Calculator

Three inputs and an optional battery. You get ranges, with every assumption next to the result. This is an illustration, not a quote.

Fictional company. Every figure on this site is an illustrative example.

Your illustrative result

Based on example tariffs. Your tariff and local rules decide the real figure. This is not a savings estimate and not a guarantee.

The system is sized to what you use, not to the roof.
Without a battery, only the power you use in the moment is counted as self-used.

Where a year of your electricity comes from

Time of day

A sunny day on a 3.4 kWp roof with no battery (illustrative)

The roof makes more than the house can use.

13:00: the roof makes 2.7 kW. The house uses 0.6 kW of it and 2.2 kW goes to the grid.

SkyClearBright cloudOvercast

Day totals: 22.5 kWh generated, 6.1 to the home, 0.0 to the battery, 16.3 to the grid.

A sunny day on a 3.4 kWp roof with no battery (illustrative)
Time	Generated (kW)	Home (kW)	Grid (kW)
06:00	0.0	0.0	-0.3
07:00	0.4	0.4	-0.6
08:00	0.9	0.9	-0.0
09:00	1.5	0.3	1.1
10:00	2.0	0.3	1.6
11:00	2.4	0.3	2.0
12:00	2.7	0.6	2.1
13:00	2.7	0.6	2.2
14:00	2.7	0.3	2.3
15:00	2.4	0.3	2.0
16:00	2.0	0.3	1.6
17:00	1.5	0.3	1.1
18:00	0.9	0.9	-0.3
19:00	0.4	0.4	-0.9
20:00	0.0	0.0	-1.3

Battery: positive is charging, negative is discharging. Grid: positive is exported, negative is imported.

A consultation is optional. Your result can be included in the request.

Listed in order of weight: your inputs, then fixed constants, then caveats.

in1 Your consumption: as entered. Sets the largest system we will suggest.
in2 Your roof area: as entered. Sets how many kWp fit, at 0.18 kWp per m2.

A1 Specific yield: 900 / 1,020 / 1,200 kWh per kWp per year. What one kWp makes in a year in the cooler north, temperate centre and warm south, after typical losses.
A2 Panel density: 0.18 kWp per m2. Includes spacing between rows and edges.
A3 Sizing rule: at most 100% of yearly use. We never suggest a system bigger than you consume.
A4 Direct use: 30% of generation. The share used as it is made, with no battery.
A5 Battery usable: 90% of nameplate. A reserve protects battery life.
A6 Round-trip efficiency: 90%. Energy lost charging and discharging.
A7 Full-cycle days: 250 per year. Days the battery is fully cycled.
A8 Generation range: -12% to +10%. Weather between years and small differences in orientation.
A9 Import price: EUR 0.28 per kWh (0.22 to 0.34). An example; your contract differs.
A10 Export price: EUR 0.07 per kWh (0.05 to 0.09). An example; rules and prices vary.

C1 Not included: costs, incentives, degradation, shade, snow. Real projects need a site review.