Cars during tests on dynamometer have to have a big fan blowing constantly on radiator. If you provide a big enough fan, you can run it as long as you want, even with big load. I was once doing 140km/h on autobahn uphill with fully loaded passenger car with additional fully loaded roof trailer. The car managed half an hour of fully open throttle without any problems, but If your home needs to use 40kW for more than a hour, maybe investing in proper generator is a better solution. In typical applications, engine will not be running at full power.
