Solar power to an immersion heater

Hello Express842 and welcome to Solar Panel Talk

First off an inverter will not work directly connected to solar panels. It is usually looking for a steady voltage source like a battery.

Next what is the voltage and amp rating of that heater? That value will help you determine the amount of panel wattage and battery size you need to run it.

Solar panels produce DC, and the thermostats in water heaters are only rated for AC. So an inverter is an expensive and lossy workaround. inverter needs batteries and it gets real expensive fast.
Or go straight DC and develop some Solid State Relays or DC rated contactors to fit your application. Both ways have pitfalls

Simplest is to go simple Grid interTie and have enough panels and inverter to offset the heaters usage

Then there is the heat pump style water heater, slower recovery, but much more efficient than resistance coils. Not free either !

That should work as long as the water tank's immersion heater is purely resistive and has direct hookup with proper non-electric temperature cutoffs and your off-grid solar inverter can put out the proper voltage range for immersion heater.

Hi, thanks for the advise so far, I am looking at using a Solax Power Solar Grid Tie Inverter 2.2Kw which fits with advise above.

This may be a silly question but if the panels (via the Grid Tie inverter) are generating 1000w AC but the thermostat on the immersion heater clicks off due to the water being hot enough, other than wasting the electricity being generated, what would happen

It it's grid-tied then excess power will flow back to the grid.

Yes it would work with the caveats explained above. It would add complexity and I am not sure what the payback would be. Do you know how much your resistive element water heater is costing you in kWhs and dollars? Heating water can be as much as 20% of energy consumption so anything that reduces that will have a financial payback.

To go outside your original question have you looked at the alternative of a heat pump water heater?. The payback can be quantified because a HPWH will consume significantly less power than a resistive element water heater. For me changing out a water heater is significantly less effort than mounting 4 to 6 solar panels and connecting them to an inverter. The first thing I did when I moved into an all electric home was to replace the old water heater with a HPWH.

You have several choices.

The best way, as has been explained, is to use a grid tie inverter and send the power to your house system. As an example, if you have 6 240 watt panels, you'll get about 1000 watts sent to the grid. Your hot water heater will take between about 3000 and 9000 watts. When it's on the hot water heater will take way more than you produce. However, when your hot water heater thermostat is off (which is most of the time) you will take zero power. As long as the daily average your hot water heater takes is less than the daily average you produce with the solar system, your solar power system will cover the hot water usage. (This assumes net metering, which is the most common utility scheme nowadays.)

The second best way is to use an MPPT heater controller. TechLuck makes one. There are a few others that I can't find now. What they do is take the DC your panels produce, convert it to a better voltage for the resistive element, and then feed it to the hot water heater. This means there's no grid involved; the power goes straight from the panels to the hot water heater element. Downside is that you lose the power you don't need. And if you need hot water at night you might be out of luck. However, it will work during blackouts.

Third way is to connect the panels directly to the element. This is problematic for several reasons, including:
1) Standard hot water heater thermostats cannot "break" DC current, so you'll destroy the thermostat. To get around this you'll need a better relay, SSR or other DC rated switch.
2) Again you lose the power you don't use.
3) You will never get a perfect match between panels and element resistance, so that will waste even more power.

If you are looking at the lowest energy approach, it's probably a combination of a heat pump water heater and a grid tied solar array. But that of course is more $$$. Solar is now cheap enough that grid tie + resistive heat is probably the lowest cost.

Jflory2 your first option is what I’m planning but I was going to have it completely stand alone, ie using a grid tie inverter but not tied to the grid, simply providing power to the immersion heater. I appreciate this may be inefficient when the water is hot and I’m still producing electricity, but what will happen under these circumstances?

The only problem I see is a grid tie inverter is looking for the GRID and may not work tied only to the immersion heater.

A grid tie inverter relies on the grid for frequency and it runs tests so you would need a grid tie inverter with some connection to the grid and the ability to be programmed for no export. GT inverters will not operate standalone.
I agree with the conclusion of jflorey2 about cost of equipment. However, based on operating cost of a HPWH their initial high cost has a payback of a few years compared to resistive element water heaters. Since they move heat instead of creating it they are three times more efficient that resistive element water heaters according to the Department of Energy. That payback time is true in places like California and Hawaii which have high rates for electricity.

Thank you everyone. Really helpful post.

I have a , yet to be connected , 600w DC element to fit the tank once it has been installed .
Bought from E Bay uk , they are available , so could you not connect one of these instead of going through an inverter ?
I await more experienced opinions ?

The eternal search for PV hot water. And the answer is always "Kid, you'll shoot your eye out." Which is unfortunately true, there are ways to burn yourself up. Solar people don't understand electricity and only want to copy something easy, like buy it from amazon. The controls are actually quite simple and cheap. Here is one that cost $15 to build, China should be cranking these out. Even someone with a very small camp PV system can harvest excess PV right from the panels that also feed a charge controller.
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