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](https://www.stiebel-eltron.com/en/home/products-solutions/dhw/instantaneous_waterheater/convenience_instantaneouswaterheater/dhb-e-lcd/dhb-e-18-21-24-lcd.html) 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](https://www.geewiz.co.za/mecer-inverters/22813-mecer-axpert-3000va3000w-24v-pure-sine-wave-solar-invertercharger-1200w-pwm.html). 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.*