Solar Panels, Dynamic Prices and Heat Pumps: When Heating Pays Off

If you combine solar panels, a dynamic energy contract, and a heat pump, you can save significantly on your energy bills. The trick lies in heating at the right moment: specifically, when your own solar energy is abundant or when the market price is low. At the same time, you want to maintain comfort and avoid unnecessary wear or peak loads on your system. 

In this article, we explain how to make smart choices, which variables really matter, and how to organize this effectively in practice with Homey.

The Three Knobs You Turn

To understand the potential, you need to look at three main factors. First, solar panels deliver the best return when you use the generated electricity directly yourself. Second, a dynamic tariff offers price fluctuations per hour—sometimes electricity is very cheap, and sometimes it is very expensive. Finally, the heat pump converts electricity into heat with a season-dependent efficiency (COP), which is higher at mild outdoor temperatures and low supply temperatures.

Your result therefore depends on:

• How much solar power you have now and will have in the coming hours.
• What the hourly price will be in the coming hours.
• How efficiently your heat pump is currently running.

Self-Consumption Over Solar Feed-in

With favorable energy net metering conditions, it used to matter less when you consumed energy. However, now that net metering is being phased out in many regions and feed-in compensation is falling, it is increasingly worthwhile to use your own electricity directly.

Heating your home or producing domestic hot water during peak solar hours increases your self-consumption. On the one hand, this reduces purchases at high prices; on the other, it prevents you from having to feed excess energy back into the grid for a lower compensation.

COP and Outdoor Temperature

It is important to remember that the COP of your heat pump rises when the outdoor temperature is higher and the required supply temperature is lower. This means that preheating during mild parts of the day is often more efficient than waiting until the cooler evening.

If you heat the building mass (floor, walls, buffer tank) during the day with a low supply temperature, you benefit from a favorable COP and potentially low or even negative hourly prices.

Thermal Mass Heating Without Overshooting

Homes with underfloor heating and heavy concrete floors have a lot of thermal mass. You can calmly bring that mass up to temperature during the day. Focus on small setpoint steps—for example, 0.5–1.0°C—and limit the maximum supply temperature.

If you overdo it, you will get significant temperature overshoot later. Consequently, the heat pump will have to run unnecessarily to get rid of the excess heat, which reduces the overall benefit. In contrast, in lighter homes with fast radiators and little mass, you should plan shorter heating periods closer to the desired comfort levels.

Domestic Hot Water: Produce It When the Price Is Low

We usually demand domestic hot water in peaks: morning and evening. However, with dynamic prices, you can heat the storage tank well in advance during cheaper hours. For instance, you can set Homey to turn on your water heater when electricity prices are low, and use it later when you actually need it.

It's also important to pay attention to legionella cycles and hygiene settings; schedule the weekly boost at an hour with a low price or high PV production. Set a maximum temperature that suits your comfort and safety, and limit unnecessary restarts to save energy. This prevents bacteria growth while ensuring the high-energy "thermal disinfection" phase doesn't spike your utility bill or strain the heat pump compressor during peak hours.

When Heating Does Pay Off

During the Day With Sun and Little Feed-In Compensation

If your PV panels produce a lot around midday and the feed-in price is low, it pays to lightly pre-heat your rooms well before the evening. You avoid buying electricity at higher evening prices and make use of a good COP.

At Low, Predictable Hourly Prices

If the price drops significantly between 11:00 and 15:00 or deep in the night, then plan a modest heat boost within your comfort limits. Even though nights are cooler (resulting in a slightly lower COP), extremely low or negative prices can more than compensate for the efficiency loss.

Ahead of Cold Peaks

If you know it will cool down significantly around 18:00 and the price will peak, then start earlier with a gentle supply temperature. This way, you avoid expensive, inefficient "sprints" in the more expensive time slot.

When It’s Better Not To Heat for a While

High Prices and Little PV

When energy prices spike or clouds block your solar panels, Homey can shift your heating strategy to a temporary "eco" mode. This involves postponing large heat demands, such as reheating a domestic hot water tank or boosting slow-responding underfloor heating zones. By maintaining only basic comfort levels during these expensive windows, you prevent high utility bills while keeping the house comfortable until prices drop or the sun returns.

Very Cold, Windy Moments

If the COP collapses due to freezing weather and the price is simultaneously high, preheating just outside that peak is often more favorable. You will need to let Homey plan in advance for such weather and adjust your schedules accordingly.

Full Tank, Little Demand

Overheating a buffer tank works counterproductively because you lose heat while you don’t need it yet. Aim for a narrow band around your desired comfort level, at levels that your home actually needs.

Avoid Peak Loads

Even if the price is low, your grid connection and fusing still set limits. If PV production is high and you are also charging an EV while the heat pump draws significant power, the peak current can shoot up. Therefore, spread large consumers where possible. Use maximum power limits or soft-start features, and let the control system decide which device is allowed to run when.

FAQ

How do I determine a good price threshold?

Look at your average contract price and feed-in compensation. Choose a threshold clearly below that, for example, the lowest 25% of prices in a week. Test for two weeks and adjust until your comfort and costs are in balance.

Does it make sense to heat when the night price is low but it’s colder outside?

Yes, provided the price reduction is proportionally greater than the efficiency loss due to a lower COP. Keep the supply temperature low and limit yourself to a small thermal charging step. At extremely low or negative prices, this is indeed often favorable.

Is preheating with radiators also smart, or only with underfloor heating?

It can be done with both, but underfloor heating benefits more thanks to its mass and low supply temperature. With radiators, you work shorter and closer to the usage moment. Consider low-temperature radiators or fan convectors for better efficiency.

When is the best time to produce domestic hot water?

During sunny hours with PV surplus or in price dips. Make sure the tank is hot at your peak usage moments. Schedule the legionella boost weekly during a cheap hour.

How do I prevent short cycling of the heat pump with short charging windows?

Use a buffer tank or ensure sufficient water content, set a minimum run time and off time, limit the number of simultaneous zones, and choose small setpoint steps. A lower supply temperature also helps significantly.

Does it make sense to combine solar panels with a home battery?

Yes, especially when feed-in compensation is low and prices fluctuate strongly. However, smart heat buffering via the floor and buffer tank can already capture a large part of the benefits without a battery. Start with that approach and then evaluate whether a battery still adds extra value.