A heat pump and solar panels: what the roof covers in winter, what it does not, and how to read your own numbers

A heat pump and solar panels work in opposite halves of the year: the roof peaks in summer, and the heat pump does nearly all its work from October to March. On a 4 kWp south-facing London roof, those six months make only 32.8% of the year’s output. So the roof helps the pump in winter but does not cover it. The overlap is in spring and autumn, and your own production history tells you how big it is for your house.

What a 4 kWp roof actually makes, month by month

What a 4 kWp south-facing roof at 35° makes in an average year, according to PVGIS, the European Commission’s free solar calculator:

Arithmetic on those two columns, which is all the shape you need:

• London: December + January + February = 510.14 kWh, 12.8% of the year. June + July + August = 1,382.45 kWh, 34.8%. July is 3.3× December (482.40 ÷ 146.48).
• Dublin: December + January + February = 485.58 kWh, 12.9%. October to March = 1,239.73 kWh, 32.8%, the same share as London’s 1,302.36 kWh. The best month is May (478.92 kWh), not July, and May is 3.6× December.

On an Irish or western British roof, “summer peak” is loose language: the year is front-loaded into late spring. Your own roof will differ with tilt, orientation, shading and location, and you can run the same calculation for it in PVGIS.

Solar panels and a heat pump in winter: the mismatch, without the exaggeration

Two things are true at once.

The first is the table above. In the darkest quarter the roof supplies roughly an eighth of its annual output, spread over the shortest days, while the heating load is at its maximum. Nothing about panel technology changes that; it is the sun angle.

The second is that a heat pump is not a resistive heater, so the load it adds is smaller than the “it will drink your solar” framing suggests. In the Electrification of Heat Demonstration Project, an official trial that installed 742 heat pumps and metered every part of each system, the median air source heat pump turned one unit of electricity into 2.8 units of heat (an SPF of 2.80), and still about two and a half (2.44) on the coldest days of the year.

Floor heating and the shoulder season, where the overlap is real

The overlap is in March, April, September and October. In the London column, March makes 341.81 kWh and September 373.61 kWh. Both are strong generating months in which the heating still runs, especially in a house with underfloor heating running long, low-temperature cycles rather than short high-temperature bursts. In those months more of what the roof makes is consumed in the house instead of exported.

How much more of your own power you use depends on your heating pattern and your roof; your meter and inverter data will show it, as set out further down. A battery shifts when the roof’s output gets used, not how many kilowatt-hours the roof makes in December, so the table above stays the ceiling.

The heat pump grant that exists today, and the dates attached to it

In England and Wales, the Boiler Upgrade Scheme pays part of the cost of replacing fossil-fuel heating with a heat pump or a biomass boiler. It does not cover Scotland, Northern Ireland or Ireland; if you live there, look up your own scheme before you plan around these amounts:

• £7,500 towards an air source (air-to-water) heat pump
• £7,500 towards a ground source heat pump, including water source and shared ground loops
• £2,500 towards an air-to-air heat pump
• £5,000 towards a biomass boiler
• Until March 2027, £1,500 more towards an air source or ground source heat pump if your home is heated by oil or LPG and has no mains gas connection. Ofgem, which runs the scheme, puts that at £9,000 in all, from 21 July 2026 until 31 March 2027.

Two limits matter before anyone quotes you a “complete package”: one grant per property, and no grant for a hybrid heat pump system. If the plan is to keep the boiler for the coldest weeks, that is a hybrid.

What it would cost you: sizing the question from your own production history

Every “should I add panels for the heat pump” conversation reduces to a comparison, and you already own one side of it. On a standard grid-tied system with no export limit, your inverter’s daily kWh describes the roof alone: it does not move when the heat pump runs, when the immersion fires, or when someone charges a car. If your inverter is export-limited or hybrid, production can be curtailed when the house is using little, so read it alongside your meter.

So work in this order:

1. Export twelve months of daily production from the manufacturer app or a monthly report. The mainstream portals (SolarEdge, Enphase and Growatt among them) report production day by day; find the daily figures in your own app, or read them off whatever report it emails you.
2. Lay your months next to the PVGIS column for your city, scaled to your kWp. If your roof made 2,400 kWh on 3 kWp, that is 800 kWh/kWp; the London model above is 3,973.45 ÷ 4 = 993 kWh/kWp on an unshaded south-facing 35° roof. A gap is not automatically a fault (tilt, orientation and shading all move it), but it is the first measured number in the conversation.
3. Take your winter months specifically, because those are the ones the heat pump will lean on. In the London model December is 146.48 kWh on 4 kWp, about 4.7 kWh a day averaged across the month. Whatever your own December comes to, that is the ceiling the roof can offer the pump on an average winter day, before any of it is used elsewhere in the house.
4. Check any month that looks wrong against the weather before you conclude anything about the heating system: how to check your solar production against the weather sets out the comparison. If the shortfall survives the weather check, the question is the array, not the pump, and solar panels producing less than expected is the next step.

The trial above metered each part separately. At home, the roof’s own record does that job, so read it before you put anything down to the heating.

The reason to do it in that order is that weather is the first explanation owners reach for, and it is sometimes the wrong one. One owner, describing a string that was producing nothing while the inverter showed no fault at all:

Since installation the generation figures have been slightly lower than before but I put this down to cloudier weather and deterioration of the solar panels. However when I checked the inverter I noticed that whilst string 2 was working fine, string 1 had the correct voltage (360v) … but the ampage was zero and hence not generating any output. The inverter appeared to be working fine and had no error messages.

Why a stopped inverter costs more once the pump is in

Before the heat pump, a dead inverter cost you the export credit and a slightly higher bill. After it, the same fault costs the same lost generation plus a heating load that keeps drawing from the grid regardless. The failure is no louder than it was: the pump goes on heating the house, the app may still open, and nothing tells you.

That is a March example on a small array; the same three weeks in June are worth more, and a fault that starts in autumn and is found at the annual reading is worth a great deal more. Whatever the size of the figure, nothing in the house announces it. If your system is quiet, the triage order is inverter offline or not producing, and if you are not sure the panels are doing anything at all, how do I know if my solar panels are working is the shorter check.