Electrical question: 3 source automatic load transfer (mains + 2 backups)

APC makes ATS, Properly designed anything is possible. Large data centers leverage ATS and only need 2 sources. The battery is only used temporarily while mains is out which gives time for the generator to kick in. IMHO you only need 2 sources not three. A crude diagram below.

Generator-----> input 1 ATS Switch -----------> output----> ups--->LOAD
Mains----------> input 2 ATS Switch ------------>output----> ups--->LOAD

You'll need to trigger the generator somehow based on the condition you need (pressure switch, loss of power, solenoid.....etc)

The ups will never go on battery unless BOTH of the ats input sources are zero volts. The UPS will be on bypass when mains or generator is running.

1KW is a very small load. Less than 15A.

Describe your application? House? Shed? cottage? In most cases you'll need a ESA permit if you're touching mains. This will cost a small fortune.

Yes, APC makes 220v ats as well.

Its for the house. Mains are currently disconnected, there was damage to mast by snow few weeks ago, so permits will be required regardless.

I will be going there soon and need some power to exist. If this works, I may leave the house off grid for now. Forget the mains, transfer switches between mains / backup are plenty available.

There is few appliances needed continuously like lights, phone signal booster and laptop. Do not need generator every waking hour just to consume 30W from it. Generator will be needed very infrequently and very sporadically mostly only in few minute intervals - e.g. flushing, shower, tankless water heater, electric stove, power tools, etc.

The water well pump has dual 30A breakers, so I figure 6kW is needed just for that, add other things and need heavy duty 8-10kW 400cc engine generator. But most of the time will need <50W.

Something like this must already exist. People do not run generators just to have a light, do they ?

ramon2 wrote: ↑ Is there an automated transfer switch that supports 3 sources - mains, battery backup and generator ?

When mains are not present, ATS should default to battery backup up to 1kW for small loads like lights, chargers, TV, etc. Whenever higher load is detected, e.g. well pump, refrigerator compressor, heating, etc. - all these are automatic by pressure switch or thermostats - ATS should momentarily start generator and keep it going until load drops.

Not sure if this is even possible with single phase inverter and did not see many two phase inverters out there.

FYI power is either single phase or three phase but there is no “two phase” which may explain why you can’t find that. If it’s 240V you’re after that is still a single phase system.