No mystery math.
A useful estimate starts with assumptions you can see and change.
1. Add up device energy
For each device, energy in Wh = running watts × active hours. We add those amounts across the selected devices. Active hours are total time at the entered watts, not necessarily the entire outage. Default device values are illustrative and should be replaced with measurements.
2. Account for losses and idle draw
Estimated battery use = device Wh ÷ conversion efficiency + inverter idle watts × outage hours. The starting efficiency is 85%; the starting battery-side idle draw is 8 W. Both are assumptions. We assume the inverter remains on for the whole outage when at least one selected device has positive use. If no device consumes energy, idle draw is zero.
3. Add a planning margin
Target nominal capacity = estimated battery use × (1 + margin). The default margin is 20%, so the target is 1.2 times estimated consumption. This is an extra-capacity margin, not a claim that 20% of a battery remains at the end. We round the large displayed target upward to the next 50 Wh and suggest an illustrative capacity class at or above the unrounded requirement.
4. Screen the output separately
Running output is the sum of running watts for selected devices with positive active hours. The startup screen adds the largest additional startup demand to that running total. It assumes one load starts at a time. It does not model simultaneous motor starts, surge duration, output waveform, power factor, temperature, manufacturer protections, or wiring compatibility. The screen is not a compatibility certification.
5. Estimate runtime for an existing battery
Runtime = nominal battery Wh ÷ (1 + margin) ÷ average modeled battery watts. Average modeled battery watts = estimated battery use ÷ planned outage hours. This projects the same duty cycle beyond the chosen interval; it does not promise that a laptop or phone will keep charging at the same rate. The remaining-capacity margin is applied consistently with the sizing calculation.
Changing or sharing a plan
Changing the outage duration scales each device’s active hours proportionally. Share links carry the values you entered, so anyone with the link can see them. Save on device keeps the plan in the browser you are using. Reset removes that saved plan. You can also print a copy to keep with your equipment.
Limits and better inputs
The planner is for general household plug-in loads. It excludes solar charging, wall-circuit installation, medical backup qualification, battery aging, temperature derating, and guaranteed internet availability. Use manufacturer documentation and a supervised real-equipment test. Measure your equipment to replace illustrative values with useful inputs.