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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.

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  • \$\begingroup\$ ”The water heater's max temperature is set to 45°C.” Nothing to do with EE but 45 degrees is just about sweet spot for legionella bacteria growth. It would not be legal (at least here) to run a water heater that cool. \$\endgroup\$
    – winny
    Commented Jun 29, 2022 at 18:05
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    \$\begingroup\$ Very interesting @winny, I didnt know that! Given that the water never stands still at that temperature and only heats up when you open the tap and it flows (similar to a gas geyser), I'm assuming that's not an issue? That being said- will investigate further. \$\endgroup\$
    – Pangolin
    Commented Jun 29, 2022 at 18:22
  • \$\begingroup\$ Back-of-the-napkin sounds close. Now keep in mind that 18kW from 240VAC implies 75A... so that much power from 24V, with a 100% efficient inverter, would draw 750A for those 20 minutes. Wire probably won't do... we're talking big buss bars. \$\endgroup\$
    – rdtsc
    Commented Jun 29, 2022 at 18:56
  • \$\begingroup\$ Could be exceptions but here you’ll be thrown in jail for manslaughter or deadly neglect (doesn’t translate well to English). No real concern for healthy young people but the elderly and people with respiratory problems are at risk and there are people dying from it every year. \$\endgroup\$
    – winny
    Commented Jun 29, 2022 at 18:59
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    \$\begingroup\$ @winny it's an on-demand water heater. There's no tank full of 45° water to grow bacteria in. \$\endgroup\$
    – brhans
    Commented Jun 29, 2022 at 19:01

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It would cost far less to install a bottled gas (LNG, propane, etc.) tankless heater such as one of these or these.

  • A 20 kW inverter, alone, is over US$1,000.
  • Using lead-acid batteries, providing water for ~1 hour total would require the equivalent of nine of these batteries, about another US$1,900.
  • The useful lifetime of a stationary-duty lead-acid battery is perhaps 10 years, so consider the cost of replacements. Of course, you could use a longer-lasting, but more expensive alternative, such as Ni-FE, or lithium chemistries -- much higher initial cost, with less frequent replacement.
  • Bottled gas, on the other hand, could last 100 years or more unused, and then is cheaply replaced when actually used. If you have reliable piped gas in your location, that might be another option.
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  • \$\begingroup\$ Yes, I think you're right. This will also mean the energy load on an inverter system for the remaining appliances will be significantly less and cheaper to do (TV, screens, kettle, etc). \$\endgroup\$
    – Pangolin
    Commented Jun 30, 2022 at 11:21
  • \$\begingroup\$ Also, thanks for the link to a South African site for the gas geyser! \$\endgroup\$
    – Pangolin
    Commented Jun 30, 2022 at 12:11
  • \$\begingroup\$ BTW, though I regret having to suggest adding to greenhouse gases, it only adds CO2 when running during an outage, and is more efficient than using *fossil/ fuel to make electricity, and then heat. (I assume you'd just put the gas water heater in series, preceding an existing electric heater.) \$\endgroup\$
    – DrMoishe Pippik
    Commented Jun 30, 2022 at 13:39

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