Gfci for 24 V

[K8MHZ]

Even 500 watts would be more than 20 amps @24 volts and 500 watts won't heat much water.

Agreed, folks, think about a single burner 'stove' like this:

It's 1300 watts. How long would it take to heat enough water to take a shower in with? I don't think a 500 watt heating element would do anything.

A standard electric water heater is 4KW or so. Think about trying to heat water from stone cold with one fast enough to take a shower with.

 

[K8MHZ]

Mira brand from the UK uses 9,000 watts. Just for reference, that would be 375 amps at 24 volts. I think there was a typo. 37.5 amps at 240 on 10AWG seem much more plausible.

Edit to add: Cheaper versions and brands use 7,000 watts and recommend a 6mm2 conductor (10AWG).

 

[K8MHZ]

500 watt water heater, 120 volts:

 

[Electric-Light]

Mira brand from the UK uses 9,000 watts. Just for reference, that would be 375 amps at 24 volts. I think there was a typo. 37.5 amps at 240 on 10AWG seem much more plausible.

Edit to add: Cheaper versions and brands use 7,000 watts and recommend a 6mm2 conductor (10AWG).

How do they work? It must not be true on demand or it's a super low flow.

It's 7*F per GPM per kW

You need 7.9kW per GPM at 55*F rise.

True on-demand at 2.5 GPM needs 20kW with a 50F supply temperature to get 104F shower.