Which is more efficient to power a load? Prius or generator?

First I would verify what the wheelchair charger really draws and for how long. Most of those chargers are 120vac input with 24vdc output at 4 amps. The 120vac input may only be 1 amp so the actual power needed is much smaller. Even then, manufacturer specs are usually the maximum output ratings and not the actual current used in your application. This does not matter much when you plug into a 120vac receptacle but it's very important when you are setting up a generator or battery backup system.

Actually measuring the AC current with a Kilowatt meter (link attached) from start of charge to end gives you a better idea of the real power needs. You can also measure the DC current load with an amp meter.

You may only need a smaller and much quieter Honda portable generator like the EU2000i to do the job and keep the peace at the same time. Standard generators are noisy and sometimes hard to start.

Now, for hybrid connected inverter options (this is tricky without actually knowing true amp draws needed):

First, you could get a 12vdc inverter but it probably would not work well, especially if you really need 3500 watts. That's almost 300 amps at 12vdc! Or you could get a specialized hybrid HV inverter that converts the Prius 200vdc battery to 120vac. Now we are down to around 18amps at 200vdc input for 3500 watts. Either of these cases make a noisy and large 3500 watt generator look better.

My guess is that you really need something like 96 watts for the wheelchair (24vdc x 4 amps) and not 3500 watts. So you may need only 3% of the 3500 watts, reducing the 12vdc amp draw to maybe 8.5 amps or the Prius HV battery amp draw down to 1/2 amp.

So if your 12vdc inverter only needs to be 100 watts or less it might work with a direct fused 12 vdc battery connection. If you tried to go the HV inverter route, the options are fewer. A company is out there but it looks like they are shutting down.

Deriving power from the Prius means the car has to be on (Ready Mode) and it will start the gas engine periodically, maybe every ten or fifteen minutes depending on load. When it runs in this mode, it will discharge the batteries only slightly before it restarts. The engine won't fully charge the battery, but it will just add enough charge to keep things alive. Actually driving the car fully charges the HV which in turn charges the 12v battery through the car's own inverter system.

So, bottom line, verify the actual loads. If you just doing this for one or two nights, buy extra wheelchair batteries. You might find out a third option would work, buy a large jump starter battery system (with a full size 500 amp hour battery) that could charge the wheelchair several times before it needs to be charged by a 12v input or maybe you bring along a 200 watt RV solar charger to keep it going. Or else the Honda Portable generator would be my choice.

Prius Whole house Converdant Inverter | PriusChat

Prius Generator Conversion Process | PriusChat

 

Depending on where you're camping, some sites don't allow generator operation at night for very good reasons. Most reasonably priced 3500w generators are commercial grade and are very, very loud.

Possible third option: purchase a spare wheelchair battery.

 

I believe the Prius has an internal electrical overhead of somewhere around 400 watts, so for light loads its raw efficiency won't compete favorably with an efficient generator sized to match the load.

But it will be quieter and have much less toxic exhaust than most generators. And may well be less costly. So for infrequent use, it could well be less expensive overall.