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Battery storage
A battery shifts electricity over hours, not weeks. It raises the self-consumption of a photovoltaic system by carrying the midday peak into the evening – and reaches its limit exactly where generation is insufficient for days in winter. This role determines both sizing and assessment.
Characteristics
A battery is described by its usable capacity in kilowatt hours, the charge and discharge power in kilowatts, the state-of-charge limits and the degradation, that is the loss of capacity over its service life. Usable and nominal capacity differ because the depth of discharge is limited to protect the cells. Power determines whether a short generation peak can be absorbed at all; in small systems it is rarely limiting, with commercial load peaks it certainly is.
Charging and discharging strategy
In the simulation the battery follows the usual self-consumption strategy: it is charged from surplus generation once the current demand is covered, and discharged as soon as generation is no longer sufficient. Free capacity, state-of-charge limits and power act as constraints. The state of charge can be displayed as a separate curve in the electricity chart – the clearest check of whether a battery is too small, adequate or oversized: if it stays full for days in summer it is too large; if it never charges fully it is too large for the array; if it cycles daily between the limits, it fits.
Sizing
As a guide, a usable capacity in the order of the average daily consumption, in residential buildings often about one kilowatt hour per kilowatt of installed PV capacity. These rules of thumb do not replace the calculation, because the benefit depends strongly on the load profile: a commercial operation with high daytime consumption barely needs storage, a household with evening load considerably more. Calculate three or four sizes and watch when the gain in self-consumption flattens – this knee marks the economic limit far more reliably than any formula.
Alternatives and interaction
Before enlarging the battery it is worth comparing thermal storage: a heat pump in PV-optimised operation shifts heat generation into the midday hours and uses the buffer tank – thermal capacity costs a fraction per kilowatt hour compared with electrical. An east-west arrangement of the modules also raises self-consumption without additional storage cost. Only when these levers are exhausted does the battery become the best option.
Modelling in EPOS-Plan
Batteries are taken from the catalogue or created manually and assigned to the project; in the simulation configuration they take part in the electricity system. The evaluation reports self-consumption ratio, self-sufficiency, the share of battery discharge in covering demand and the remaining feed-in – month by month in the self-sufficiency analysis and as a time series in the electricity chart. All values can be exported as CSV files to compare several storage sizes in table form.