Photovoltaics and heat pumps: what the combination really delivers

A photovoltaic system on the roof and a heat pump in the cellar: that sounds like a house that heats itself. It is not quite that simple. The two go well together, but mainly in spring, summer and autumn. In the depths of winter, the majority of the electricity still has to be purchased. If you know this beforehand, you won't be disappointed afterwards.

Why the combination makes sense in principle

klimaaktiv explicitly describes the combination as highly recommended. The reason: a heat pump turns one kilowatt-hour of electricity into several kilowatt-hours of heat. Every self-generated kilowatt-hour that goes into the heat pump is therefore multiplied. Viewed over the whole year, according to klimaaktiv, a significant proportion of the heat pump's electricity can be generated on your own roof.

Then there is self-consumption. You do not have to buy in electricity that you use yourself. The heat pump is a major consumer that can make good use of part of the surplus. According to klimaaktiv, only what is left over after the household, heating and hot water is fed into the grid. klimaaktiv lists a higher self-consumption rate and greater cost-effectiveness of the PV system as advantages.

An honest expectation for the winter

This is where the most common misconception lies. The heat pump needs the most electricity precisely when the sun provides the least.

The klimaaktiv guide gets specific: on an average December day, a PV system only delivers around 1 kWh of electricity per kilowatt-peak of installed capacity. In the depths of winter, your own solar power only covers a minimal part of what heating and hot water require. The rest comes from the grid. The second klimaaktiv brochure also lists November, December and January as months with a relatively low electricity harvest.

It is different at the beginning and end of winter: then the PV system can certainly deliver more than the household needs, and the heat pump can use this electricity.

A good test before planning: klimaaktiv recommends having the planning team calculate how much the PV system will deliver and how much electricity heating and hot water will need on a typical winter day. As an example, the guide mentions 21 January with a daily average of 0 °C. Then you will see in black and white what to expect in winter.

Shift hot water heating to lunchtime

The simplest and most effective lever costs nothing: set the time window for hot water heating to the time when the sun is shining. klimaaktiv suggests the early afternoon as an example.

Why this works: a hot water cylinder is a thermal storage unit. According to the guide, a 300-litre cylinder can absorb around 9 kWh of heat, for which the heat pump needs around 2 to 3 kWh of electricity. This amount can easily be covered from your own roof on a sunny day. The guide also notes that in the sunnier months, storing energy as hot water is a more economical option than storing it in a battery.

This can also be worthwhile without a PV system. According to E-Control, from 1 April to 30 September between 10:00 and 16:00, the consumption-based part of the grid usage fee is 20 per cent lower if your smart meter measures quarter-hourly values. Find out more in the article Electricity tariff for the heat pump.

The house as storage

With underfloor heating, the screed also stores heat. The guide gives a rule of thumb: one square metre of floor corresponds to a water storage capacity of around 10 litres, with a 5-degree temperature difference. On sunny days in the transitional periods, the heat pump can therefore heat slightly more at midday, and the house releases the heat over the evening.

You shouldn't overdo it. If you overheat the rooms significantly, you lose more heat and won't feel any more comfortable. It is best to coordinate with your installer to find out how much leeway makes sense.

The control system: from simple to automatic

There are three levels:

1. Time programme. Hot water and, if necessary, a slight increase in heating at midday. This works with practically any control system and requires no additional technology.

2. SG-Ready. This is a label for heat pumps whose control system, according to the guide, can be integrated into an intelligent power grid (smart grid). Via this interface, an energy manager, for example, can send a signal to the heat pump when there is surplus solar power. How the heat pump reacts to this depends on the device and its settings. Whether your device has this interface is stated in the technical specifications, or you can ask your installer.

3. Energy management. A dedicated system continuously measures generation and consumption and controls the heat pump, hot water and, if applicable, the wallbox. The guide states that a dedicated control system is required to optimise operation for self-generated electricity use. According to the guide, intelligent control can increase the self-consumption share of a PV system by around a fifth.

For most households, level 1 already brings a large part of the benefit. Levels 2 and 3 are particularly worthwhile if there is already an energy manager or a wallbox in the house anyway.

How large the PV system should be

This question belongs in the planning stage. For a detached house with a heat pump, klimaaktiv recommends cost-effective PV sizes between 5 and 10 kWp, which corresponds to a roof area of around 25 to 55 square metres. A larger system primarily increases the surplus in summer, not the yield in December.

What you should look out for

• Efficiency remains more important than self-generated electricity. Solar power is not unlimited either. A well-adjusted heating curve saves more in winter than any control system because it works in the months when the PV system barely delivers.
• Do not turn up the hot water temperature unnecessarily. Higher temperatures lower the coefficient of performance (COP). Ask your installer which temperature makes sense for your system.
• Measure the result. You can see whether the combination is working from the meter readings, not from a gut feeling. How to do this is explained in the article Checking your electricity consumption yourself.

Public subsidies for PV systems and heat pumps are constantly changing. You can find the current status for your federal state in the subsidy overview and in the Förder-Manager (subsidy manager).