The system size results from three variables: your electricity consumption including future demand, the usable roof area and the yield per kWp. As a rough rule of thumb, around 1 kWp per 1,000 kWh annual consumption applies – but roof area and self-consumption are more decisive. In the Zurich region, 1 kWp delivers a guideline value of around 950 to 1,100 kWh per year. (As of: July 2026)
Key points in brief
- Three variables determine the system: consumption, roof area and yield per kWp.
- Rule of thumb as a starting point: around 1 kWp per 1,000 kWh annual consumption – with reservations.
- Conversion: 1 kWp requires about 5 to 7 m² roof area and delivers around 950 to 1,100 kWh per year (Zurich region).
- Future consumers such as a heat pump and e-car belong in the calculation, not just current consumption.
- The rough calculation does not replace planning – the binding size is provided by the roof survey.
In what steps do I calculate the system size?
The dimensioning takes place in three steps. First you clarify the demand, then the available area, and finally you bring the two together.
| Step | Question | Result |
|---|---|---|
| 1. Demand | How much electricity do I need – today and in the future? | Target annual consumption in kWh |
| 2. Area | How much roof is available? | maximum possible output in kWp |
| 3. Alignment | How do I reconcile the two? | sensible system size in kWp |
Guideline value logic, as of: July 2026. The binding design is carried out using the real roof data.
The most important thought first: a system does not have to match your annual consumption exactly. Production and consumption diverge over time, and the system delivers less in winter anyway. The goal is not perfect coverage on paper, but a size that fits the roof and consumption profile.
How do I find my electricity consumption – and future demand?
You can read your current consumption on the electricity bill: the annual consumption in kilowatt hours is decisive. As a guideline value, a single-family home without a heat pump and e-car is often around 4,000 to 5,000 kWh per year.
Looking ahead is crucial. If a heat pump is added, the demand increases as a guideline value by several thousand kilowatt hours. Depending on mileage, an e-car adds another four-digit amount. Anyone considering such purchases for the next few years is better off factoring them in straight away – otherwise you are planning for a house that will soon no longer exist. The articles Combine heat pump with photovoltaics and Charge e-car with solar electricity show how strongly these consumers influence the system.
How do I convert from kWp to roof area, modules and yield?
Three simple conversions are sufficient for the rough calculation:
| From | to | Guideline |
|---|---|---|
| Service | Roof area | around 5 to 7 m² per kWp |
| Service | Yield | around 950 to 1,100 kWh per kWp and year (Zurich region) |
| Service | Number of modules | kWp divided by the module output (~0.4–0.45 kWp) |
Guideline values, as of: July 2026. The actual yield depends on orientation, inclination and shading.
An example for the number of modules: An 8 kWp system corresponds to about 19 modules for 420 watt modules and roughly needs 40 to 55 m² of roof area. The article Watt-peak simply explained clarifies what watt-peak, kWp and kWh exactly mean; how many modules are actually needed is explained in the article How many solar modules do I need?.
Calculation example: This is how large the system for a typical single-family home will be
Let's take a family with a current consumption of 4,500 kWh. A heat pump and an e-car are planned for the next few years.
| Item | Guideline |
|---|---|
| Current consumption | 4,500 kWh |
| Surcharge heat pump (planned) | approx. 3,500 kWh |
| Surcharge e-car (planned) | approx. 2,500 kWh |
| Target annual consumption | around 10,500 kWh |
| Rule of thumb starting point (1 kWp / 1,000 kWh) | around 10 kWp |
| Required roof area (5–7 m²/kWp) | around 50 to 70 m² |
| Expected yield (950–1,100 kWh/kWp) | around 9,500 to 11,000 kWh |
Guideline values, as of: July 2026, without obligation. Your real roof and consumption data as well as an individual quote are decisive.
The calculation provides an order of magnitude of around 10 kWp – provided the roof has the space. This is exactly where it shows why the rule of thumb is only a starting point: If the roof is only sufficient for 8 kWp, one plans with 8 kWp and optimises self-consumption. If the roof offers space for 12 kWp, there is little to stop you using it.
Why is the largest sensible system often the best choice?
Because the fixed costs of a system hardly depend on its size. Scaffolding, travel, electrical work and registration apply largely equally whether you install 8 or 12 kWp. Each additional kWp is therefore comparatively cheap – the price per kWp falls, as the article What does a solar system cost? shows.
In addition: A low feed-in tariff does not make a large system unprofitable as long as you increasingly use the electricity yourself. The key figure that really counts is not the system size, but the self-consumption rate. The article Self-consumption or feed-in? goes into detail about the trade-off between self-use and feed-in.
Where does the rough calculation end – and the planning begin?
The rules of thumb bring you to the right order of magnitude, nothing more. The real roof is needed for the binding design: exact area, orientation, inclination, shading by chimneys or neighbouring trees and the position of the skylights. The portal sonnendach.ch provides an initial indication of the area potential – how to read it correctly is explained in the article Using sonnendach.ch correctly.
At the latest when things get specific, the on-site survey replaces the estimate. The system size that fits your house – and the figure on the quote that you can count on – emerges from the real roof data and your consumption profile.
The rule of thumb «1 kWp per 1,000 kWh» is a good conversation starter, but we regularly see how it misleads. Anyone who only factors in current consumption and leaves out the heat pump and e-car will end up with a system that is too small. Conversely, there is no point in trying to calculate a large number for a small roof. In the end, the area almost always wins: we first look at what the roof yields and then check whether the demand matches it – not the other way around.
Frequently asked questions
How many kWp do I need per 1,000 kWh consumption?
As a rough rule of thumb, around 1 kWp per 1,000 kWh annual consumption. That is a starting point, not an exact value: roof area, orientation and the proportion of self-used electricity shift the result.
How many kWh does 1 kWp deliver in Switzerland?
In the Zurich region as a guideline value around 950 to 1,100 kWh per kWp and year. An ideal south-facing roof is at the upper end, shaded or unfavourably oriented areas below.
Should the system cover my entire consumption?
Mathematically it can often do this, practically never at all times. Production and consumption diverge over time, and the system delivers less in winter. The goal is high self-consumption, not complete coverage on paper.
Do I calculate with the current or future consumption?
With the future one. Heat pump and e-car significantly increase the demand. Anyone who plans them for the next few years is better off dimensioning the system accordingly right away.
How much roof area do I need per kWp?
As a guideline value, around 5 to 7 square metres per kWp, depending on the module output. A modern module with a good 400 watts measures almost two square metres.
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Sources: Swiss Federal Office of Energy SFOE, EnergieSchweiz, Swissolar, sonnendach.ch (SFOE / swisstopo).
Last updated: 9 July 2026 · Author: ecoEn editorial team
