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Photovoltaics • Storage • kWh

Calculate Storage Size

Calculate a rough size for a PV battery storage in kWh.

Calculation Values

Note: This calculator provides a technical estimate for guidance only. For electrical installations, standards, protective measures, installation method, ambient temperature, grouping, protective devices and manufacturer specifications must also be checked.

Formula Used

Storage size = daily consumption × night share × autonomy days ÷ usable share

This calculation provides a rough sizing for the usable energy demand.

Frequently Asked Questions

Is a bigger storage always better?

No. Oversized storage increases costs and may not be fully utilized.

What does usable storage share mean?

Battery storage is usually not fully discharged. The usable share accounts for this limitation.

Do I need my load profile?

For precise storage planning, the load profile is very helpful because consumption and PV generation need to align over time.

Practical guidance

Size a PV battery around shiftable consumption

Battery size should follow evening and night demand that can actually be recharged from daytime PV, not annual consumption alone.

Input values explained

Daily consumption
Prefer an average based on measured data. Dividing annual use by 365 is only a rough estimate.
Evening and night share
Estimate the consumption outside PV generation hours that could in principle be supplied by a battery.
Backup duration
The number of days sets the desired energy reserve. Multi-day full autonomy is rarely the economic target for a grid-connected PV system.
Usable battery share
Only part of gross capacity should be cycled routinely. Use the manufacturer's usable-capacity or permitted depth-of-discharge figure.

Example: 10 kWh daily consumption

Half of 10 kWh is used outside PV hours, one day of coverage is wanted and 90% of gross capacity is usable.

10 kWh × 0.50 × 1 ÷ 0.90 ≈ 5.56 kWh gross capacity

About 5.6 kWh is indicated. A larger battery adds little if the PV array cannot refill it on typical days.

Capacity is not power

kWh is stored energy; simultaneous operation of a hob, heat pump or charger also depends on charge and discharge power in kW.

Calculation limits

  • Efficiency, degradation, reserve and seasonal generation are simplified or excluded.
  • Economics also require cycle count, investment cost and energy-price assumptions.