Is photovoltaics the ideal self-sufficiency solution for tiny houses?

When it comes to photovoltaics, opinions differ. Some claim that the hype is over, others speak of the energy solution of the future. Is a PV system worthwhile financially at all? Are there currently any grants?

We spoke to industry insider Ruben Kunze, solar module planning expert, and were able to clarify a few questions about this.

Ruben Kunze is an expert in the field of PV systems

Hello Ruben, thank you for taking the time for us today and the important topic of sustainable energy. Is it still worth getting started in 2021 and if so, what do I have to look out for when choosing my PV system?

Absolutely! From a financial point of view, it is worth investing in photovoltaics for your own family home. On this scale, the legislature allows self-generated electricity to be used without paying off electricity surcharges. The limit here is 30 kWp installed capacity.

In concrete terms: the kilowatt hour of solar power generated from the roof costs the operator of the PV system around 8-10 ct/kWh. With the current electricity prices of approx. 28-30 ct/kWh, the savings per kilowatt hour are considerable. The investment costs of a photovoltaic system are usually amortized after 8-10 years, especially due to this electricity price saving.

In contrast to 10 years ago, the income from the excess feed-in plays a rather subordinate role in the amortization of the system. Today, photovoltaic systems are optimized for self-consumption. In other words, ideally an east-west system for the power requirement before and after work. South-facing investments are, of course, still profitable.

To buy a system: A prospective buyer no longer has to deal with the technical details of a photovoltaic system. The production standards for solar modules (also from China!) and other system components are now so high that you can assume a system service life of more than 25 years, regardless of the manufacturer.

Incidentally, systems that were installed around the turn of the millennium are still running today without any significant loss of performance. And that despite the fact that the production standards were significantly lower then than they are today.

PV systems are not only worthwhile for single-family houses, but also for apartment buildings and commercial systems

One hears again and again that the efficiency of a PV system is poor. To what extent is that true and are there already newer modules that are significantly more efficient?

Here, too, a lot has happened in recent years. Standard solar modules on the market today have a module efficiency of around 20%. With solar radiation of 1000 watts per square meter, a standard module (1.6 square meters) converts up to 320 Wp into usable electrical energy. In order to generate the same average electrical energy over the year, an average trained person would have to pedal non-stop 365 days a year at a moderate pace (per module!)

Approximately 15 modules are therefore sufficient in a single-family house to generate more electricity over the year than is consumed in the entire house (assuming 4,500 kWh annual electricity consumption). In general, however, I always recommend large systems. On the one hand in terms of the energy transition, on the other hand there are fixed costs during construction, which can be specifically reduced with a larger system size. From a system size of 10 kWp (approx. 30 modules), the electricity is sufficient in summer to charge an electric vehicle and/or to operate a heat pump.

A lot of energy is consumed in the manufacture of solar cells. In addition, environmentally harmful substances are used, such as nitrogen trifluoride, which is known to be a strong greenhouse gas. Is that correct?

Manufacturing industry generates greenhouse gases, especially CO2. This undoubtedly also applies to photovoltaics. Converted to the service life of a module, however, only 50 g/kWh of solar power is emitted. With lignite we are talking about 1070 g/kWh.

However, I often hear the claim that a solar module consumes more energy during production than it produces during its entire service life. This is of course complete nonsense. According to Fraunhofer ISE, a PV module in Germany pays for itself energetically after just 2-3 years. With a life expectancy of less than 25 years, the PV module thus produces a multiple of the electrical energy used.

Ruben Kunze plans and installs large-scale PV systems on Germany's roofs.

One claim made by critics is that solar energy cannot guarantee a constant supply of energy. Is that true and are there alternative solutions for a secure energy supply?

We need different technologies for the success of the energy transition. Wind power generates an enormous amount of electricity in winter and photovoltaics in summer. This worked well for a long time because renewable energies were kept small politically. In the meantime, however, renewables produce almost 50% of Germany's electricity requirements. Since these are weather-dependent energy generators, excess electricity must be temporarily stored and fed back in when the power is low.

With the national hydrogen strategy, the federal government is finally providing reliable framework conditions so that industry can invest in this segment. The temporary storage of hydrogen sounds nice at first, but is associated with large energy losses (approx. 40% overall efficiency). In addition, it is an extremely volatile gas that can only be kept in the system with great effort. Despite everything, I support this decision.

However, in order for hydrogen to really become a mainstay of the energy transition, we have to make significant progress in expanding renewable energies. We're talking about ten times the annual increase that we're seeing today. That's why we can no longer afford to have visual reservations about wind turbines in Bavaria. For the energy transition to succeed, we need a large-scale expansion of wind power and solar energy.

Is solar energy or a PV system the right solution for a tiny house? Why?

I am also convinced that photovoltaics is a good technical solution for tiny houses. Approximately 14 solar modules fit on your roof. You can get through the day well even in winter. With the excess electricity in summer, additional electrical consumers, possibly even a small electric car, can be charged.

A PV system is a good technical solution that, depending on the region, can even ensure self-sufficient living.

How big does my PV system have to be designed so that a tiny house with approx. 30 square meters and two residents can be supplied independently at any time?

If the hot water requirement is to be covered by the photovoltaic system, I would exhaust the available roof area as much as possible. With a heat pump, the water can be heated efficiently using solar power. Since there is no connection to the power grid with a self-sufficient supply, a small battery storage system must of course also be installed. The orientation of the modules depends on the roof area, since the modules have to be firmly attached to the roof.

In principle, the roof area of your tiny house is very well suited for solar power generation. If you live in the property all the time in winter, you may have to sweep the snow off the modules from time to time. Of course, this does not slip off the roof by itself thanks to the module installation parallel to the roof. In this way, good self-sufficiency can also be achieved in winter.

Are PV systems stable enough to withstand any weather situation in Germany?

Yes. For large systems, the operators take out cheap insurance, e.g. against hail damage. In principle, solar modules from every manufacturer are sold globally and therefore have to withstand all kinds of weather extremes. Wind suction forces of 200 kg/sqm are no problem for solar modules. That is an enormous value. The glass and frame are also extremely dimensionally stable. For example, I once dropped a hammer from a meter on the construction site with the blunt side onto a brand new solar module. That didn't bother the glass surface. But that should not be an invitation to imitate.

How long does a PV system like this last? Can you simply replace the modules afterwards, or is it advisable to replace the entire system? What about the recycling of solar modules?

Modern solar modules have a life expectancy of around 25 - 30 years. Of course, the efficiency of the modules decreases slightly for each year of operation (approx. 0.2% p.a.). A broken module must be replaced with a module with the same cell voltage. For large systems, we therefore always store a few modules in the basement, which can be used in the event of a module defect. In small systems, however, a defective module can usually simply be removed from the line. The failure of a module is generally extremely rare.

The subject of recycling solar modules is only just picking up speed. I have already described the reason for this. Plants that were built around the turn of the millennium are still producing electricity today. Companies such as PV-Cycle or the Reiling group of companies have relatively high recycling rates. Glass and aluminum make up the largest part of the solar module. Together with the tinned copper cable, these components make up around 80% of the solar module. These raw materials are relatively easy to recycle. I'm rather skeptical about the remaining 20% - rear wall foil, cable insulation, connectors, silicone, silicon, etc. After several decades of work, this proportion would have deserved a place of honor on the hazardous waste.

Personally, I find the discussion about the recycling of solar modules somewhat disproportionate. Especially when you consider the garbage that you produce in everyday life (waste paper, plastic waste, disposable batteries, a new mobile phone every three years, car, etc.).

One of Rubens' major projects over the roofs of Munich

What can the German government do even better when it comes to solar energy? What problems are currently slowing down the expansion of solar energy?

When it comes to solar energy, the full potential must finally be used. In Germany we have a huge potential for apartment buildings that remains almost unused. The reason for this are regulatory requirements that lead to high investment costs. In addition, tenants still have to pay a kind of self-consumption tax on the electricity they generate themselves.

You could also build solar systems on private roofs yourself or are you already doing so? When are you going to start your own business or does it already exist?

I have already installed systems up to 10 kWp myself. Physically, however, you can hardly keep up your job until you retire. That's why I decided in 2019 against starting my own company in the solar sector. For this I am on the verge of founding the company elsewhere. The topic is at least as exciting.

Dear Ruben, thank you for taking the time to answer these important questions in such detail. We wish you all the best for the future and always hold the hammer tight!