or approve a load connection.
For monitoring, note large ordinary load changes with timestamps. They help explain why a live runtime field moved. A sudden change caused by stale SOC or a missing pack is a data issue and should not be interpreted as a new forecast.
Recalculate at useful checkpoints
Choose checkpoints around real changes: after cooking, before bed, or when weather diverges from the plan. Compare actual energy used with the earlier blocks. Keep the difference so tomorrow’s assumptions can improve.
If a critical requirement depends on continuous power, do not rely on this rough worksheet or a consumer dashboard as the sole assurance. Use the system’s documented design and operating plan. For ordinary household planning, the worksheet’s value is showing which assumptions matter and when the current plan stops fitting.
Sources & further reading
Documentation checked October 10, 2026.
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Build an overnight reserve plan