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Generators

What Size Generator to Run battery charging only?

Researched from published standards and manufacturer specifications. Updated .

Quick answer

Running battery charging only draws about 750 watts continuously and needs about 750 watts to start. That calls for a generator rated at roughly 2,000 starting watts. With a soft starter fitted, 2,000 watts is enough.

Generators are sized by the startup surge, not by the running load, and this is where most undersized purchases come from. A 13,500 BTU air conditioner runs on about 1,400 watts, so a 2,000 watt generator looks like plenty. It is not, because the compressor needs roughly 3,000 watts for the fraction of a second it takes to get turning.

This load is largely resistive, so the running figure and the starting figure are close together and a soft starter would not help. Work through your own combination on the generator size calculator.

How many watts to run battery charging only?

Generator requirement for battery charging only
ConfigurationRunning wattsStarting wattsGenerator to buy
As is750 W750 W2,000 W
With a soft starter750 W750 W2,000 W

Convention Source: Typical published running and locked rotor figures for this equipment class. Your unit publishes its own data plate figures..

Is a soft starter cheaper than a bigger generator?

Not in this case. This load does not have enough motor surge for a soft starter to change the generator size you need.

The second benefit is weight. Generator weight rises steeply with output, and a generator too heavy to move alone is a generator that stays in the garage.

Does it need to be an inverter generator?

For an RV, yes, for two reasons. Inverter generators produce clean, stable output that is safe for electronics, chargers and variable speed motors, where a conventional generator's output can be rough enough to cause problems. They are also dramatically quieter and throttle down when the load is light, which matters in a campground with neighbours and quiet hours.

Look for a TT-30R outlet if you have a 30 amp rig, which lets you plug in without an adapter. And insist on a carbon monoxide shutoff sensor: it is the single most important safety feature on a generator used anywhere near where people sleep.

Could a battery and inverter do this instead?

Possibly. At 750 watts running, this load pulls about 73.5 amps from a 12 volt bank, so an hour of use costs roughly 74 amp-hours. A large lithium bank and a 2,000 watt inverter can cover it, silently and with no fuel.

Size the alternative on the inverter sizing calculator and the battery bank calculator.

Generators that start battery charging only

Frequently asked questions

What size generator do I need to run battery charging only?
About 2,000 starting watts. The load runs on roughly 750 watts but needs around 750 watts to start, and the starting figure is what sizes the generator. A unit rated only for the running load will stall or shut down trying to start it.
Will a 2,000 watt generator run battery charging only?
Yes. This load needs about 750 watts including startup, which is inside a 2,000 watt unit. Leave a little margin for altitude and heat, both of which reduce a generator's real output below its rating.
Can I run two small generators in parallel instead?
Yes, and it is a common approach. Two matched inverter generators joined with a parallel kit combine their output, and each one alone is light enough to carry. It also means you can run just one for small loads, which saves fuel and noise. Both units must be compatible models and the parallel kit must be the one the manufacturer specifies.
Where is it safe to run a generator?
Outside, well away from the rig, with the exhaust pointed away from windows, vents, doors and neighbouring rigs. Never in a garage even with the door open, never under an awning against the rig, and never in a pass-through compartment. Carbon monoxide is odorless and kills quickly. Fit a working CO alarm inside the rig and choose a generator with a CO shutoff sensor.

Generator safety. A generator must never be run in an enclosed or partly enclosed space, including a garage with the door open, under an awning against the rig, or in a pass-through compartment. Carbon monoxide is odorless and it kills. Keep the exhaust pointed away from windows, vents and neighbours, fit a working CO alarm inside the rig, and never refuel a hot engine.