We had considered it already a few years ago – when we decided to live and work in the middle of a dusty and noisy construction site for a few months:
The upper part of the roof is inclined by 30° – which is the optimum angle for photovoltaic panels – whereas the windows in the steeper roof surface works as ‘solarthermal collectors’ in winter.
But we had no photovoltaic modules installed as the scope of our project had already been extended from The roof should be repaired! to Replace the roof truss, replace the gas boiler, add insulation … add a second ‘open space office’ storey to the house!
The price an end-user paid for a turnkey PV system was about 3 times higher than today (German source). Prices of PV systems became lower everywhere but systems are still more expensive in the US.
But today an Austrian home owner only pays about € 2.000 per kWpeak (rated power) for a turn-key system, including photovoltaic modules, supporting construction, inverter, installation, and paperwork with utilities.
A ‘small’ 5 kW PV generator yields about 5.000 kWh per year. The system is more economical if you consume as much energy as possible in your home as you pay much more per kWh (~ € 0.19) than the utility pays you for energy fed into the grid (~ € 0,06 / kWh). A typical Austrian home needs about 3.500 kWh electrical energy per year – heating not included. We use about 7.300 kWh because of the heat pump; we believe that we will be able to use more than 50% of the power generated.
The payback period of the investment will be longer than 10 years, but I’d rather compare yearly profits with other ‘save’ investments: If we use 3.500 kWh of our solar energy, we would save € 665. Adding € 90 for sales of the remaining 1.500 kWh results in + € 755 / year – equivalent to 7,6% of the investment costs. Running costs are typically estimated to be 1-2% of the investment costs; so yearly profits are still more than 5%.
It is considered unlikely that prices of modules will plummet even more, so we decided we finally do it this spring!
We chose these black modules, also for aesthetic reasons:
Installation started exactly at the day of the grid-threatening solar eclipse. Our unusual – and high! – mansard roof was the first challenge, to be met with an ad-hoc innovation:
On the upper roof surface there was not too much space to walk besides the modules. We started crafting theories about those guys being super-human life-forms, equipped with spider-man-style bionic gloves and shoes, genetically engineered for that type of work – like the special agents in the Bourne movies.
Then the sun was about to set, and they were still working …
… and they still kept working until all modules had been installed – long after sunset! They worked with lamps on the roof! Or their eyes had been enhanced with super-sensitive camera implants.
The image also shows that the modules are not visible from the ground. But the Chief Engineer can enjoy watching the modules from his office desk:
So we will use the sun’s energy in two different ways now:
- Generating electricity with our new panels, to power the heat pump – among other appliances. 25% of heating energy is electrical energy.
- Harvesting energy from the ambient air via convection with the unglazed solar thermal collector. 75% of heating energy stems from this ‘ambient’ energy from the heat source, the combination of solar collector and water tank.
elkemental Force 
I’m quite envious of your setup Elke. Is there any storage capacity or are you either using the electricity or selling it ‘down the line’?
We have considered to buy a battery, too, but those are far from economical today (they would most likely reach end of life before payback time). Actually, we mainly profit from using the power directly in our house – we will only sell a (hopefully) minor fraction of it.
Costs per kWh we buy from the utility (€ 0,19) are three times what they would pay for our PV energy (€ 0,06/kWh) – so the profits in my calculation are mainly from the savings … power we don’t have to buy. Since the heat pump and the office constitute a continuous and rather high base load, we are confident we can use a lot of our power. In addition, we are going to play with our control system and try to align power generation and usage of power better.
A battery would help us to use even more, but since we can use so much without it anyway, the benefits would be so small, making a battery even less economical. But we have picked an inverter that should allow for a rather easy upgrade / replacement in the future, when storage systems get cheaper. Companies like Tesla enter the market for batteries for homes, so perhaps this will trigger some innovations.
I feel your excitement! I didn’t realize that when you described the roof on your home that it was something you added through renovating. I’d love to have the Chief Engineer’s desk by the window. Can you imagine the poetry that would be born from that view? Maybe that’s why I’m not that much of a poet… no such view! :)
Indeed – this view is poetic! In the fields behind the garden you can sometimes see a herd of deer :-)
Yes, the ‘roof project’ was rather an addition of another half of a building – quite an adventure (It’s good you don’t know all that in detail before!). I remember looking at weather forecasts in fear. It had not rained for a full month before The Day of Destruction and chances for a dry roof rebuild became smaller and smaller. Amazingly, we were lucky and it hardly rained for about 2 more weeks until the new roof was water-tight. The carpenters mounted a huge heavy plastic sheet to the unfinished second floor every evening, like a circus tent.
Hi there Elke, much to think about here. Here’s a synopsis:
1-That roof of VERY steep. I doubt even gunpoint would get me up there :-)
2-I’m impressed 5kW on average is a great return. I’m not sure, though, that you’d get anything like that where i am owing to the excessive cloud cover and fog. Worth an experimental try though.
3-Again I’m impressed by the pay back into the grid being so widespread. In NL it’s virtually unknown as very few people use it owing to the factor I just mentioned.
Overall, I am very impressed with what you have, and more than a little envious. As far as I am concerned you and Siggi have built what should be a model of sustainability for th whole world.
1000 kWh / 1kW is the rule of thumb here, maybe more. Our roofs are oriented south-west and south-east, so yields might be a bit lower – but more evenly distributed over the day which helps optimizing the percentage of power we can use ourselves.
Feeding in to the grid had only been lucrative a few years ago when tariffs had been subsidized. Now you only get the stock exchange price of power which is less than you pay yourself – so you have to use most of the power yourself. Our profit is equal to the power we don’t have to pay for. This has triggered some innovations by vendors, as intelligent energy management / load shedding systems (as evaluating weather forecasts, as discussed on the previous post).
Our collector for the heat pump is a bit exotic, but many houses here use PV panels or solar thermal collectors for hot water (or both). Many governmental benefits for home builders have been tied to installing some sort of green energy in recent years. France just passed a law, making PV or ‘green roofs’ mandatory on commercial buildings – I think green energy for new buildings will also be mandatory here soon.
One more though on comparing PV in North America and Europe: Costs of building a PV system are twice as high in the US as I learned from the linked article, but power is twice as cheap ($ 0,10).
If you only profit from using your own solar power (as we do now), the rate of return is just one fourth – and thus perhaps zero as profits would be equal to the (low) maintenance costs.
In Germany, ‘Europe’s PV poster country’, costs of electricity are even higher than in Austria, about € 0,30 / kWh. This makes PV even more attractive (but there are experts who say that power is so expensive because of the focus on PV).
Congratulations!
Thanks :-)
Seriously, you guys are an inspiration! I love it.
Thanks a lot for the kind comment!!