# Photovoltaics - Ratio Storage-Power

Calculator for the ratio of the capacity of an energy storage and the nominal power of a photovoltaic system. Storage capacity is the maximum energy content of a battery. This is often given in kilowatt hours (kWh), the symbol for energy is E. The nominal power of a photovoltaic system usually is given in kilowatts (kW), the symbol for power is P. The unit of the ratio is hours. So the ratio tells, how long at maximal power the battery takes to load from totally empty to totally full, or for how many hours electricity can be stored.

The ratio is a theoretical value for how quickly the memory could be loaded under ideal conditions. The charging time in practice is always longer because ideal conditions never last long, even if they are achieved. Please enter two values, the third value will be calculated. r=E/P

Example: a battery with the storage capacity of 1.5 MWh is connected to a photovoltaic system with a nominal power of 20 kW. Then it can store for 75 hours at peak power. The sun doesn't shine for 75 hours continuously, so the real duration is higher. Nonetheless, the ratio is given as 75. Such a ratio is not necessarily realistic, it only serves to illustrate how the calculation is done.

A storage device as described above would be far too large and expensive for private use. For domestic systems, a ratio of 1 to 1.5 is usually recommended; for very small systems the ratio can be somewhat higher. This is the situation in 2024, where storage is still expensive, more expensive than photovoltaic modules. If storage becomes or has become cheaper afterwards, a higher ratio may also be worthwhile.

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