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Which inputs really determine the result

A result is only ever as good as the details behind it. Five things decide whether the final figure has anything to do with your house.

• Roof area, orientation and tilt. For a south-facing roof on the Swiss Plateau, the best tilt is 30 to 35 degrees, which is where most gable roofs already sit.
• Shading from trees, neighbouring buildings, chimneys and roof windows. No online calculator can see it, and it explains why two identical roofs on the same street pay off differently.
• Your electricity consumption and when it occurs. That is the biggest lever in the whole model.
• The electricity price charged by your grid operator and the payment it makes for the electricity you feed in.
• The price of the system itself, including scaffolding, electrical installation and connection.

Where our data comes from

The basis is Sonnendach, the federal solar cadastre. It knows the geometry and the solar irradiation for practically every Swiss roof. What it does not know is your roof in detail: the condition of the covering, the shading from the neighbour's tree, the position of the windows and the existing electrical installation.

That is why our calculator works with the values for your address and not with a Swiss average, and why an on-site visit still comes at the end. Between the result and the quote there is always a check by a person.

Why your consumption profile tips the balance

Every kilowatt hour you use yourself replaces electricity at the grid tariff. Every kilowatt hour that flows into the grid earns only the feed-in payment, which is several times lower. Whether a system pays off therefore depends less on the size of the roof than on how much electricity stays in the house.

A heat pump, an electric car or a battery moves that share up considerably. If you are planning something like that, say so in the calculator, otherwise it works out the house you have today and not the house you will have in a few years.

How to spot a calculator you cannot trust

Some calculators are there mainly to collect addresses. A few patterns give them away.

• The result only appears once you have entered your name, phone number and email address.
• Only a single figure is shown, without the assumptions behind it.
• Payback periods of just a few years, without consumption and tariff ever being asked for.
• No statement of whether subsidies, the tax deduction and VAT are included.
• The data is passed on to several providers, all of whom then call you.

What a calculator cannot do at all

No calculator can see the condition of your roof, the load-bearing capacity of the substructure, the space for the inverter or the headroom left in your house connection. It also does not know the requirements of your municipality, nor whether your building is protected.

It gives you a reliable order of magnitude for deciding whether the subject is worth pursuing. The binding figure comes in the quote after the site visit.

What to have ready

Three things get you to a usable result in two minutes: your address, your electricity consumption for the past year in kilowatt hours from your electricity bill, and an idea of whether a heat pump or an electric car will be added in the next few years.