Use a generator that has enough power for both Tagtool and projector. For example, if you have a projector that uses 300 Watts and your Tagtool computer uses 300 too, a 1000 Watt generator will be good enough. Just make sure you turn everything off before you run out of gasoline, to avoid damaging the projector's bulb.
You can also use a car battery to power your projector, used together with a DC to AC power inverter. In our test we could get around 30-45 minutes of projection time (3000 Lumens) out of a standard car battery with a 600W inverter. Again make sure you turn everything off before you run out of power, to avoid damaging the projector's bulb.