Solar water heating project

[TenMile]

So I've had the C-Dory 25 now since the beginning of July and have had a chance to make some fixes and modifications. Thought other members would be interested in what I've done with water heating.

Given that we want to cruise for long stretches, adding a solar panel to the hardtop was a given. The boat has 2 x 100Ah Lead Acid house batteries plus a start battery that I link together with an ACR. I ended up installing a 190w bifacial panel and wiring it to a Victron SmartSolar 75/15 charge controller. On a sunny day I see a maximum of about 10A from the panel and it's usually able to top my system up in a few hours.

One of the cool features of the Victron charge controller is that it has a built in Load Control function that can divert power to a load when the batteries are charged up to a maximum of 15A at 12V.

Our boat came with a 4 Gal water heater that was only powered via 110V when plugged into shore power. We rarely (if ever) plan to be at the dock but would like to be able to take a warm shower. I ended up swapping the heater element and thermostat out of the heater and used a 150w 12V element instead along with an STC-1000 digital thermostat plus a 40A 12V relay.

Here's how it all works. The STC-1000 is directly connected to my DC panel and acts like a switch that opens/closes a relay. It has a temperature sensor that I have heat taped to the cylinder inside the water tank (same place the thermostat connected to). It turns on until the water reaches 45C (~115F) and it will allow the water to cool to 40C (~105F) before it turns on again. The output of the STC-1000 is connected to a 40A DC Solid State relay and will open/close the relay switch. The SS relay is connected to the Load port on the Victron on one side, and a 150w DC water heater element inside the tank on the other.

When the boat batteries are charged, the load turns on and if the water is below 40C, the water heater element is switched on. I find that the draw on the system is about 11A or almost exactly 150w. Overnight the water will usually cool to about 32C and takes about 60-90 mins to heat back to 45C. Water in the fresh tank is just around air temperature so low 20's in summer. If we fully drain the hot water it takes about 2.5hrs to heat again. For us this means we can have a hot shower at the end of a day, or a warm-ish shower in the morning. The water heater has the standard safety valve and I've fused it.

The Victron load control has a number of custom settings but one of them prioritizes Battery Life so every few days it ensures that the batteries are fully charged.

The system will make hot water when the sun is out, when the engine is running or when plugged into shore power/charger as all 3 of those charge sources will trigger the Load to switch on. Will let everyone know how this system holds up over the long term. Total cost was very inexpensive -- was already going to do the solar panel / charge controller and already had the ACR installed so just needed the DC 150w element ($35), STC-1000 ($20) and 40A relay ($20).