All electric equipment on the boat depends on the charge from the engine and battery bank or that there are charging plugs available at the pier. In other words, Wallas wants you to use as little battery power as possible. At maximum power, a 22 Dt takes only 0.9 A to give you 2.2 kW of heat. Wallas heaters are designed so that they keep the heat even when the power consumption is optimized as low as possible. Learn more
How much is less?
A traditional boat light of 12 W takes 1.0 A, while Wallas 22 Dt takes only 0.9 A at maximum. This is to say that with 1 Ah you can get 2.44 kW of heat. (Note that Ah is an energy unit and not a time unit).
With your 120 Ah battery, you could run Wallas 22 Dt around a week without loading your battery.