Photovoltaics

How two figures become a system

The visitor enters the roof area and their consumption. Everything else is calculated — a chain of formulas building on one another:

• What fits on the roof? The usable area divided by 1.8 m² per panel. On a flat roof only about half counts: racked rows need spacing, otherwise they shade each other.Area × coverage factor ÷ 1.8 m²
• What does the household need? Consumption plus wallbox plus heat pump. As a rule of thumb, about 1 kWp covers 1,000 kWh of annual use.(Consumption + 2,500 + 3,500) ÷ 1,000 ÷ 0.44 kWp
• The recommendation is the smaller of the two. A roof that offers more than is needed does not get filled — unless you want it to.min(roof, demand)
• The yield follows from system output, orientation and pitch. 950 kWh per kWp is the usual average for Germany.kWp × 950 × orientation × pitch
• Self-sufficiency is capped by the battery, not by annual consumption. In December even twice the system helps little if the electricity cannot make it through the night.min(battery cap, yield × self-consumption share ÷ consumption)
• The payback period from total investment and annual savings — electricity used yourself saves 35 ct, electricity fed in earns 8 ct.investment ÷ (self-consumption × 0.35 + feed-in × 0.08)

Three things to try

• Set the roof shape to flat roof. Pitch and orientation disappear — with racked panels the mounting fixes both, so the question would be pointless. At the same time the panel count drops, because the rows need spacing.
• Heat pump to "yes". The demand jumps by 3,500 kWh and the recommendation grows with it — until the roof sets the limit. That is exactly where you see whether the area is enough.
• Choose "fill the roof". Yield and savings rise, but so does the payback period: the extra electricity is fed in and earns 8 instead of 35 cents. That is the honest answer to "just fill the roof".