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Amperecalc

Solar & storage

The rooftop array, fully costed.

A rooftop array is a 25-year investment. This calculator treats it like one: energy flows, revenue, cost and return — with battery, heat pump and electric car as optional loads.

How we calculate

Calculation model

  1. 01

    Yield is modelled as for balcony solar, but with loss assumptions for string inverters and longer DC runs.

  2. 02

    Self-consumption is simulated hourly; battery, heat pump and EV shift load into the generation window.

  3. 03

    Revenue combines avoided grid purchase with the statutory feed-in tariff.

  4. 04

    Economics are reported as net present value and internal rate of return over the service life.

Good to know

  • Systems up to 30 kWp on residential buildings are exempt from VAT on supply and installation (zero rate).
  • Income from systems up to 30 kWp is exempt from income tax.
  • The feed-in tariff is guaranteed for 20 years plus the year of commissioning.

Law & subsidies

What applies in Germany

Regulation determines the limits and tariffs this calculator works with.

Zero VAT and income tax exemption up to 30 kWp

Supply and installation of PV systems up to 30 kWp on residential buildings carry no VAT, and the income is exempt from income tax.

VAT
0 % up to 30 kWp
Income tax
exempt up to 30 kWp
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The zero rate covers modules, inverter, storage and installation. You pay the net price without needing to reclaim input tax — the old optimisation via standard taxation is obsolete.

The income tax exemption applies per taxpayer for up to 100 kWp in total and usually removes the need for a trade registration.

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.

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.

How large should the array be?

As a rule: as large as the usable roof area allows. Because the feed-in tariff sits above the marginal cost threshold and the cost per kWp falls with system size, a larger array is almost always more economic than one sized narrowly to self-consumption.

Is a battery worth it?

It depends on the price per kWh of capacity and on your consumption. Below roughly €600/kWh it becomes economic in most households. The storage calculator shows the threshold for your case.

What is the difference between self-consumption rate and self-sufficiency?

The self-consumption rate says what share of your generation stays in the house. Self-sufficiency says what share of your consumption comes from your own system. A large array has a low self-consumption rate but high self-sufficiency.

Battery calculator

The economically optimal capacity, not the largest one.

Payback calculator

Net present value, internal rate of return and break-even for any energy investment.

Heat pump calculator

Running cost, investment, subsidy and payback in a direct comparison.

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

See all calculators