Budget power for battery recovery

Generator battery charging calculator

Estimate a charger current target while other AC appliances run. The calculation checks generator real power, apparent power, charger output, and the battery's remaining charge-current allowance.

Use a compatible AC-to-DC battery charger. Generator AC watts are not battery DC amps. If a load is specified in only one AC power unit, convert between watts and VA using its power factor. After checking the budget here, estimate recovery time from the available battery-side current.

Enter sustained ratings and simultaneous loads

Generator and other AC loads

Use continuous ratings available on the actual outlet and configuration, adjusted per the manual for conditions such as altitude. Do not enter a peak marketing rating. For single-phase AC, VA = RMS volts × RMS amps. If you do not know the VA capacity, consult the generator manual. The 20% headroom is an editable planning allowance.

Charger and battery

Use charging voltage, not just nominal battery voltage. Subtract other simultaneous charging sources from the manufacturer/BMS charge limit before entering the remaining allowance. Defaults are illustrative; verify the actual charger efficiency and power factor.

Formula and assumptions

remaining W = generator W × (1 − headroom / 100) − other W
remaining VA = generator VA × (1 − headroom / 100) − other VA
DC current limit = min(remaining W, remaining VA × power factor) × efficiency fraction ÷ charging V
target A = min(DC current limit, charger rating, remaining battery allowance)

Negative budgets are treated as zero charging capacity. Apparent-power magnitudes are added conservatively; the model does not assume cancellation between loads. Motors, nonlinear loads, thermal derating, and generator regulation can require a lower actual setting.

Victron's generator FAQ explains why charger input limits and power factor matter when a generator supplies charging and other loads.

Worked example

An 1,800 W / 1,800 VA generator with 20% reserve leaves 1,440 W / VA. After 300 W and 400 VA of other loads, the charger has 1,140 W and 1,040 VA available. At 0.9 power factor and 90% efficiency, 14.4 V charging is limited to 58.5 A by generator VA. A 60 A charger and 50 A remaining battery allowance reduce the target to 50 A: 800 W / 888.9 VA charger input, or 1,100 W / 1,288.9 VA total.

Generator charging FAQ

Why do watts and VA give different limits?

Watts describe real power; VA describes voltage times current. At power factor below one, the charger needs more VA than watts. A generator or outlet current limit can be reached before the real-power budget is exhausted.

Will this include motor starts or inverter PowerAssist?

No. This is a steady-state budget. Starting events and inverter-assisted modes need equipment-specific assessment, and the remaining battery allowance must account for other active charging sources.

Can I run a portable generator inside the RV?

No. Exhaust contains poisonous carbon monoxide. Operate portable generators outdoors in accordance with the generator manual, away from openings and occupied areas. Follow manufacturer instructions for weather protection, connections, and carbon-monoxide alarms. See Honda's generator safety manual.

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