Solar - Twin Immersion Question

I'm getting a solar system with battery and immersion controller.

House has 4 bathrooms/ensuites all fed from the hot water tank, hence we've got a large water tank with 2 immersion heaters in it, but there's only 2 of us living here.

I'm getting a SolarEdge system and they only do 3 or 5kW controllers. The immersion heaters are 3kW so I'd need 2 x 3kW controllers if we power both heaters. The question is do I need to power both if there's only 2 people using the hot water?

Is anyone else in this situation and did you have one or two immersion controllers installed?

Can you replace one of the heaters with a 2kW version? Alternatively, timers exist to control both elements which allow only one to be on at a time.

Not solar but my Mum's Megaflo tank had 2 elements but only the top one was ever used.

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Originally Posted by Kingstepper

Can you replace one of the heaters with a 2kW version? Alternatively, timers exist to control both elements which allow only one to be on at a time.

Not solar but my Mum's Megaflo tank had 2 elements but only the top one was ever used.

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I did think about replacing one heater but I've had a bad experience with a tank before which split whilst removing the heater.

A twin timer may be an option though.

I don't see why you'd need to power both. The hot water will stratify so feeding the lower one would give you more hot water in total. The higher one would probably be sufficient in your situation.

We have an Eddi & you can set it which you prefer, once up to temp it will select the 2nd immersion heater & so on.

Installer has confirmed 1 controller connected to the lower element. 3kW surplus in summer will heat tank and in winter there won't be enough surplus to run 2 heaters.

Isn’t it best to use top element first?

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Originally Posted by Kingstepper

Isn’t it best to use top element first?

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I don't know but they are a well regarded (Which approved installer) so I'd like to think they know what's the best option.

I suppose I could swap the mains and solar feeds over if I wanted to check which performs best?

The top one is there as a catch up to provide a relatively small volume of hot water in a hurry. It is not intended to heat the whole tank. But you want the solar to heat everything if there is an excess, so you want it on the lower one, all the time.

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Originally Posted by sweets

The top one is there as a catch up to provide a relatively small volume of hot water in a hurry. It is not intended to heat the whole tank. But you want the solar to heat everything if there is an excess, so you want it on the lower one, all the time.

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Not necessarily. If you are on a tariff with a decent export rate (e.g octopus flux or agile) you may not want to be wasting solar heating excess water when your could be getting three times the cost of gas or more sending it to the grid.