Analyzing One Year of Solar & Heat Pump Data

It is now 1.5 years since we replaced our gas boiler with a heat pump and installed a solar panel system. We had heard a lot of horror stories about misconfigured heat pumps and the excessive costs that they may cause, so I invested into detailed planning and a custom simulation of the photovoltaic system.

Now everything has been up and running for the first full calendar year, and it’s time to check if my assumptions were correct or if I only did some wishful thinking with the numbers.

Part I: Simulation vs. Reality

Electricity Demand

For the general electricity demand, I could already rely on actual data, since I had been measuring the electricity consumption of our household for a while.

What was much less clear was the heat pump demand, since it depends on many factors like the outdoor temperature, the control algorithm of the heat pump, how we use the house, etc. So I did the estimation based on historic weather data of Deutscher Wetterdienst and a simple linear approximation of the heating demand with fixed Coefficients of Performance (COP) of 4 for heating and 2 for hot water. Further, I assumed that the heating would be evenly distributed throughout the day. These assumptions felt so simple, that I was surprised to see how accurate the results were:

The heating electricity is lower than expected, the hot water demand is higher and overall I underestimated the demand by 9.4%. This is fairly acceptable from the planning perspective. I’m just not happy with the amount of energy wasted for hot water. The culprit turns out to be the drinking water circulation and we may consider removing the circulation in the future.

For the total electricity demand the simulation was even more accurate. I expected 10 MWh, but the real demand was 10.5 MWh, which is only a 5% difference. Also the seasonal pattern looks very similar:

Photovoltaic Production

For the photovoltaic production, I used PVlib for a detailed estimation. The final system we’ve built consists of two arrays of photovoltaic modules with different orientations and a 10 kWh battery.

The simulated production was 17.3 MWh, while the real production was 16.9 MWh, which is only a 2.5% difference.

Matching demand and production

The problem with solar power is that it poorly correlates with the heating demand. So it is quite interesting to look at the energy flows throughout the year.

As it turns out, there’s barely a difference between simulation and reality. As can be expected, the rising demand in the winter months triggers the majority of the grid imports. During summer, the household is almost completely self-sufficient and exports a large amount of electricity.

We achieved a self-sufficiency of 64%, which is only 3% lower than the simulated value. That means that instead of importing 4 MWh of electricity and 20 MWh of gas per year, we only need 3.6 MWh of external electricity and no gas for the same task.

Part II: The Financial Verdict

All of the above looks very promising. The simulation was accurate, so also the financial side of the coin should be a success story. But let’s have a detailed look into the numbers.

Profitability of the Heat Pump

The first investment was the heat pump. It was necessary to find a replacement for the gas boiler, and heat pumps seem to be the most future-proof option given the need to replace fossil fuels altogether.

The achieved energy efficiency of the heat pump is rather mediocre, with a Seasonal COP of 3.91 compared to an expected 4.8 according to the data sheet. This turns out to be an effect of excessive water heating, which is much less efficient than heating the house. The lesson for me is that a circulation should be avoided. However, the overall financial effect of the change is still good.

For the financial evaluation, I calculated the Net Present Value (NPV) of the heat pump compared to a gas boiler alternative. I kept it conservative and used current prices: an electricity price of 0.32 €/kWh and a gas price of 0.11 €/kWh. For a 20 year lifetime, the following parameters are delivered:

So the heat pump provides annual savings of €840.39 and a NPV of €2,502.88 over 20 years despite the higher initial investment. For now, this only holds true thanks to a 15,000 € government subsidy. The full installation costs of the system were 37,000 €.

This is under the assumption of constant prices. Often times, the gas price is predicted to increase in the future because of CO2 taxes. On the other hand, it is subject to discussions in which direction the electricity price will move in short and long term. A sensitivity analysis shows how the ratio between the gas price and the electricity price affects which heating system is better from the purely financial perspective.

Under current market conditions, the heat pump offers a slight financial advantage. When combined with its environmental benefits and more consistent heating performance, it emerges as the superior choice for us.

Profitability of the photovoltaic system

Calculating the financial parameters of the photovoltaic system is much more complex because the energy flows throughout every day including battery charge and discharge need to be considered. But thanks to the detailed simulation and recorded real values over the last year, these flows are available and can be used to calculate the financial parameters.

The following table shows the parameters of the real system measured last year, the simulated system and a house without solar power:

As can be seen, the solar panels and battery drastically reduce the electricity bill. Although the consumption of the house is rather high, the electricity costs are very low now.

Assuming a discount rate of 3% and a system lifetime of 25 years, the investment clearly pays off:

• Initial Investment: €25,500
• Annual Savings: €2,976
• Payback Period: 8.57 years
• Net Present Value: €26,316

Summary

After one year of operation, the data confirms both good accuracy of the simulation and profitability of the investments.

The custom simulation was remarkably accurate:

• Total electricity demand: 5% deviation
• Heat pump electricity: 9.4% deviation
• PV production: 2.5% deviation
• Self-sufficiency: 3% deviation

This demonstrates that detailed simulations using tools like PVlib can reliably predict real-world performance, giving confidence for investment decisions.

Both systems are profitable despite the significant upfront costs:

• Heat Pump: €840/year savings vs. gas boiler, 11.9-year payback period, NPV of €2,503.
• Photovoltaic System: €2,976/year savings, 8.6-year payback period, NPV of €26,316.
• Combined Effect: Reduced annual energy costs from €3,360 to €385 while achieving 64% self-sufficiency.

My key lessons are:

• The investments are predictable. The simulation was accurate enough to make confident decisions.
• A PV system such as ours would be profitable even without the heat pump. A quick simulation showed ~90% self-sufficiency in this case and still a high NPV.
• Heat pump efficiency matters: Carefully choose the right system size and tune the parameters to maximize efficiency.
• The cost advantage of a heat pump depends on the price ratio between gas and electricity, but especially with PV production, the profitability is robust.
• The modernization is both environmentally friendly and financially sound.

For anyone considering a similar investment: yes, it’s possible to simulate the outcome beforehand with high accuracy, and yes, both systems are genuinely profitable — not just wishful thinking.

You can find the analysis code and data used in this blog post on GitHub: https://github.com/chr1st1ank/blog-assets/tree/main/code/pv-evaluation.