Green Building Forum - Solar PV to ASHP (not Grid Tied)

Solar PV to ASHP (not Grid Tied)

Modelmechanix — Thu, 03 Jul 2025 16:54:28 +0100

Hi all.

One for the experts here.
I have sufficent south facing space for a 3-4kWh Solar PV Ground Array on my rural property. No planning issues where I live in the north of Ireland. The proposed array site is within 10m of where I intend on installing a 6kWh A2W Mono Heat Pump. I intend to use the heat pump for UFH only as I have ample DHW heat served by other means.
Essentialy for it to heat/preheat the floor screed via UFH during the day whilst PV is generating. The screed will cool overnight and charge again where PV is available the enxt day. Thats the line of thinking anyway.
The UFH has multiple input methods so where there are low PV production days or multiple poor days in a row, I can fire the wood boiler stove or boiler pellet stove to cover the heat needs.

So I am wondering, is it possible to have the solar array, via AC inverter supply the A2W heat pump without a grid connection?

As I understand it, the inverter can supply the AC but I don't fully understand if when PV generation vaies, how that would affect the A2W heat pump. Would it work at all, or short cycle? Would the compressor start up amps requirement cause an issue if the PV wasn't generating sufficent power?
I have read several other threads but none seem to discuss how it's done or controlled, maybe its a straight forward wiring connection and I'm over analysing here.

I would be very greatful for any advice or guidance, even if its a simple - it wont work matey!

Cheers fellas,
Karl

Solar PV to ASHP (not Grid Tied)

Peter_in_Hungary — Thu, 03 Jul 2025 19:04:33 +0100

Can you do a grid tied system? If so what sort of feed in options are available? I ask because grid tied is easier when running motors.

Terminology - kWh is a measure of electricity used. 1000w for 1 hour, 2000w for 1 hour is 2kWh, 1000w for 2 hours is also 2kWh.

PV is quoted as kWp which is the maximum output that a PV array can produce under ideal conditions.

Heat pumps are quoted in kW which is the maximum output that that heat pump can produce under ideal conditions.

Have you done an estimate of what your proposed PV array (I assume 3-4 kWp) can produce by month. This will give you an idea about how much energy you can produce in the winter when you need the heating.
Look here
https://re.jrc.ec.europa.eu/pvg_tools/en/
Zoom in on the map to your location and fill in the boxes and click 'visualise results' for a chart. The results by months are in a graph and in kWh.

Motors don't like running at either under or over voltage. in addition motors also have a higher start up current. So unless your PV is producing enough power to supply the start up current it wont start. (probably sit there humming and getting hot to the point of tripping the thermal or smoking). So extra controls will be needed to prevent the motor trying to start unless the power conditions are right.
Or
Have a system with a battery where the battery is charged by the PV and the battery (through the inverter) runs the motor (heat pump)
Or
Others may know of alternative solutions.

If you do the sums on setting up the PV system to run a heat pump I wouldn't be surprised if the numbers arn't feasible.

Do you already have the heat pump?
If not I would carry on using the wood stove (I don't like pellet stoves) and use the proposed PV for household power, grid tied or not.

Solar PV to ASHP (not Grid Tied)

fostertom — Sat, 05 Jul 2025 07:11:26 +0100

If running heating is (one of the) primary aim(s) of your PV, then orient the PVs so as to maximise winter solar capture. That will reduce its summer capture, and will reduce the annual total capture - but often the summer capture is excessive anyway. Maximising winter capture means much more vertical panels (even fixed on elevations not roofs) so as to be perpendicular to solar radiation arriving at lower elevation angles (though no radiation arriving at below 20o elevation is worth having). This assumes you actually receive that low-elevation winter radiation, not shaded by surounding trees/buildings/mountains - if not then you won't capture much winter solar however you orient the panels, so supplying heat pump is a non-starter.

Solar PV to ASHP (not Grid Tied)

philedge — Sat, 05 Jul 2025 07:52:56 +0100

You can use PVGIS to get a pretty good forecast of your likely generation at you location.

I think there's a general view that in winter you typically get 25% of the max summer generation so with a 4kw system you'll get 1kw on a typical winter day. I don't think that would be enough to power a 6kw heat pump??

Solar PV to ASHP (not Grid Tied)

fostertom — Sat, 05 Jul 2025 09:46:17 +0100

Is that 25% true when a system is winter-optimised, as described in 2-previous?

Solar PV to ASHP (not Grid Tied)

owlman — Sat, 05 Jul 2025 13:41:00 +0100

Use a home storage battery with a hybrid inverter as an interface between the PV and the ASHP. Also link the batteries to cheap rate overnight tariff...easy. As PiH suggested.

Solar PV to ASHP (not Grid Tied)

philedge — Sat, 05 Jul 2025 16:33:08 +0100

PVGIS suggests that there's negligible difference in generation between panels mounted at 45 and 90 degrees in Jan and Dec for a 4kw system at our location- 150kwh total for each month

Solar PV to ASHP (not Grid Tied)

Peter_in_Hungary — Sat, 05 Jul 2025 16:57:47 +0100

Posted By: philedgeI think there's a general view that in winter you typically get 25% of the max summer generation so with a 4kw system you'll get 1kw on a typical winter day.

For my system PVGIS forecast 233 kWh for December and 1010 kWh for July.
The actuals for last year were 275 kWh for December and 1095 kWh for July.
And for 2023 we got 261 kWh for December and 1094 kWh for July-
So for me the actuals were just about spot on 25% for summer max vs. winter minimum.

Solar PV to ASHP (not Grid Tied)

fostertom — Sat, 05 Jul 2025 21:36:17 +0100

Posted By: philedgenegligible difference in generation between panels mounted at 45 and 90 degrees in Jan and Dec

That's because the winter optimum for south facing would be about 67.5o!

If, with a system optimised for max annual take, minimum winter take = about 25% of maximum summer take, then a winter-optimised system will give a larger minimum winter take, but smaller maximum summer take - a larger %age of a smaller total - so a ratio of say 40%?

Solar PV to ASHP (not Grid Tied)

Jeff B 2025 — Sun, 06 Jul 2025 00:02:44 +0100

Posted By: fostertomIf running heating is (one of the) primary aim(s) of your PV, then orient the PVs so as to maximise winter solar capture. That will reduce its summer capture, and will reduce the annual total capture - but often the summer capture is excessive anyway. Maximising winter capture means much more vertical panels (even fixed on elevations not roofs) so as to be perpendicular to solar radiation arriving at lower elevation angles (though no radiation arriving at below 20o elevation is worth having). This assumes you actually receive that low-elevation winter radiation, not shaded by surounding trees/buildings/mountains - if not then you won't capture much winter solar however you orient the panels, so supplying heat pump is a non-starter.

Our average PV generation (14 years worth of data) for the months of November, December and January are 75, 36 and 55 respectively. Our panels are roof mounted at 45 degrees and the sun is so low that the light is partially blocked by my neighbour's trees. They are deciduous trees but I guess the trunks and branches are enough to do the damage.

Solar PV to ASHP (not Grid Tied)

philedge — Sun, 06 Jul 2025 07:58:36 +0100

PVGIS shows a slight increase in Dec and Jan production for panels at 67 degrees but it's maybe a kwh per day.

I think the chances of running a 6kw heat pump off a small PV array are pretty slim without a grid connection to either supply the bulk of the load for the pump or charge batteries off peak and run the pump off the batteries.

Solar PV to ASHP (not Grid Tied)

WillInAberdeen — Sun, 06 Jul 2025 10:44:13 +0100

Is the house off-grid?

Like others said, you need a battery and off-grid inverter to collect together all the bursts of PV during the day, when the weak sun comes between the clouds, and consolidate
them into stable power that keeps the ASHP happy for a couple of hours.

Just to be aware that it will not make very much heat each day, maybe less than a pound/euro, so unlikely to pay back the cost of ASHP and battery. Unless you can use grid electricity the rest of the time?

Modern heatpumps are inverter speed-controlled so startup currents are no longer such a problem, but like all electronics they don't like a stop/start power supply.

The people on the Camelot forum are more electricity focussed than here, and may give you better advice.

Solar PV to ASHP (not Grid Tied)

stonecold — Sun, 09 Nov 2025 17:30:41 +0000

What wall and floor U values do you have? If you have or are aiming for EnerPHit clearly it is a different situation compared to a period house with no wall insulation. I am not sure on the ground array capacity.

I can tell you that for my solar PV we have a gateway installed to allow generation to continue off-grid, and have multiple batteries we charge in the day which will be used for a 8kw ASHP next year - it is a 9.8kwh peak solar PV roof system and with zero floor insulation and 0.60 U walls, it will not be enough for year-round self-sufficiency but should be enough for some time with heating on during power cuts (up to 10-12 hours max I think). There strings come down from the roof into a solaris inverter which I think converts it to A/C, and through isolation switches to the gateway.

Using daytime generation to heat a dedicated solar PV compatible hot water cylinder is sometimes suggested as an alternative to batteries and would be better than forcing the heating higher in the daytime (if you have significant heat loss). https://www.installeronline.co.uk/plumbing/hot-water-storage-as-a-thermal-battery-what-installers-need-to-know/

On solar PV with 2 people home all day you will still have am and pm spikes - the high usage appliances are the kettle, microwave, any water heating including showers not on a mixer, hobs and oven plus washing machine. Setting the washing machine on in daytime only helps reduce evening consumption.

It will also depend on where you are, solcast has irradiance maps showing different parts the UK.

I think official MCS calculations will be your best guide if your question was about capacity rather than electronics.