Rooftop photovoltaic system are becoming more and more popular. Good! But when planning such a project for your house, there are some things you should consider to achieve good results. This text isn’t intended to give you an exact cookbook. Every project is different and there’s no “one size fits all” approach. Instead this text is intended to give you an overview and recommendations to make good decisions for your specific project.
In this text I mostly write about a photovoltaic system on the roof of residential buildings. There are other options, of course. You might use your garage, backyard or whatever, but in this text I’ll focus on the most common case.
Photovoltaic vs. solar thermal.
There are two ways to use the sun’s radiation in your house: a solar thermal system to heat up water or a photovoltaic system to generate electricity.
Solar thermal systems are proven technology - in fact US-president Jimmy Carter installed one on the Whitehouse in 1979. These systems heat up water. It can be used as warm water for shower, etc. or to support the building’s heating, often combined with a short term heat storage.
Besides being a robust and proven carbon-free technology, it has some disadvantages. The main disadvantage is that it only produces heat, which is not as versatile as electricity and there’s no practical way to feed it into any kind of grid. But during summer, a significant surplus is inevitable, which will be mostly wasted, except if there’s a specific use for it, e.g. if you have a swimming pool.
A key challenge for all solar systems is the capability to store surplus energy from the summer to be used in the winter. Thermal long term storage technologies exist, but they are not very widespread and often not possible due to constructional conditions.
The other and most most common form of using solar radiation is photovoltaic.
Photovoltaic systems generate electricity, the most versatile form of energy. It can be used in your house for own consumption or it can be fed into the public grid for which the power utilities pay a compensation. My house does both: own consumption as much as possible and the surplus is being fed to the grid, which provides some extra revenue to pay off the investment.
A photovoltaic only installation also comes at a cost advantage: you need to pay installation costs only once. Therefore larger systems come at a lower price per kWh.
Recommendation: For new installations photovoltaic-only is the recommended choice. Combine it with a heat pump and electric car, if possible. If you have a well working solar thermal system already, keep it. It can be integrated with a heat pump as well. Only if you have a high demand of heat, e.g. a pool, a new solar thermal system might be worth considering.
In this text I’ll focus mostly on photovoltaic systems going forward.
On-roof vs. in-roof.
Basically there are two options to install a photovoltaic system on your roof: in-roof systems basically replace roof tiles or are tiles themselves, which replace the existing tiles or are being added on top of them.
In-roof photovoltaic systems replace the tiles and therefore could potentially save money if you plan a completely new house or a new roof. You save the money for the classic roof tiles.
They are also often described as more aesthetic, but that’s a matter of taste. Watching from below, there’s not a huge difference. Sometimes preservation orders by the authorities might make it the preferred choice for old houses.
In-roof systems come at a disadvantage, however. Modules get hot when exposed to the sun. The ventilation behind in-roof systems is significantly worse than on-roof systems, resulting in higher temperatures, lower efficiency and less power output.
On-roof systems are installed on top of the existing tiles and leave the existing roof mostly untouched. An assembly system is attached to the roof tiles to hold the modules.
On-roof systems are the common choice for existing houses. They leave the existing roof mostly intact. Installation is rather simple with a variety of proven assembly systems on the market. They also have a much better air circulation behind the modules caused by the chimney effect (hot air rises to the top and causes a cooling air stream behind the modules).
In areas with snowfall and a rather flat roof, there’s a risk however. The resulting forces of a massive snow cover presses onto certain roof tiles, where the hooks are installed. This bears the risk of breaking these tiles. Therefore these tiles can be replaced by metal tiles, which can withstand much higher forces.
Recommendation: If your roof is good and doesn’t need a replacement, choose an on-roof system. It’s the most common and cheapest choice. Proven, more efficient and rather simple to install. If major snowfalls are a possibility in your area, choose metal tiles. If you plan a whole new house or a new roof, you might want to look into an in-roof system, but compare prices carefully to a standard roof with an off-the-shelf on-roof assembly system.
Size of photovoltaic system.
Start with a simple rule: as big as possible! Fill up the space you have.
However, there might be cases where dormers or other structures are in the way. Often these spaces can be used too, but that might not be simple and come at an extra price. Ask installers for offers, look carefully in their offers. It highly depends on the structural conditions.
Dormers might also cause a different direction of the modules, different exposure to the sun, which might require the use of additional MPP trackers or module optimizers. See the chapter below, discussing this matter.
So, no clear recommendation here. It depends on your house.
High performance vs. value-for-money modules.
Some photovoltaic modules have a higher power output than others. Due to their higher efficiency, they’ll harvest more energy in the same given area. But they come at a higher price. So, is it worth it?
There’s no simple answer, because it depends on your situation. In most cases residential photovoltaic systems are mostly paid off by own consumption, even more as prices for electricity are rising. Other considerations come into play: Do you use a heat pump? Drive an electric car? Other major consumers of electricity?
It also depends on the available space on your roof. A big roof obviously can harvest more energy, but a huge surplus in summer won’t justify the price of high performance modules. You won’t get much more money for feeding the extra surplus into the grid.
Therefore there’s no clear recommendation here, just a rule of thumb:
You have a relatively high consumption and/or a small roof? —> high performance modules might pay off through own consumption.
You have a large roof? —> rather solve the problem with mass, modules with a high value-for money might be the better choice from a financial perspective.
Glas-Glas vs. Glas-Foil modules.
The most common glass-foil modules are being installed on the back side of a glass layer and covered by a foil. The glass on the front protects the actual photovoltaic cells from mechanical stresses.
Some modules use a glass-glass structure, which means the actual photovoltaic cells are embedded between two glasses. They are more stable than glass-foil modules and can absorb higher mechanical stresses. Therefore they have a longer life expectancy and a higher probability to withstand extreme conditions like hail.
However, glass-glass modules come at a higher price and given the progress of Phtovoltaic technology, it’s not guaranteed they’ll be longer on your roof than glass-foil. In 20 years you might want to replace them with something new and more efficient anyway.
Regarding the robustness against weather impacts, glass-glass is definitely better than glass-foil. But there’s an alternative to prevent you from a financial loss: insurance! Your homeowners insurance might already cover it or you can easily buy extra insurance to cover it. See section below for some more recommendations.
In some of the German states (Bundesländer) different distance rules for glas-glas and glas-foil might apply if installed on semi-detached and terraced houses. Glas-glas might lead to less safety distance required, providing more space to fill with modules. Double-check what the rules actually are. Installers have an incentive to sell you the pricy glas-glas modules.
Recommendation: if you prefer a long life and robust modules, glass-glass might be an option for you. But compare the price of glass-glass modules carefully with the cost of an appropriate insurance. The insurance might turn out to be the cheaper option and is highly recommended in any case. Check which rules apply for distances to a neighboring house.
Snow Guards.
Snow guards are a structure on your roof to reduce or prevent snow avalanches coming from your roof.
In combination with a photovoltaic system, they can be a problem. They occupy space, which you might prefer to use for a row more of solar modules.
Also avalanches are to some degree desirable for photovoltaic systems: the sooner the snow slides down, the sooner your modules are clear again and can harvest energy. A snow guard might slow down this process and keep the snow longer on the modules.
Unfortunately avalanches can be dangerous and if they threaten to hit public ground, e.g. if your house is directly at a public walkway, the house owner might be liable for harm being done by them.
Recommendation: Examine carefully if avalanches could do harm when coming down your roof. Only if you can safely deal with them without the risk of anyone being hurt or any damage done, remove the snow guards, use the extra space for more modules and let the avalanches clear your modules.
Module optimizers.
Let’s first look into some theory what a module optimizer is.
A photovoltaic module has its maximum power at a certain combination of electric current and voltage. It’s called the “Maximum Power Point”. Several modules are being connected as a series circuit. This so called “string” shares the same Maximum Power Point. A/C converters usually offer two or more inputs which optimize the MPP for the attached string and are called "MPP trackers”. Each MPP tracker is being optimized individually.
In some cases a building structures or tree might cause shades on some modules of a string, e.g. a chimney. This will reduce the whole string’s performance. Module optimizers could come into play here. They optimize the MPP for every single module in a string and therefore can prevent a whole string temporary loosing some performance due to small shades on single modules.
The actual benefit of module optimizers might be small, if any. Installers like to sell them anyway to make extra money. They can also fail and will increase the overall failure rate of your system.
Recommendation: In most cases module optimizers are not necessary and offer little to no advantage. Only in special cases with different shades and module directions, they might offer some advantage. If in doubt, don’t install them.
Battery storage.
Battery systems provide a short term storage capacity for your photovoltaic system. Their main use case is to be charged during daylight hours and to provide electricity during the night. By doing so, they will increase the amount of self used energy significantly.
The following graph shows the state of charge of my battery system on a day in September 2022. The battery got charged starting early in the morning to late afternoon, while energy output surpassed energy consumption. Later that day the consumption was higher than the production and the battery got discharged. No power from the grid needed during this 24hr period.
In several internet forums there have been long debates regarding the financial feasibility of batteries. As the energy prices in Germany and other countries are really high meanwhile, this debate is settled now.
Recommendation: Home storage batteries will improve the percentage of self-used energy from your roof. Given the current energy prices, they’ll be worth the investment in most cases.
Type of battery storage.
The output of photovoltaic modules and batteries is always direct current, while your house and the public grid uses alternating current. Therefore DC must be transformed into AC to use it.
This leaves two options to attach the battery to your house installation: AC-coupled or DC-coupled. See the following illustration for the difference.
AC-coupled batteries have been the standard for some time. Both PV modules and battery have a dedicated DC/AC converter. Energy harvested by the PV modules must be transformed into AC current and then again into DC current to charge the battery. Two converters cause higher losses and more costs for installation. They are more flexible in terms of the battery’s location, as it can be installed basically everywhere in your house, using the existing cable infrastructure.
More modern battery system use DC coupled batteries. They need only one DC/AC converter, while the PV modules can charge the battery without a DC/AC conversation in between. The disadvantage is the lack of flexibility regarding battery installation. A dedicated DC cable ins need to connect them. Luckily this won’t be a problem in most cases as DC/AC converter and battery are often installed in close proximity. Some systems offer a DC/AC converter completely integrated with the battery in one housing, which looks nice, is space-saving but less flexible.
Currently there are two types of battery chemistry on the market for home storage: lithium-ion and lithium iron phosphate. Lithium-ion has a higher energy density, resulting in smaller batteries. Lithium iron phosphate is more robust, but lower energy density, resulting in bigger batteries. As storage space and weight is not a big issue in most people’s basement, lithium iron phosphate is a good choice for home storage, but the differences in practice aren’t big.
There have been reports about fires cause by lithium-ion. As far as I understood, this was due to a problematic production batch of one producer. Well designed lithium-ion batteries are a safe option and run reliably for years in thousand of installations.
There are rumors that the fire department will not fight the fire in a house with photovoltaics and battery. Don’t believe this. It’s nonsense.
Recommendation: DC-coupled batteries are the recommended option. Consider an AC-coupled battery only if you need the installation flexibility. Both lithium-ion and lithium iron phosphate batteries are fine if you have enough space for the latter one.
If new battery chemistries become available on the market, I might have to re-write this.
Size of battery.
That’s a hard question because most people don’t have an exact energy consumption profile of their house, just the annual consumption. If you have such a consumption profile, great! You might be able to do a more precise prognosis. Everyone else needs to work with an estimation.
As prices per kWh storage capacity are high, battery size is highly relevant. You don’t want to spend too much money on it as smaller batteries tend to have a higher grade of utilization. Let’s look at the following graph, depicting the utilization of my battery system with net storage capacity of 13,5 kWh.
The graph shows that only in 5 months within a 24 month period, the utilization was at or above 90%. In the majority of months the utilization was at or below 70%. In other words: a cheaper 10kwh battery would have performed equally in 19 out of 24 months.
Recommendation: As a rule of thumb, choose ~1.5 kwh storage capacity per family member without a heat pump, ~2 kwh per person with a heat pump.
If battery prices change, this will change too, of course.
Battery as a backup source.
Some photovoltaic / battery systems can run without a grid connected, but not everyone. It’s an extra option which requires extra hardware and installation efforts.
To use the battery as a backup, you will have to reserve some capacity which remains charged and will not be used during normal charge-discharge cycles. The battery needs to be bigger therefore.
The hardware, battery size and installation efforts will come at an extra price. It’s your decision if it’s worth it for you or not to improve your family’s resilience in cases of grid failure.
There are options to either supply all three phases in your house with backup power, or only one phase or even a simple solution to provide backup power to one socket only at the battery system. These options will have different price tags, of course.
Recommendation: Photovoltaic system with battery can be enabled to run in “island mode” as a backup for possible grid failures. Choose this, if you want to improve your family’s resilience and add 2-3 extra kWh storage capacity to your battery.
Do I need an insurance?
Yes.
There might be hail or a fire or other natural disasters. Home owners should have an insurance for their house, covering these risks. Some insurances cover damage to the photovoltaic / battery system too, some don’t. Check your insurance if and what it covers. In case it doesn’t cover the photovoltaic / battery system, there are insurances on the market which offer additional coverage at reasonable prices.
Recommendation: The photovoltaic / battery system is part of the house, might be damaged by natural disasters and you should insure it appropriately.
What about taxation in Germany?
It’s annoying bureaucracy.
Not anymore since 1.1.2023 thanks to a new law. You need to register as a “small business owner” (“Kleinunternehmer”) and you’ll be fine. No value added tax, no income tax on the profits. Simple. A good law.
Conclusion
Do it! Build a photovoltaic system on your roof! Find installers to offer you options.
Hopefully this text gives you some helpful recommendations to make good decisions once you have offers on the table. Don’t hesitate to contact me for questions. I’m happy to help.
Comments are welcome as always and I’m looking forward to interact with my readers.
With sunny regards,
Andreas
It is also recommended to include the PV in the liability insurance. This covers e.g. the case when a PV module is torn off the roof in a storm and third-party damage is caused. As a rule, this extension of liability insurance is very inexpensive.