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Photovoltaics

In energy planning photovoltaics is rarely a question of annual yield but of simultaneity: what matters is how much of the generated electricity is used on site. EPOS-Plan therefore calculates the electricity side in quarter-hourly steps and compares generation and demand at the same instant.

Yield model

The yield follows from the irradiance on the tilted module plane, the module rating and the temperature-dependent efficiency. From solar position, tilt and azimuth the angle of incidence is determined for every hour and from it the irradiance on the module surface; irradiance and ambient temperature give the cell temperature, which reduces efficiency. Typical losses are 0.3 to 0.4 per cent per kelvin above the reference temperature, which is why yields on hot summer days are lower than proportional. Inverter losses are taken into account.

Orientation, tilt and sub-arrays

Orientation and tilt shift not only the annual total but above all the daily distribution. A south-facing array at 30 to 35 degrees delivers the highest annual yield, an east-west arrangement a wider generation window with peaks in the morning and afternoon – often the higher self-consumption at the same demand. A shallower tilt favours summer, a steeper one winter and the transitional season. Several sub-arrays with different orientations can be created separately and are added in the balance, so comparing south against east-west is straightforward.

Self-consumption, self-sufficiency and storage

The balance follows the usual order: generated electricity first covers simultaneous demand, then charges the battery and only then is exported as surplus. If generation is insufficient, the battery is discharged first and the remainder is drawn from the grid. This yields two key figures that react in opposite directions: the self-consumption ratio states which share of generation is used on site and falls as the system grows. The self-sufficiency states which share of demand is covered by own generation and rises with system size. Economically the self-consumption ratio matters most, because electricity used on site replaces the full purchase price while exports are remunerated considerably lower.

Interaction with the heat side

The most effective lever for more self-consumption is often not the battery but the heat pump: in PV-optimised operation it runs preferentially at midday and uses buffer tank and building mass as thermal storage. Because thermal storage capacity is far cheaper than electrical, it is worth calculating this variant before enlarging the battery. An immersion heater or electric boiler can also serve as a surplus consumer – albeit with the poorest efficiency of all options.

Sizing in practice

Start with the load profile: if a measured year in quarter-hourly values is available, any further model refinement is secondary. Then calculate the pure PV system in two or three sizes and read off self-consumption and surplus; only afterwards add storage and heat pump operation. A look at a single summer day in the electricity chart is more illustrative than any annual figure. Shading by neighbouring buildings, trees or roof structures is not captured automatically and has to be accounted for through a reduced module count or a deduction.