Home › What solar saves › In detail

The calculation, down to the panel

For those who have their figures to hand. Fill in what you know and leave the rest: every field starts from a value with a stated source.

Let us read your annual bill or tariff sheet

A PDF of up to 10 MB. We only take your consumption, prices and tariffs from it and delete the file straight away. How we handle it.

Your consumption

Consumption per yearkWhIt is on your annual bill. 3,500 kWh is the reference household of the Flemish regulator (Vlaamse Nutsregulator).

Is anyone at home during the day?

Hardly evereveryone works or studies away from homeA few days a weekfor example working from homeMost of the timeretired, working from home, young children

Do you have a day/night tariff?

No, one tariffsingle-rate meterYesdual-rate meter

Additional consumers

These come on top of your consumption above. Leave empty whatever you do not have.

Electric car: kilometres per yearkmElectric cookingkWhIf you do not know, leave it empty: it is then already part of your annual consumption.A heat pump is being added

The installation

Number of panelsOutput per panelWpThe SunPower P7 we install is 460 Wp, our standard panel (Trina) 455 Wp.Roof orientation

Home battery and smart control

Battery capacitykWhWe count 200 full cycles a year, as measured on home batteries in real use.With smart energy management (Jullix)

Tariffs

Price per kWh€All-in. Starting value: €0.34, what a prosumer saved on average in 2025 per kWh used on site (Vlaamse Nutsregulator).Feed-in payment per kWh€Starting value: the weighted average for 2025, 3.16 cents (Vlaamse Nutsregulator).Capacity tariff per kW per year€Fluvius Halle-Vilvoorde, 2026 tariff: €56.04 excluding VAT, €59.41 including 6% VAT (Fluvius tariff list, November 2025). Only in Flanders.Your average monthly peakkWLeft empty: 4.24 kW, the average for households in Flanders (Vlaamse Nutsregulator).

Your installation

— kWp

generates — kWh a year

Total consumption, everything included	—
Used straight away	—
Via the battery in the evening	—
Fed into the grid	—

Electricity you do not buy	—
Feed-in payment	—
Lower monthly peak	—

Off your bill

— per year

And the payback time? It is in your quote, after the site visit. By then we know what fits on your roof and what it costs.

Add this calculation to my appointment or request

Book a site visit Request a quote Call 02 486 93 02

How we calculate

The rules behind this calculation

Output: number of panels × output per panel × the yield per kWp from PVGIS for Lembeek, with 14% system losses and a 35-degree roof pitch.
What you use straight away: a measurement by Vlaanderen.be in 1,000 Flemish households whose panels generate as much as they use: 28% self-consumption on average, 27% in households that use most in the evening, up to 35% for those who make the most of their own solar power. Your answer to "at home during the day" picks one of those three; with smart control we count at least 35%. With a heat pump the measurement found 22% instead of 28%, and we scale accordingly. The percentage applies to the smaller of output and consumption.
The battery fills up with what is left over and returns it when the sun has gone: at most 200 full cycles a year, as measured on home batteries in real use (Nature Energy, 2024), with 11% round-trip losses. Even with smart control we do not count extra cycles: we found no solid source for that.
Day/night tariff: the day rate applies on weekdays from 7 am to 10 pm, the night rate at night and all weekend. Solar power used straight away is therefore counted 5 days in 7 at the day rate and 2 in 7 at the night rate; power from the battery at the night rate.
The monthly peak: without smart control we assume the charger and the heat pump run at least once a month at the same time as your usual peak, but never above 8 kW: according to Fluvius, households with an electric car or heat pump peak between 4 and 8 kW. With smart control the car charges when everything else is quiet, and never slower than the 6 amps a car needs as a minimum (1.4 kW single phase, 4.1 kW three phase). A battery then helps out up to its discharge power. Without smart control we do not count the battery for the peak: in winter it is usually empty. Never below 2.5 kW, the minimum in Flanders. The difference times your capacity tariff is the saving.
What is not included: what smart control earns from dynamic hourly prices, maintenance, an inverter replaced after fifteen years, panel degradation, and interest if you borrow.

You can change every starting value. The result remains an estimate on paper: only a site visit tells us how your roof really is.