A system can have enough energy on paper and still fail on power, timing or control limits. Check the whole operating sequence, not one equipment size.
1. Define the outage boundary and critical loads
List the circuits that must remain available and the operating states that can occur at the same time. Separate continuous energy in kWh from instantaneous and starting power in kW.
Include the electrical dependencies of water, heating, communications and controls. A fuel appliance or well may still become unavailable when its pump, fan or ignition loses electricity.
2. Use a low-sun production scenario
Estimate production for the relevant season and location. A simple array-power × peak-sun-hours × net-yield calculation is useful for scenarios, but it is not an hourly PV model.
NREL PVWatts estimates PV energy from solar resource and system inputs and explicitly includes system-loss assumptions. Use an appropriate modelling tool or measured history to establish the scenario, then keep uncertainty visible.
NREL · PVWatts Calculator ↗3. Walk the battery through the period
Start from a realistic state of charge, reserve the protected minimum, and apply loads and charging in time order. Daily totals can hide an overnight shortfall or an inverter power limit.
Confirm battery discharge power, charger power, temperature limits and the controls that decide which loads remain connected. Keep a separate margin for forecast error and battery ageing.
4. Define the generator trigger and recharge target
Choose a trigger state of charge, a recharge target and a sustained charge power that the battery, charger and generator can all support. Add household loads that will run during charging before checking generator capacity and fuel rate.
Operate combustion generators only as the manufacturer and local safety rules require. Never treat the fuel calculation as permission to place or run a generator in an enclosed or attached space.
DOE · Home guide to emergency preparedness ↗5. Test island operation before relying on it
Solar panels alone generally do not provide outage power. DOE notes that the system needs suitable inverter and storage arrangements to operate independently from the grid.
Run a controlled test of transfer, critical loads, solar charging, low-state controls and generator charging. Record actual power, energy, fuel use, alarms and elapsed time, then update the plan from evidence.
DOE · Solar and resilience basics ↗Keep four limits visible
| Limit | Unit | Question it answers |
|---|---|---|
| Period energy | kWh | Is total generation plus reserve enough? |
| Instantaneous power | kW | Can sources and inverter carry the load now? |
| Battery state | % or kWh | When must loads shed or charging begin? |
| Fuel autonomy | L and h | How long can the backup plan continue? |
Worked example
Three low-sun days with critical loads
A 4 kW array sees 1.8 peak-sun hours/day at a 72% net yield. Critical loads use 5 kWh/day.
Solar = 4 × 1.8 × 0.72 × 3 = 15.55 kWh Demand = 5 × 3 = 15.00 kWh Period balance = +0.55 kWh
The small positive total is not a resilience margin. Check the hourly sequence, starting power, battery reserve and a worse weather case.
What to have ready
- Measured critical-load energy and starting power
- Seasonal solar scenario or monitored production
- Battery usable range, power and temperature limits
- Backup trigger, fuel plan and safe operating procedure
Common mistakes
- Using annual-average sun for the worst season.
- Treating kWh balance as proof that peak kW is adequate.
- Assuming grid-connected solar will operate in an outage.
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.