In its simplest form this will be a DC generator which can be used to charge backup battery banks which can be used for backup power if your main power source is down. With a bit more work you can fit a DC to AC inverter which would enable you to power your electrics straight from the generator.
The thing to bear in mind is – your average mower motor will not be as powerful as a commercial generator. That doesn’t mean this is useless, just that it has its limitations. It is a good option for charging battery banks and it would be a good back up for solar panels and wind turbines when the sun is not shining and the wind is not blowing. If you fit a DC to AC inverter you could also directly power your lights and other low power devices.
Check out the links below for details and instructions.