I'm looking at installing a battery + inverter system for my apartment to keep things powered during frequent scheduled powered outages. I live in South Africa, where we have 2.5hrs "load shedding" twice daily.

The apartment has a Stiebel Eltron DHB-E 18/21/24 instantaneous water heater installed, instead of a traditional water tank-based geyser.

I'm trying to determine what the requirements should be for the inverter and battery system to enable the water heater for <20mins during a power outage. The water heater's max temperature is set to 45°C.

My primary concern is the peak power consumption of the water heater, and making sure the inverter can handle it.

Ideal would be to use an inverter like this Mecer Axpert 3000VA/3000W 24V Pure Sine Wave Solar Inverter/Charger.

Back-of-napkin math points to the heater using ~18kW, which, to my understanding, means that a 18kW inverter is needed. Is this correct, or is there some other way to interpret this?

What size inverter is needed to handle safe operation of the Siebel Eltron water heater mentioned above?

Note: I don't have an easy way to measure current draw during water heater operation.