Waste solar to hot water tank

[FlyingScot]

I’m tempted to add complexity (automation). For now, I made small changes to allow one water heater to work from the inverter bus. We leave that inverter on 24/7. For us, it’s a matter of turning on the breaker manually. I replaced the 1500w heating element rated for 120VAC with one rated for 240VAC. Thus it became a 375w element. It draws approximately 30 amps DC. Of course it takes four times longer to heat. Nothing is free. To heat 24L to the cutoff from Caribbean ambient temp takes 3-3.5 hours. Roughly 100 amp hours at 12v nominal.
On our catamaran the starboard water heater feeds our sink and shower plus the galley faucet. Two showers and some dishes typically use all of the hot water.
We find that we love having solar power so we are about to rip out our 1440wp and install 2175wp bifacials. We also have 900ah Firefly agm. Our old Xantrex inverter will be replaced with a Victron a multiplus 3000. That may tempt me to swap the heating element back to the full 1500w. The good thing about slow heating is less battery drain if you forget to turn it off. But also means it takes more time for a hot shower if you forget to turn it on.

[JustMurph]

That sounds like a pretty cool setup. No regrets here doing the automation on my system using the Cerbo GX. You use the generator start/stop function with an inverted output.

[mako]

Whether you feed your element via an inverter or directly off DC (they have 900 watt 24v elements) I would not want to draw down the battery, especially with a larger water heater.

I would think that you could pull a signal off your charger when it goes into float, switching a relay to directly feed the heater element, if working off DC. A nice and simple power dump.

[admiralslater]

Thanks for all the information. Here is a question that shows my ignorance. What happens if you apply 300 watts to a 1500 watt element. Also is it possible to meter the out put from an inverter that does not have the full capacity to put out 1500 watts

[Adelie]

I expect that if you applied 300W to a 1500W element it would get warm rather than hot.

[gregabbiss]

Originally Posted by admiralslater

Thanks for all the information. Here is a question that shows my ignorance. What happens if you apply 300 watts to a 1500 watt element. Also is it possible to meter the out put from an inverter that does not have the full capacity to put out 1500 watts

300W would barely heat the element so I would suspect nothing at all.

No you can't really limit the draw if you connect a 1500W element to an inverter that can't manage that it will trip the breaker at the best. Not a good idea.

Same result as turning on kettle et al and exceeding your inverters load, it will cut off the power at some point. But do not rely on safety devices, design for load, never connect knowingly too large a load, there is always the chance your safety devices, breakers, fuses, wont' stop your boat fire.

[JustMurph]

Originally Posted by admiralslater

Thanks for all the information. Here is a question that shows my ignorance. What happens if you apply 300 watts to a 1500 watt element. Also is it possible to meter the out put from an inverter that does not have the full capacity to put out 1500 watts

The short answer is you can't, and no.

The longer answer is that it is possible if you could lower the voltage going to the element, or you increased the resistance of the element. Neither of which is a practical option. The element has a resistance. You apply a voltage to the element and it will pull a current that is equal to the voltage / resistance. The power is the voltage * the current.

When an inverter reaches its limit, what that really means is that it can't maintain the voltage at the current that is being drawn. So the voltage sags, the current increases and the inverter will shut down.