Heating your home

I haven’t published anything in the last few years due to all the work in and around our new house, and there are half a dozen drafted articles waiting for me to finish them up. But the topic of this article is too important to leave idle. Municipal elections are coming up here, and many Germans have lots of wrong information about the various forms of heating, and the physics, economics and politics behind it. This is a massive problem, because the average heating unit in Germany has an expected lifetime of about twenty years. As a result, up to 50% of all the heating units in Germany will need to be replaced in the coming ten years, with the owners having to face the consequences of their choice for another twenty years after that. And since the political landscape in Germany has a very strong influence on the availability and economics of heating technologies, what-ever political party the people vote for today has a direct impact on what is available in the future, but also on what that means to the voters financially in the coming thirty to forty years.

Data for the win – lots of data

That being said, I want to use the data that has accumulated during the last three and a half years to shed some light on the subject. Our house isn’t a top-notch passive house, but it has decent thermal insulation on the walls, windows with triple glazing, floor heating and an air-water heat pump for domestic hot water and heating. There is a 12.21 kWp PV generator on the roof along with a 13.8 kWh battery for those dark hours. I have data on the energy consumption of the entire house along with a few specific consumers, but also on the generated energy from the rooftop photovoltaics as well as the stored energy of the battery. All this data has a 15 minutes resolution and spans the 43 months (or 1.310 days) from June 1st 2022 to December 31st 2025, giving me an enormous 83,001,600 data points.

I also have the energy performance certificate (EPC) of our house and the heat pump’s energy monitoring data for the entire runtime. Unfortunately, the energy monitor of the heat pump and the data it has gathered is rubbish. According to that, the heat pump has generated just about 6.4 units of heat for every unit of electric energy. This is far beyond anything that specific device could possibly deliver even under perfect conditions, so that data is bound to be at least partially false (more on that later).

The things I do know for sure are that my house has consumed a total of 25,683 kWh of electricity and I also know the consumption of my washing machine, my fridge, our workstations and my server (I’m a nerd, nerds can have servers) along with the base load of the house which is roughly 120 W. All the energy that is not accounted for so far will include stuff like the oven, the TV and radio, Christmas lights, but also the heat pump. This gap represents about half of our entire electric energy consumption, weighing in at 12,813 kWh.

Making sense of the heat pump energy monitoring data

The energy monitoring data of the heat pump looks as follows. Note how the calculated Coefficient of Performance (COP) is quite a bit high, so at least one of the columns must be false.

COP = Generated heat energy / consumed electrical energy

A quick peek into the EPC will help get to the bottom of this. According to that, the house requires 2,457 kWh/year or 8,804 kWh/43 months of heat energy for domestic hot water and another 7,097 kWh/year or 25,431 kWh/43 months for heating. The EPC’s estimations aren’t actually that far off from the measured heat energy provided by the heat pump. According to the heat pump’s technical specification, the COP for hot water is expected to be around 3.75 (SCOP 55) whereas the COP for heating should be around 4.75 (SCOP 35).

Just to be sure, here is the generated heat energy calculated according to what the heat pump can provide and assuming that the stated consumed electrical energy is correct. Unfortunately, these numbers are as plausible as are the others, and I have a hard time finding a workable answer.

One more bit of information that may help here is the way I use and store energy in my house. For once, I love a cosy bathroom and I like to keep them warm all day. As a matter of fact, my home is heated to a steady 22-24 °C, whereas the EPC expects an average of 20 °C. I also use the hot water tank as an energy storage to save money by dumping any excess solar power directly into hot water. This is good for my bank account but bad for our energy efficiency and therefore results in a higher energy consumption. Over all, I would expect the generated heat energy in my house to be quite a bit higher than what the EPC estimates. Going forward and with all that in mind, the heat energy data given by my heat pump should be pretty spot on and the energy consumption calculated from that fits nicely into the energy gap.

Economical comparison of heating technologies

With all that data straightened out, the next logical step is to compare different heating technologies with an economic point of view and based on market values in Germany. I looked up the current system installation costs, as well as energy prices and efficiencies of each technology, using google and a bit of determination. For each technology, the cheapest system costs were chosen along with the highest feasible efficiency. The energy cost is calculated from the current average fuel price and the energy density of the specific fuel.

Electricity costs are currently an average of 31.6 cents per kWh, with an average of 21 cents for heat pump contracts. Grid injection pays around 6 cents per kWh, give or take, depending on your energy supplier. My heat pump’s consumption ratio of grid drawn electricity to excess solar energy is roughly 20:80 for hot water and 70:30 for heating. As it turns out, the size of the battery is actually not as important as one would think, and my 13.8 kWh battery has no real benefit over a common 5 kWh battery. The heat pump simply uses more energy to heat my home than what the photo voltaic can provide on most days during the cold months, and what little is generated during the day is consumed almost instantly.

If you pour all those numbers into a diagram, you’ll end up with something like this. It’s immediately obvious how the heat pump and photo voltaic combination blows everything else right out of the water, it’s not even a contest. The grid powered heat pump also smashes every single fuel based heating technology, except when you are heating with wood, and you also happen to own a piece of forest where fuel is basically free.

Combining the system installation costs with the running costs for fuel and electricity gives yet another interesting diagram, this time showing the economics during the system’s lifetime. Again, the heat pump and photo voltaic combination is leading by a land mile and can quickly compensate for the high initial investment. All combustion based heating technologies eventually match each other at around 22 years of runtime, with the obvious exception of the one where you own a forest and fuel is free of charge. Of course, this data does not account for any changes in fuel cost in the future. It’s a save bet that oil and gas will increase in cost over time whereas electricity is getting cheaper each year. With that in mind, the spread between heat pump and combustion is going to widen quite a bit more in the coming years.

What now?

Everybody has to decide for themselves, but here are a few guidelines for decision-making:

• If you hate having money, go with oil or natural gas.
• If you hate wasting money, go with a heat pump.
• If you hate wasting money a lot, add a photo voltaic generator and maybe a battery pack to the shopping cart for good measure.
• If you own a forest, a water-bearing chimney may be a good choice for heating. Domestic hot water should probably be done using a hot water heat pump.
• Go with pellets only if you can’t have a heat pump (or if you are for some reason madly in love with compressed wood).

And if you don’t find yourself aligned with the first bullet point, go ahead and vote for a political party that recommends heat pumps, renewable energies and climate protection.