Daily kWh sizes the energy budget. Simultaneous watts and motor starting requirements size a different part of the system.
1. Measure the loads you need to keep
List appliances, their measured or documented power and the hours they actually operate. A 100 W device running for ten hours uses 1 kWh. A heater at 2,000 W for half an hour also uses 1 kWh, but needs much more instantaneous power.
For fridges and pumps, use measured daily energy or realistic cycling time. Nameplate power multiplied by 24 hours often overstates daily use. Separate essential loads from loads you can postpone. Record motor starting requirements from the equipment documentation rather than applying one universal multiplier.
2. Work from the limiting season
Solar modules generate DC power, and the system also needs suitable conversion and protection equipment. Storage can shift energy to times when the sun is unavailable. A standard grid-connected solar installation should not be assumed to provide outage backup.
Use a location-specific production estimate for the relevant month, with shading and orientation considered. Our solar tool uses peak-sun-hours and a stated overall yield factor for a first energy balance. It does not simulate weather sequences or guarantee off-grid reliability.
DOE · Solar system design basics ↗3. Budget usable energy and losses
Nominal battery energy is not all available at the AC outlet. Account for permitted usable fraction and conversion efficiency. A 10 kWh nominal battery at 80% usable fraction and 90% delivery efficiency gives an illustrative 7.2 kWh to loads.
Check continuous and surge power separately, along with temperature limits and the manufacturer’s permitted charging conditions. Battery capacity for two days does not explain how it will recharge on day three. After sizing the energy balance, test a low-production sequence with the installer.
4. Plan the generator and the outage routine
CPSC advises operating portable generators outdoors, at least 20 feet (about 6 m) from the house, with exhaust directed away from occupied buildings and openings. Never operate one in a garage, even with the door open.
Have a qualified electrician specify approved transfer equipment for house connection. Do not backfeed an outlet. Consider fuel availability, refuelling procedure, maintenance, CO alarms and the loads the generator can actually start.
CPSC · Portable generator safety ↗Keep these quantities separate
| Quantity | Unit | Used to decide |
|---|---|---|
| Daily energy | kWh/day | Solar energy and battery budget |
| Simultaneous running power | kW | Inverter or generator continuous output |
| Starting requirement | W or VA, manufacturer specified | Motor-starting capability |
| Autonomy | Hours or days | Energy required without charging |
| Peak sun hours | Equivalent full-sun hours/day | First-pass PV production |
Worked example
A small essential-load scenario
Assume essential loads use 6 kWh/day. Plan two days without charging, 80% usable battery fraction and 90% delivery efficiency.
Nominal battery = 6 × 2 / (0.80 × 0.90) = 16.67 kWh
At 3 peak sun hours and an assumed 0.78 solar yield factor, 6 / (3 × 0.78) = 2.56 kW DC only balances the average daily load. Extra energy is needed to recover a discharged battery.
What to have ready
- Measured daily essential-load energy
- Simultaneous and starting power
- Monthly solar estimate with shading
- Battery limits and recovery plan
- Safe backup-power arrangement
Common mistakes
- Confusing kW with kWh.
- Sizing off-grid solar from an annual average alone.
- Forgetting pump starting and battery recharge energy.
Reference check: 5 September 2026. Sources include US public agencies. Principles are general; regulations and design values must be checked where you live.Worked examples use stated hypothetical inputs.