Connecting solar to your hot water tank

immersun, solar pv, hot water tank

If you want to make the most of your solar pv panels, the next step up from running around turning things on and off is a solar hot water diverter.

You can only have one of these if you have a hot water cylinder. If you have a combi boiler then this will work for you.

The way these gadgets work is by a little sensor clamp over your electrical incoming supply cable. This clamp constantly measures whether you are importing or exporting electricity. As soon as you start exporting electricity this clamp sends a signal to the immersion element in your hot water tank to turn on. But here’s the clever bit: only to the amount of electricity that you are exporting.

So say you are generating 2.5kW on your solar pv. You have the oven on and this is using most of it at 2.2kW. You are exporting 300W. Your immersion will automatically turn on and deliver 300W worth of heat into your hot water tank. You decide that your chips are taking a bit long so ramp up the oven a few degrees and the load shoots up to 4kW. As your solar generation has not changed the immersion automatically turns off until such time as your chips are done, you turn the oven off and you are exporting electricity again.

This will go on until you have a nice full tank of hot water. When no more heat is required, the built in thermostat within the immersion element will tell it not to turn on anymore. Of course, if you run off hot water (and cool your hot water tank down) before the sun goes down (and you stop generating electricity) then the process starts all over again.

These units are brilliant and frankly a total no-brainer if you have solar pv and a hot water tank. It will take a big bite out of your solar electricity export and at the same time save you a pile of cash on your gas bill (or whatever your current fuel supply is). In fact, I have even been advising people that if you have a solar pv system installed then you can do without solar thermal (and this is from a solar pv AND solar thermal installation company-what am I thinking??!). This will do nearly as good a job for a fraction of the cost.

So which one? There are a few on the market and the basic job that most of them do is the same. The industry standard is probably the Immersun. What is great about this is that it has a nice display which gives you real time data about whether you are importing or exporting.

It’s major drawback is that it does not come with wireless signal as standard to tell the immersion unit to turn on and off. This means that you have to either a) put the immersion unit in the fuse board cupboard-not ideal as they can often get pretty crowded; b) run a cable from the incoming meter to the cylinder cupboard or c) buy their wireless signal enabler for an extra £80. If the cylinder is in the same place as the fuse board though which it sometimes is then this won’t be an issue.

The other good one is Power Diverter. This is a good bit cheaper, comes with wireless communication as standard but doesn’t give you the data you might like. It will simply flash green if the immersion is being switched on by the solar or red if it isn’t. There are others on the market which are perfectly good so if your installer suggests one and is confident with it then I would just go with that.

Installed cost for one of these babies by the way is somewhere between £350-£500.

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