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!
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.