Methodology / worked examples
LoadDay Power Planner guide
Start with the loads that matter during an outage, then separate their daily energy from their instantaneous startup demand. This keeps a large peak-wattage appliance from hiding an otherwise workable battery plan.
Build a useful load list
Use the device nameplate, a plug-in meter, or a documented operating estimate. Enter peak watts at least as high as running watts. Hours per day describe energy use, while surge watts describe inverter sizing.
daily Wh = running W x hours/day
A 60 W laptop used for 8 hours contributes 480 Wh per day.
- Keep essential and comfort loads on separate rows.
- If a device cycles, use a measured average or a clearly labeled estimate.
Translate battery size into delivered energy
Nominal battery energy is reduced by the usable fraction and conversion losses. The planner applies both assumptions before estimating runtime and planning-horizon capacity.
delivered Wh = battery Wh x usable fraction x efficiency
A 1,000 Wh battery at 85% usable and 90% efficient delivers about 765 Wh for this estimate.
Check the inverter separately
The sum of running watts is a continuous-load check. The sum of entered peak watts is a conservative simultaneous-start check. Real products can specify different limits for AC, DC, and short surge duration.
required battery Wh = daily Wh x planning days / (usable fraction x efficiency)
If the load list uses 1,000 Wh/day for two days, the same assumptions require about 2,614 Wh nominal battery energy.
Limits and assumptions
- Results use your entered load measurements and assumptions; they do not certify an electrical installation or guarantee a manufacturer runtime.
- Battery aging, temperature, duty cycle, DC loads, and power-station reserve behavior can change real runtime.
- Do not connect generators or transfer equipment without following the equipment manual and applicable electrical requirements.