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Amperecalc

Solar & storage

How much storage do you actually need?

Every additional kilowatt hour of storage delivers less than the one before it. This calculator draws the saturation curve for your system and marks the point where more capacity stops paying.

How we calculate

Calculation model

  1. 01

    For each month a typical daily profile of generation and load is built and simulated hour by hour.

  2. 02

    Surplus charges the battery up to its usable capacity, deficit discharges it — with round-trip efficiency and depth of discharge.

  3. 03

    The simulation is repeated for every capacity to trace the self-sufficiency curve.

  4. 04

    Economics follow from the storage cost per kilowatt hour cycled over the cycle life.

Good to know

  • Practical rule of thumb: about 1 kWh of usable capacity per 1,000 kWh of annual consumption, capped at the array size in kWp.
  • Usable capacity is not rated capacity — depending on the system there is 5 to 20 percent of reserve between them.
  • A battery raises self-sufficiency but lowers the return per euro invested compared with the array alone.

Law & subsidies

What applies in Germany

Regulation determines the limits and tariffs this calculator works with.

The feed-in tariff is guaranteed by statute

Surplus from systems up to 10 kWp earns a statutory rate, guaranteed for 20 years plus the year of commissioning.

Surplus up to 10 kWp
around 7.9 ct/kWh
Guarantee
20 years + commissioning year
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The rate falls every six months for new systems; once set, it stays fixed for the whole term. Full feed-in earns higher rates but rarely pays where self-consumption is high.

During extended periods of negative exchange prices, newer systems lose the tariff for those hours. So far this affects a low single-digit percentage of annual output.

Bidirectional charging: not banned, but still awkward

Feeding electricity from an electric car back into the house is not prohibited in Germany. What blocks it is certification, metering and the treatment of levies.

This is the most frequently misreported point in the field. It is not "forbidden" — it is regulatorily unfinished, and the position is moving quickly.

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A bidirectional wallbox exports into the house circuit and therefore counts as a generation unit. It must be certified to VDE-AR-N 4105 and registered with the grid operator. For a long time, certified vehicle and wallbox combinations simply did not exist.

The second obstacle is metering: vehicle-to-home needs a metering concept that separates drawn from returned electricity, otherwise the same kilowatt hour risks being charged grid fees, levies and taxes twice.

Third, § 14a of the Energy Industry Act applies: wallboxes above 4.2 kW are controllable consumers, which earns reduced grid fees but lets the operator throttle them.

For the calculators here that means we deliberately do not model vehicle-to-home as a revenue item. Anyone counting on it should get the certification of their specific vehicle and wallbox pairing confirmed in writing first.

This compilation is orientation, not legal advice. Rules in this field change several times a year — check the linked primary source before you invest, or ask a certified specialist.
See all regulatory topics

Frequent questions

Answered briefly.

Why doesn't self-sufficiency rise linearly with battery size?

A battery can only move one day's surplus into the night. Once it is large enough for the average night, extra capacity only helps on a few especially sunny days. The curve therefore saturates.

Can a battery make me fully self-sufficient?

Not on an annual basis. Winter demand cannot be covered by household-scale storage, because December yields roughly a tenth of June. Realistically, 55 to 80 percent self-sufficiency across the year.

What does a stored kilowatt hour cost me?

Storage price divided by cycles times usable capacity times efficiency. The calculator reports this figure directly — if it is below your electricity price, the battery pays.

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All results are model calculations based on your inputs and statistical reference data. They do not replace individual professional planning.

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