Battery capacity tells you how much energy is missing; charger and source limits determine when it will actually be restored.
1. Start with the energy deficit
Subtract the starting state of charge from the target and apply that fraction to nominal capacity. Keep the planned reserve visible: routine operation should not quietly consume energy reserved for a long outage.
State-of-charge estimates have uncertainty. Battery management systems may protect capacity that is not shown to the user, and available capacity changes with temperature, age and discharge rate.
2. Find the real charging limit
The smallest limit in the chain controls charging: source output, charger rating, cable and protection limits, battery-management limits and the battery’s accepted power. Concurrent house loads can reduce what remains for the battery.
A generator should be evaluated at the combined load and suitable operating conditions. Solar output changes with weather and time, so nameplate power is not constant charging power.
3. Estimate time, then add real-world allowances
Divide stored energy by average battery-side charging power and charging efficiency for a first estimate. Real batteries may reduce charge power near the target state of charge or at unsuitable temperatures.
DOE notes that solar panels alone do not normally provide resilience during an outage; suitable controls and storage are needed. Map the operating states before relying on the system.
DOE · Solar and resilience basics ↗4. Commission the recovery plan
Test a representative recharge while recording source power, house load, battery power, temperature and elapsed time. Confirm shutdown and ventilation requirements for any generator.
Write a simple outage rule: which loads stay on, when charging begins, the target state of charge and who checks fuel or weather. Repeat the test after major battery or firmware changes.
Why calculated and actual recharge time differ
| Limit | What to record | Typical effect |
|---|---|---|
| Charger rating | Sustained kW | Caps input |
| Concurrent loads | House kW during charge | Reduces battery share |
| Charge taper | Power near target SOC | Extends final stage |
| Temperature | Battery and ambient °C | May restrict power |
| Source variability | Solar or generator output | Changes average power |
Worked example
Recharge from 20% to 90%
A 15 kWh battery is charged at an average 3 kW with 92% efficiency.
Energy added = 15 × (90 − 20)% = 10.5 kWh Time = 10.5 / (3 × 0.92) = 3.80 h
Actual time can be longer because of taper, temperature, house loads or source limits.
What to have ready
- Nominal and usable battery capacity
- Starting and target state of charge
- Sustained source and charger power
- Concurrent critical loads and temperature limits
Common mistakes
- Dividing capacity by nameplate source power.
- Ignoring loads running during recharge.
- Assuming charge power stays constant near full.
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.