Methodology
Three things decide whether an estimate is useful: the formula, the constants, and where the inputs come from. All three are written out here so you can disagree with any of them.
The formulas
Daily energy per device
watts x hours x duty cycle
Duty cycle is the share of the hour a device actually draws power. A fridge compressor might run 35 percent of the time, so its nameplate wattage overstates its average draw. A router runs continuously, so its duty cycle is 1.
Recommended capacity
(daily Wh / inverter efficiency) x reserve margin
The inverter converts battery DC to household AC at a loss. Dividing by an efficiency figure of 0.80 to 0.90 accounts for that. The reserve margin of 15 to 35 percent covers battery ageing, cold weather, and the gap between a lab figure and your house.
Surge check
max(single device surge) <= station peak output
Motor-driven devices draw a brief spike on startup. This is checked separately from capacity because a station can hold ample energy and still fail to start a compressor.
Continuous output check
sum(device watts) <= station continuous output
If you run several devices at once, the simultaneous draw must stay under the station rating. Repeatedly tripping an inverter shortens its life.
Default constants
- Inverter efficiency
- 0.85
- Reserve margin
- 1.20
- Usable capacity
- 0.85
Usable capacity appears in the reverse calculator. Stations are not discharged to zero in normal use, and the last few percent deliver falling voltage.
Where inputs come from
Device power ranges
Manufacturer documentation where available, otherwise published engineering ranges. Each device entry records which it is.
Station specifications
The manufacturer product page. Each model record stores the source URL and the date it was verified.
What we do not claim
We do not publish our own runtime test results. Every runtime figure on this site is calculated, not measured.
Known limitations
- Temperature is not modelled. A lithium battery loses capacity in the cold, and the effect is significant below freezing.
- Battery ageing is not modelled per unit. Cycle life varies with depth of discharge and heat.
- Device wattages are ranges, not constants. A fridge in a hot garage draws more than the same fridge in a cool basement.
Use thecalculatorwith these constants, or browseverified station specs.