The sizing formula
Daily device energy in Wh is the sum of quantity × running watts × hours per day × duty-cycle fraction. Duty cycle reduces energy, not the instantaneous power needed while a device runs.
Our defaults are 85% inverter efficiency and a 20% unused reserve. They are explicit planning assumptions selected for this tool, not measured properties of a model. The reference calculator also uses these default percentages, but our reserve is calculated as a fraction left unused, not as an additive markup. Daily hours, backup days and solar assumptions are user choices.
Idle draw is battery-side and assumes an inverter left on for 24 hours per day. We add that energy after converting the device load. The default 0 W omits idle consumption because we have not sourced a model-specific idle figure. Lower your expectations or enter a documented value rather than treating zero as a real rating.
Continuous and startup power
With all loads together, continuous output is the sum of quantity × running W for active rows. Startup demand is the largest single-unit start plus every other concurrent running device, including other units in the same row. We use total startup W, not extra W above running. We do not model multiple simultaneous starts.
With big loads at once switched off, all unmarked devices remain concurrent and only one individual marked flexible device runs at a time. We compare the largest compatible combination of running and starting demand. This changes inverter demand only, not energy. You must ensure the entered hours can actually be scheduled.
Solar recharge
Full-nameplate recharge hours = recommended battery Wh ÷ (panel W × derate fraction). Solar days = equivalent full-sun hours ÷ peak sun hours per day. Daily harvest = panel W × derate × peak sun hours. Derate and sun hours are editable assumptions, not a local weather forecast. Loads during charging, controller limits, input clipping, charging taper and storage charging losses are not modelled separately. Check the station manual; the estimate can be optimistic.
Our editorial size classes
Compact: up to 500 Wh. Everyday essentials: above 500 to 1,500 Wh. Extended backup: above 1,500 to 3,000 Wh. Large backup: above 3,000 Wh. These are our own navigation labels, not externally verified product categories, electrical standards or promises about device compatibility.
Worked-example assumptions
The sizing guide uses watts from PowerStation Size and clearly chosen hypothetical schedules. Outage: 150 W × 35% × 24 h + 12 W × 24 h + 9 W × 5 h = 1,593 Wh/day; divided by 0.85 and 0.80 gives 2,342.65 Wh, rounded up to 2,343 Wh. CPAP: 45 W × 8 h = 360 Wh; the same allowances give 529.42 Wh, rounded up to 530 Wh. Camping: (60 W × 4 h + 9 W × 5 h) × 2 days = 570 Wh; the same allowances give 838.24 Wh, rounded up to 839 Wh. These schedules illustrate the method and are not researched typical usage claims.
Sources and specification checks
Preset running W and the fridge duty and surge come from the cited device library, checked on 5 November 2026. Other device startup values are unverified placeholders equal to running W; replace them. Model capacity, continuous output and surge are stored with manufacturer links in models.json. Our source notes record verification limits and omitted claims.
Food safety uses FoodSafety.gov. Solar yield planning links to European Commission PVGIS. Electrical and building rules vary; confirm the current requirements with local authorities and qualified professionals.