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An east-west roof produces less than a south-facing roof with the same power, but it spreads production over more hours: more in the morning and evening, less at midday. We calculated what this means for self-consumption and for the battery with PVGIS hourly series for Kassel [4] and the BDEW standard load profile [1].

Production on the two roofs

With PVGIS data for the year 2005, 1 kWp produces 981 kWh in a year with modules facing south tilted at 35°, and 811 kWh with half the modules facing east and half facing west tilted at 15° [4]. On this east-west roof, production is therefore about 17% lower.

Same house, 6 kWp

A house using 4,500 kWh in a year, 6 kWp:

Without a battery, east-west covers almost the same share of consumption while producing less: more of its production falls in the hours when the house is using energy. With a battery, south catches up, because it has more surplus to shift to the evening.

Same house, 10 kWp

With a system that is large compared with consumption, self-sufficiency without a battery is the same for both orientations, and the east-west curve flattens sooner.

Only the shape of the day

To separate the effect of the shape from that of the quantity, we redid the 6 kWp case giving east-west the same yield per kWp as south, 981 kWh/kWp: self-sufficiency without a battery is 39% against 37% for south, and with 5 kWh it is 64% for both. The shape of the day matters mostly without a battery; with a battery, how much energy is produced matters more.

Try it with your data

The calculator uses the same two shapes and your yield per kWp: a south-east or south-west roof lies between the two, and the closer of the two gives an indication, not a measurement. Different tilts and shading change the yield per kWp, which PVGIS estimates for your address [4].