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I am planning to replace my (broken) gas boiler with a 10 kW electric boiler to supply the central heating radiators. I am also planning to replace the existing gas hob with an induction hob (say 4 kW). Hot water for shower and taps is currently supplied from a 3 kW unvented electric cylinder.

Once the electric boiler is installed, I then plan to install grid-connected solar PV panels on my roof. But I have been told by one boiler supplier that theirs is incompatible with solar power, by another that they are unsure and I should consult with the PV installer.

None of this makes sense to me. I had understood that the grid seamlessly takes over from the PV inverter when needed. So the boiler does not experience interference?

Any comments would be most welcome.

UPDATE Many thanks for all your reassuring comments. It may be that there was some concern regarding harmonic or other interference from the output of the inverter. I will find out from the PV supplier.

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  • \$\begingroup\$ Have you got storage built in there somewhere? Why would you need the radiators when the sun is shining? Have you got a brand and model for the boiler? Link to user manual? \$\endgroup\$
    – Transistor
    May 8, 2021 at 18:49
  • \$\begingroup\$ I am in Cambridge UK and the sun is not shining much. The recommended boiler is Heatrae Sadia Amptec. There is no electric storage, I would rely on the grid. \$\endgroup\$
    – Geoff
    May 8, 2021 at 19:05
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    \$\begingroup\$ You can get solar thermal panels for direct heating of water, which may be more efficient than going via electricity. \$\endgroup\$ May 8, 2021 at 19:06
  • \$\begingroup\$ This seems like a very bad plan. Have you worked out your running costs and compared with, for example, air-water heat pumps? Air-water heat pumps have a coefficient of performance of 3 or more. That means you get 3 kWh of heat energy for 1 kWh of electrical input compared with 1:1 for direct electrical heating and probably less than that if you're heating water electrically and pumping it around the house as that will have heat losses through the floor, etc. \$\endgroup\$
    – Transistor
    May 8, 2021 at 20:31
  • \$\begingroup\$ Have you calculated the cost of running a 10kW electric boiler? I recommend you do so before getting one installed. In the UK it will likely be about £1.60 per hour. And you will need it most when the solar panels are generating the least. \$\endgroup\$
    – Simon B
    May 9, 2021 at 20:37

3 Answers 3

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But I have been told by one boiler supplier that theirs is incompatible with solar power

Almost all PV installations use an inverter (or multiple inverters) to convert the DC from the solar panels to mains voltage AC.

Once that is done, the solar power is no different from mains AC. So any boiler that can run on mains AC can also run on the (converted) power from the solar panels.

Probably the installers were assuming that the boiler would have to run directly from the DC from the solar panels. Although technically possible that's not a preferred solution as it would mean you might not have enough hot water on a dark/cloudy day especially in winter.

With the PV solution with inverter, the boiler would simply run from mains power when there isn't enough solar power available.

If a boiler supplier doesn't tell you this, seek another supplier instead of one that complains their boilers can't run on solar power. Because they can, just not directly.

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You are right. Grid-tie inverters supply standard mains AC to the grid. So, really, the solar panels and boiler are two completely separate things that don't affect each other as far as compatibility goes. The boiler will never know you even have solar panels. I suspect the boiler person misunderstood what you were proposing to do.

"Hob" as you are using it is not a common word here in the US. I am not 100 percent clear on what it means other than it supplies heat somehow for some purpose and that there are gas versions as well as electric versions.

Just a comment on system design. Solar availability and heat demand don't line up very well in most places. If you have long periods when it is cold out and the days are short or cloudy, you can't expect the solar panels to supply enough power to offset the electricity you consume from the grid. This is not necessarily a problem, but I just want to make sure your expectations are realistic.

Switching a major heating appliance from gas to electric is going to increase your electric use noticeably. Your electric bill might go up during the cold season even though you have solar panels.

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    \$\begingroup\$ UK hob = US cooktop. The part that you put saucepans on. \$\endgroup\$ May 8, 2021 at 19:36
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You are correct that adding a correctly installed grid connected solar power system to your house will likely not have any effect on electric appliances you may have. They do not know or care where the power they use comes from!

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