Size a generator for a battery charger using the charger’s AC input demand, including efficiency, power factor, and other connected loads. Battery-side volts multiplied by amps provide a useful starting point, but use actual charging voltage and verify the charger’s manufacturer requirements before choosing generator capacity.
Open the Battery Charger Generator Size Calculator.
Read the AC input label first
The most direct information is the charger’s documented input voltage, input current or VA, maximum real input power, supported frequency, and any generator-specific guidance. These values describe the load the generator must supply more directly than the battery’s nominal voltage.
Make sure the figures describe the exact charger model and operating mode. An inverter-charger may supply AC pass-through loads while charging a bank, and its total input demand can exceed the charger section alone. A selectable input-current limit may also change how much power reaches the battery.
Do not confuse a 50 A DC charge rating with a 50 A draw from an AC outlet. The two currents occur at different voltages and are connected by conversion efficiency and power factor. Copying the DC amps directly into the generator’s AC current calculation can produce a very large sizing error.
If the manufacturer provides maximum AC input data, use it to check the calculated estimate. A disagreement may reveal a nominal-voltage assumption, a low power factor, a different charge mode, auxiliary consumption, or another documented limitation.
Follow the power path from DC output to AC input
Three relationships keep the units clear:
DC output watts = actual charging volts × output amps
AC real input watts ≈ DC output watts ÷ efficiency fraction
AC apparent input VA ≈ AC real input watts ÷ power factor
Efficiency accounts for energy lost in conversion. Power factor accounts for the relationship between real power and the AC voltage-current product. They describe different effects and should not be used interchangeably.
For a single-phase charger, apparent VA divided by AC voltage gives input current. A 1,000 VA input at 120 V represents about 8.33 A, while the same 1,000 VA at 230 V represents about 4.35 A. Those are illustrative idealized current relationships, not a declaration that a charger accepts both voltages.
Compare the result with both generator watts and its current or apparent-power capability. A generator can encounter an outlet-current or kVA limitation even when the real-watt demand appears below its advertised continuous watts.
Work a 24 V bank example using charging voltage
Consider an illustrative charger delivering 60 A at 28.8 V to a nominal 24 V bank. Assume 90% efficiency and 0.80 power factor. Its DC output is 28.8 × 60 = 1,728 W, and estimated real AC input is 1,728 ÷ 0.90 = 1,920 W.
Apparent demand is 1,920 ÷ 0.80 = 2,400 VA, or 2.40 kVA. At 230 V AC, that corresponds to about 10.43 A. Applying an illustrative 20% planning headroom gives 2.88 kVA, before considering other loads or particular startup requirements.
| Step | Illustrative calculation | Result |
|---|---|---|
| DC charge output | 28.8 V × 60 A | 1,728 W |
| Real AC input | 1,728 ÷ 0.90 | 1,920 W |
| Apparent AC input | 1,920 ÷ 0.80 | 2.40 kVA |
| Input current at 230 V | 2,400 ÷ 230 | 10.43 A |
| Apparent demand with 20% headroom | 2.40 × 1.20 | 2.88 kVA |
All efficiency, PF, current, and voltage figures in this example are assumptions for demonstrating the method. A real charger may limit current, vary efficiency with output, or draw a different input waveform. Use its published specifications and supported settings.
The calculator’s nominal-voltage limitation
The embedded tool accepts nominal bank choices of 12, 24, 36, or 48 V. It multiplies that selected value by charger current, divides by efficiency and PF, and applies headroom. The result is an apparent-power estimate in kVA.
Using the example’s 24 V nominal selection, 60 A, 90% efficiency, 0.80 PF, and 20% headroom returns 2.40 kVA. Using the actual illustrative 28.8 V charge output produces 2.88 kVA with the same factors. The difference arises entirely from charging voltage.
That nominal input can therefore understate demand when output voltage is above nominal during charging. Do not assume the selected headroom automatically corrects it: an allowance may already be needed for other uncertainties, and it should not silently compensate for an identified input limitation.
Use the tool to explore current, efficiency, and PF sensitivity, then calculate with the charger’s actual maximum relevant output voltage or documented AC input. Victron’s technical specifications list charge voltage, efficiency, PF, and current separately, illustrating why nominal bank voltage alone is insufficient.
Do not enter an invented charger-current value merely to make the calculator reproduce a preferred generator size. Keep a separate written calculation with the correct voltage so the assumptions remain reviewable.
Add other loads without mixing watts and VA
List everything the generator supplies during charging. Lighting, refrigeration, tools, heating, or AC pass-through demand can compete for capacity. Include the most demanding realistic overlap rather than assuming charging always occurs alone.
For real-power budgeting, add real watts. For a conservative initial apparent-power screen, adding documented VA values can be useful, while recognizing that a detailed combined-load calculation may require more information about current waveforms and phase relationships.
Suppose the example charger requires 1,920 W real input and an additional resistive appliance draws 700 W. The real-power total is 2,620 W. The separate apparent demands are 2,400 VA and 700 VA; their simple sum is 3,100 VA before an additional planning allowance. Do not add 1.92 kW directly to 700 VA and label the result kW.
Include starting demand for other equipment where relevant. A refrigerator or pump starting while the charger is at maximum output can create a more demanding event than the charger’s steady input alone. Headroom percentages do not replace a documented motor-starting check.
Check continuous ratings and each connection limit
Choose from the generator’s applicable continuous rating, rather than treating its short peak rating as sustained capacity. Confirm the rating applies to the selected voltage, fuel, output configuration, and operating conditions.
The outlet, connector, cable assembly, transfer equipment, and input limit can each impose a lower usable capacity than the generator’s total headline output. A generator with several outlets does not necessarily make its entire capacity available through every individual receptacle.
On split-phase or three-phase equipment, check how the charger loads the available lines. A single-phase charger connected to one permitted output may create an imbalance even when its watts are well below total generator capacity. Use the generator manufacturer’s permitted loading arrangement.
Also check input quality and compatibility guidance. Frequency and voltage stability, charger startup behavior, and nonlinear current draw can affect operation. There is no universal generator-to-charger size multiplier that replaces the charger’s own requirements.
Match charger settings to the battery and available power
A charger capable of 100 A should not automatically run at that setting. The battery or bank may have a lower permitted charge current, and the installed conductors, fuses, and terminals must suit the intended arrangement. Follow the battery and charger documentation for supported limits.
An approved lower charge-current setting can reduce generator demand at the cost of a longer bulk-charge period. If an illustrative bank needs 120 Ah replaced, 60 A gives a two-hour constant-current baseline, while 40 A gives three hours. Full-charge timing still depends on acceptance, taper, and losses.
Temperature restrictions also matter. If the charger reduces output because it is hot, or the battery protection prevents charging because it is cold, a larger generator does not remove those constraints. Read the indicated condition before changing input-power assumptions.
For inverter-chargers with a supported AC input limit, understand how that limit is shared between charging and pass-through loads. A setting may protect the source by reducing charging when other demand rises, which means actual battery charge current will vary during use.
Arrange a useful verification record
Record model numbers, AC input limits, selected DC current, intended battery profile, other loads, generator continuous output, and the connection ratings. Keep the calculated real watts and apparent VA in separate columns so neither requirement disappears in a single rounded generator number.
During ordinary authorized operation, use existing displays or suitable approved monitoring to confirm whether input limits or overload warnings occur. Do not open energized equipment to obtain a more precise reading. A qualified installer can assess questionable compatibility or protective behavior.
Portable fuel generators belong outdoors at least 20 feet from the home with exhaust directed away from buildings, according to CPSC guidance. Place and connect the equipment accordingly while following charger ventilation and environmental requirements.
Does the charger draw maximum power for the whole session?
Usually the demand changes as charging progresses or settings limit output. Generator sizing should cover the maximum relevant demand, while fuel and runtime estimates should reflect the actual changing load rather than assuming a full-power draw until completion.
Is kVA the same number as kW?
Only at unity power factor. At 0.80 PF, a 2.40 kVA load draws 1.92 kW of real power. A generator and connection must support the relevant current as well as the real-power demand.
References
- Victron Energy: Blue Smart IP22 technical specifications
- CPSC: Carbon monoxide and generator safety
- Battery Charger Generator Size Calculator