To heat water without a storage tank requires one of two things or a mix of the two. Either the water flow has to be reduced to the point that the burner can heat it as it goes through the heater, or the heater must have a large burner (and increased gas consumption), (and flue vent) so tht it can keep up with the demand. Usually the burner size is increased somewhat and the flow is also reduced. To the extent that these two items are balanced, will determine whether you would be happy with it and whether it would work in your situation. An electric unit requires a massive amperage load that a temporary hookup could not even begin to supply.