Mains grid power demand is especially high during the day, so the system makes sense. Grid-connected systems convert their entire output into 230V alternating current; you don’t need low-voltage installations and normal current consumers can be used. However, there is absolutely no point in using e.g. an electric boiler with a solar cell array; a solar thermal system is much cheaper and more suitable and for this reason precedence should be given to thermal use of solar energy.
The standard for grid-connected systems in single-family homes is currently the 3-kWp system (kWp means "Kilowatt peak" and describes the maximum output of the modules under standardised conditions). Depending on cell type, there is plenty of room for its 20-30 m² area on a south-facing roof and this will largely cover the power use of a thrifty household. For an installed nominal value of 1 kWp, about 840 kWh of annual solar yield can be expected in Bavaria. In fog-free or Alpine areas, this amount can rise considerably at times, as the system on the Jungfraujoch has proved. For this reason, larger grid-connected PV systems in the Alps may one day be of interest to the electricity industry. A lot of energy companies once had problems with buying back stored energy, but the providers are now more and more prepared to pay an appropriate price for valuable solar energy, sometimes even more than the usual top tariff. At a feed payment rate of 28.74 cents per kWh with state subsidy, a photovoltaic system is an interesting investment from an economic perspective.
The cross-section of the house in the illustration shows the operating principles of a photovoltaic system: as the sun’s rays strike the photovoltaic modules, a physical reaction is caused with the result that direct current is generated. This home-produced electricity is measured and converted into mains-compliant alternating current using an inverter. This clean energy can be fed into the public mains grid using a feed meter, and under the EEG (renewable energy regulations), the system owner will receive a state-guaranteed feed payment for this. As a producer of solar energy, you will receive a constant payment for 20 years (plus the rest of the month in the year in which your system goes live) at the rate applicable in the year you install the system. The consumption meter measures the amount of power used that can be drawn from the energy provider and the feed payment for home-produced power thus exceeds the cost of the power drawn from the energy provider. For solar energy systems up to 500 kWp installed after 1 July 2010, the regulator is also offering operators of photovoltaic systems the opportunity of using their home-produced electricity themselves, either wholly or in part. Personal use of solar power is measured using a personal consumption meter and paid for at a specially calculated payment rate that differs from the payment for feeding power into the mains grid. For every personally used kilowatt-hour system operators receive a payment rate that is set for personal use for 20 years (including the remainder of the month in the year in which the system begins operation). Household electricity costs, which relevant forecasts predict will consistently rise in the years to come, are also saved. Adding up the payment for personal use and the cost savings on domestic electricity clearly shows that personal use of the solar power you generate yourself is subsidised even more than supplying power to the mains grid, as the sum arising exceeds the rate for supplying the mains grid. Personal use is thus especially interesting for system operators who have the option of using power from their own photovoltaic system on-site. Under current legal conditions, any excess power generated can still be supplied to the mains grid at the regular payment rate and additional power requirements can be met by taking electricity from the mains grid at the usual price. There is thus nothing standing in the way of a nice secure profit.
In Switzerland, most of the independent systems are to be found in remote houses or Swiss Mountaineering Club huts, and a solar array is often the most economical method of electricity production. The most important element of such a system is a current store for nights or days of bad weather. As the solar cells deliver direct current, devices drawing current are run directly at a battery voltage of 12 or 24V, making an inverter redundant. Such low operating voltages established themselves during the pioneering days of solar energy and were taken from the DC wiring looms of cars and trucks. From a modern perspective, this choice of voltage seems a little unfortunate as it causes currents that are ten or twenty times the rating of standard domestic installations, requiring expensive cables with large cross-sections.
Top-quality lead batteries are used to store electricity. A charging regulator is installed upstream to ensure these are never overcharged or completely discharged, which can have negative consequences for their already short working life. The charging regulator constantly monitors the charge level of the batteries. A Maximum Power Tracker, which consistently maintains the solar cells in perfect operating conditions Pmax (see above) is a suitable device to increase the performance of the system. Performance can thus be increased by some 10%. The module’s lightning protection should also not be forgotten and is mostly installed at raised points (roof, façade or mast); the valuable crystals would otherwise be at considerable risk from bad weather.
Devices with the most economic current draw possible should be considered. The range of low-voltage devices such as fridges, televisions, motors or even washing machines is unfortunately rather limited and these devices are also expensive. There are no such problems with lamps or the less sophisticated kinds of radios, however, although energy-hungry devices such as electric cookers, boilers and fan heaters should be avoided at all costs with such systems. These are in any case mostly small installations where power has to be consumed extremely economically.
Photovoltaik staatl. Förderung:
Im Februar beträgt die Solar-Stromvergütung 16,64 ct pro kWh. Dieser Betrag wird für Installationen ab März weiter reduziert.