Photovoltaics basics: rooftop system, self-consumption and storage

In short: A photovoltaic (PV) system converts sunlight into electricity. On a house roof it covers part of your own demand, the rest goes into the grid for a payment.

In more detail: Solar cells produce direct current. An inverter turns it into grid-standard alternating current. Output is stated in kWp (“kilowatt peak”, peak power under standard conditions).

In Depth

• Yield: In Germany 1 kWp with good orientation delivers about 900 to 1000 kWh a year. A single-family-house roof with 8 to 10 kWp therefore produces several thousand kWh.
• Self-consumption: Directly used solar power saves the full electricity price and is usually worth considerably more than the feed-in tariff. Running the washing machine, dishwasher or heat pump in the daytime raises the share.
• Storage: A battery raises self-consumption (typically from about 30 to 60–70 %). It does not always pay off; economics depend on price, consumption and lifetime (charge cycles).
• Feed-in tariff: For power delivered to the grid there is a fixed rate per kWh for 20 years, which falls regularly for new systems. Choice between surplus feed-in and full feed-in.
• Tax: For small systems a zero VAT rate applies to the purchase, and the income is exempt from income tax (mind the limits).
• Obligations: Registration in the Marktstammdatenregister and with the grid operator, meter (often a bidirectional meter), check fire protection and roof statics.
• Maintenance: Little. Modules often last 25 years and more, the inverter about 10 to 15 years. Monitoring shows performance drops.
• Planning: Clarify roof area, orientation, shading and grid connection with a specialist firm. Compare several quotes.

See also: balcony solar, heat pump