4 common misconceptions about solar energy
…and what I should have known before I bought them
Last year in September I had 16 solar panels installed on my roof. We went into the whole procedure rather naively. We already had 3 solar panels installed as they came with the house. However, our neighbors were investigating what it would save them to have their full roof covered with solar panels. When we indicated that we were also interested to consider that in the future, the installation company gave us a highly competitive offer. That persuaded us to also upgrade our installation.
Once we had our solar panels installed I turned to my good ol’ self and decided to analyse the effect of the solar panels on our energy usage. I had some assumptions about the benefits of solar panels that I’ll discuss in this article. The motivation for us to get solar panels installed was partially economically (“to save money”) and partially ideology (“consume our own renewable energy”)
Our installation contains 16 solar panels (325 wP) with a total power of 5200 wP (for those interested). It is expected to generate about 4000 kWh per year. The investment including installation was €425 per solar panel. It is installed on a roof facing the south. At the end of the day we have some shadow on some of the panels, so we have optimizers connected to each solar panel. This ensures that we capture the most out of each panel, and they don’t affect each other.
Lesson 1: You are not ‘using’ your own energy
When you purchase solar panels, the installer will calculate how many solar panels you will need to cover your own energy needs. We decided to ‘over install’, which means that we installed more solar panels than the energy that we use. As we have young children, we might switch to electric cooking, and in the future purchase an electric vehicle, we can safely assume that our energy needs will grow in the future. Therefore it was a safe bet for us to invest slightly more in additional solar panels now, so we are covered for the next decade.
We consume currently about 3500 kWh of energy on a yearly basis. Since we have an installation that is guaranteed to generate 4000 kWh we should easily cover our own energy usage. However, solar panels only generate energy during daytime. As a family we consume most of our energy in the morning and evening. That means that during the day we are most of the time providing energy to the grid and in the morning and evening we are taking energy from the grid.
Luckily in the Netherlands we have a regulation that we are still allowed to ‘even out’ the total energy generated and the total energy generated. This means that essentially we can use the energy grid as a huge battery.
Economically that is interesting as it means that we currently don’t have to pay anything for our electricity. Actually, the energy company is paying us a small fee (about €0,07) for every additional kWh we delivered to them by the end of the year. What this saves us a year? 1 kWh of electricity is about €0,22. So 3000 kWh saves us €660 a year. Additionally, we receive about €70 for the energy we generate. So we save a total of €730 a year with our solar panels.
However — the rules will change. As of 2023 it’s likely you will only be allowed to even out about 93% of your energy, and that percentage will reduce year on year until it is 0% around 2030. This makes it economically less interesting as you must consume your own electricity immediately to actually profit from it.
Lesson 2: You can’t get ‘off the grid’ when you have solar panels
Quite a bummer when I realized this. But, no worries: we can just install a home battery! With the home battery we can store the energy that we generate in our own house. This means that we don’t use the grid as a large battery. That sounds like an excellent idea! But…will it work?
In order to figure out this question, I first needed to understand how much energy we actually need. In order to get this information I installed an energy monitor in our main installation (see the section “How I gathered my data” for details). This actually monitors the energy (and gas) in real-time. With almost half a year of data (October 2020 up to April 2021) I now know the following about our energy requirements:
- On average we consume about 12 kWh of energy (slightly less). (However, this must be biased for the winter period, as I know that the previous year we consumed about 3000–3500 kWh for the full year)
- The least we have consumed is 6,3 kWh and the most is almost 20 kWh.
Okay. That clarifies our energy needs. How about the energy that we generate?
- On average we generate 8,6 kWh. (However, this is again biased as I’m primarily looking at the winter period).
- The least we have generated is 0,1 kWh (!) and the most is 32 kWh.
Oops. Our average generated is not sufficient in this period to cover for our average required. This is already an indication that we can probably not generate sufficient energy ourselves in the winter time to cover our own needs.
I also ran the scenario “what if we had a home battery installed”. Perhaps the best days can get us through the worst days?
I’ve used the data that I gathered about our energy usage to simulate charging a battery and first using that power the next day. For simplicity I’ve taken “time” out of the equation. That means that I take for granted a scenario where we would consume all the energy in the battery before we start charging it again. To simulate this I’ve taken a Tesla Powerwall (just because it looks cool), which has a storage capacity of 14 kWh of energy.
What you immediately see in the diagram is that the battery is depleted for most of the winter period (November, December) and starts to be fully charged again in the beginning of Spring. However, on bad days it is still fully depleted. To counteract that we could install a second battery to get a capacity of 28 kWh. That at least shaves of the bad days during spring (see the diagram below).
What a bummer. That means that we will never be able to fully get off the grid. We must rely on the grid if we want to maintain our current way of living.
Lesson 3: Energy is never there when you need it
If we can’t get off the grid, then we should at least try to use most of the energy that we generate. From 2023 onwards — when we can’t use the grid as a large battery any longer — that will also be the most economically favorable scenario. When you extract electricity from the grid you will pay the full price, and when you deliver electricity to the grid you will get a small fee in return. Essentially you get a 3:1 ratio: you need to deliver 3 kWh to extract 1 kWh.
The most obvious alternatives to put our electricity to use:
- Get an electric car so we can ‘consume’ our electricity as mileage.
- Exchange our gas-heating for electric heating so we consume less gas.
However, the difficulty with both of these scenarios is that we are generating energy at the wrong time.
In scenario 1 we want to charge the car with our own electricity. Problem is: we also need the car to commute to work. That practically means: leaving at 8:00 latest, returning home at 17:30 earliest. Guess what: you’re not generating enough electricity to charge your car between 17:30 and 8:00. Let’s calculate this scenario. My commute is 62km back and forth. Assume that I would have a Tesla Model 3. According to this source the Model 3 consumes on average 15,1 kWh per 100 km. That means that I would need to charge the car daily with (15,1 x 0,62 =) 9,3 kWh. Except for the weekends of course. Currently (in spring, that is), I won’t be able to reach that (see the chart below). The only option would be to install a home battery and transfer that energy during the night from the home battery to the car. But obviously that would limit our options of actually consuming the energy during the day.
For scenario 2 there are various alternatives to heat up your house with electricity instead of gas. The most efficient ‘all electric way’ is to do this via a heat pump. You can imagine this as an inverted refrigerator. You extract heat from the air, or from ground water and use that to heat up your house. Let’s assess if that would be a beneficial scenario. (Be warned: what follows might be a bit complicated and there are some assumptions in there).
I want to know how much kWh I would need to heat up my home to the same comfortable level that I have now. 1m3 of (Dutch) gas provides about 35 MJ of energy (source). That equates to 9,76 kWh. Gas heating systems are not 100% effective in converting this energy, but about 95%. We actually have a modern one, so I will use this number. That means our heating system converts 1m3 of gas to about (9,76 x 0,95=) 9,27 kWh of energy.
When we consider a heat pump this works slightly different. Through compressors and fans it extracts heat from water of air. That heat acquired then heats up water to be used in central heating, or for taking showers. This system is solely powered by electricity. For each kWh of electric energy inserted, you get a certain amount of heated water. This can also be expressed in kWh. Modern systems typically have a ratio of 1:4–5. That means if you insert 1 kWh of electricity you get 4 kWh of heat energy out. When you know how much m3 gas you use for heating you can use this ratio to know how much electricity you would need to heat your home to the same comfortable temperature. For example, if you use 100 m3 of gas for heating that would equate to 927 kWh of energy. To get the same amount of energy you will need (927 / 4 =) 232 kWh of electricity.
I’ve tracked my gas consumption for a year. About 3 m3 per month is used for cooking so I deduct that from the total per month. The remainder is used for heating (central heating and showers). If we do this conversion for our energy consumption it means that we would require about 2116 kWh per year (see the diagram below). The issue is that we would need the majority of it in the months that we produce the least solar energy. Actually, as we’ve seen before, we barely produce enough energy to cover our existing energy consumption in those months.
This basically means that we can produce a lot of electricity. Unfortunately, it’s never at the moments that we need it: We need most kWh during winter, but we actually generate most of our energy over summer. If we really want to use that renewable energy to replace other sources of energy we will need a decent energy storage solution! And I mean a serious storage solution. Let’s say at least 100+ kWh so we can fill it up during summer and reap the benefits during winter
Lesson 4: When you go full electric, you need a huge house!
I believe in technology and in innovation. So I imagine that we would figure out a way to actually get the electricity to the right place at the right time. If we solve this…could we then perhaps live of our own, generated electricity? That would be great, because it would allow us to replace gas and petrol with a renewable energy source.
For us, that would mean the following:
- Replace our petrol cars with electric cars.
- Replace our gas heating with a heat pump.
- Replace our gas cooktop with induction cooktop.
Let’s assume we get electric cars. Commuting to work is about 16.000 km a year (about 260 workdays x 62 km) for me and about 20.000 a year (about 260 workdays x 75 km) for my wife. Assume we drive about 4000 km privately (I guess it will be more actually. Probably closer to 7000 km). That means we drive 20.000 km per year, per car, needing about 15,1 kWh per 100 km. We require (20.000 / 100 * 15,1=) 3020 kWh additional energy to charge each car. So 6040 kWh in total. (For reference: that is close to what we currently consume living in our house for a full year).
As we’ve seen in the previous section changing the gas heating with a heat pump will require approximately 2116 kWh per year for heating.
Furthermore, changing to an induction cooktop would also consume an additional 350 kWh (assuming that we consume approximately 1 kWh every day). That would bring the following total (numbers rounded down):
- Regular energy usage: 3500 kWh
- Driving electric cars: 6000 kWh
- Electrical heating: 2000 kWh
- Electrical cooktop: 500 kWh
Annual electricity needed: 12.000 kWh!
We would need to triple the amount of solar panels we have. It would require changing our gas heating to a heat pump (about 4 times the size) and installing 7 (!)Tesla Powerwalls to store about 100 kWh of electricity. Well…you definitely need a big house for that!
Conclusion
After running this analysis I’m still happy with my solar panels. We are adjusting slowly to consume most energy during the day (example: run the dishwasher and washing maching during the day, instead of the night) so we actually consume our own energy. Yet, I’m more realistic on using renewable energy to replace other sources of energy. As long as storing electricity remains a problem, solar panels alone will not cover household full energy needs.
In my opinion governments and energy companies should support the transition to renewable energy by focusing their efforts on storing and transferring energy to the right place, at the right time. You can think of scenarios where I could provide electricity to a company that runs a data center and in exchange could get heated water. Or some huge batteries in a neighborhood where each household can “deliver” and “withdraw” energy as a small marketplace.
Economically our solar panels are still viable. Ideologically…well, I’m optimist: perhaps some day.
(You know what…perhaps Elon Musk isn’t that crazy after all…and someone can charge his Tesla using my energy in exchange for some bitcoin)
Disclaimer: How I gathered my data
The solar installation that I have also contains a converter from SolarEdge. This converter tracks and monitors the energy that the solar panels generate. It displays this through an app and a webportal. I’ve gathered the total kWh of energy we generated for each day in a spreadsheet.
I have installed an energy monitor (Home Wizard) on our central metering system. This tracks in real-time the consumption of electricy and gas and stores this.
With these tools I’ve calculate the following information on a daily basis:
- Total energy generated by solar panels (from my solar installation) in kWh
- Total energy used throughout the day in kWh
- Total energy generated and directly consumed in kWh
- Total energy required from the energy provider in kWh
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Remco Magielse is a product manager at CM.com. CM.com is a high tech company focusing on Conversational Commerce in The Netherlands. He has worked as a system engineer and product manager at Philips Hue. Remco has gained his Ph.D. on the dissertation titled ‘How to design for adaptive lighting environments: Embracing complexity in design’. He writes articles about product and software development, and the hard- and soft-skill required for product management. He is passionate about innovation and has contributed to approximately 50 patents.
