Responsive resistive heating for soaking up excess PV

I have a 5kWp of south-west facing PV and it’s cold downstairs in the afternoons and warm upstairs when the sun is shining. I have gas central heating.
Now the scene has been set…is there some sort of resistive heating that can be smartly controlled and moves in small steps? Ie, I might only be exporting 400W, but 400W could be a useful amount of heat and I’d rather use it locally than export it.
I could charge the car but I believe it won’t charge at less than 6A [=1.4kW].
I think I am after some sort of smart power-limiting device that I can plug a simple fan heater in to, but I’m not sure what the terminology is, or if it exists.

You’re looking for a Mk2 PV Router. Unfortunately, our contributor Robin Emley has ceased production of the kits, and the business has been taken up by a French husband & wife team.

But if you want to construct your own, there is sufficient detail in the ‘Docs’ section here, under “PV Diversion”.

Robin’s English website will remain available for at least the remainder of this year at


What kinds of load are happy to have their input power manipulated like this?

Obviously a simple resistive heating element is fine, but as you add complexity, what happens? Motor driving a fan? Could be OK? But more complex electronic controls won’t appreciate it, right?

Probably not OK - you would almost need the fan to be speed controlled from a separate supply - or it would need a heavy flywheel to maintain an average speed.

Exactly. And you can include battery chargers in this group too. This is why water heaters or thermal storage heaters are the few practical loads that can be used.

If you wanted to control, say, a battery charger (a relatively complex electronic device), the only viable way would be to use the diverter to set the charge rate, as an analogue voltage or current on a wire, or as a command word sent digitally, and allow the battery charger to regulate the power it drew in response to that instruction.

How about a simple oil filled radiator? That would also buffer the short term power variability into a smoother heat output.