Follow one roof’s sun from breakfast to bedtime
A roof makes its power at midday. A home uses it at breakfast and after dark. Move the sun and watch each kilowatt go to the house, the battery or the grid.
One sunny day on a 5 kWp roof with a 5 kWh battery (illustrative)
The roof makes more than the house can use.
13:00: the roof makes 4.0 kW. The house uses 0.6 kW of it and 3.5 kW goes to the grid. The battery holds 4.5 of 4.5 kWh.
- Solid band: straight from the roof to the house
- Dashed band: in or out of the battery
- Dotted band: to or from the grid
Day totals: 32.7 kWh generated, 6.8 to the home, 4.3 to the battery, 21.7 to the grid.
| Time | Generated (kW) | Home (kW) | Battery (kW) | Grid (kW) |
|---|---|---|---|---|
| 06:00 | 0.0 | 0.0 | -0.3 | 0.0 |
| 07:00 | 0.6 | 0.6 | -0.4 | 0.0 |
| 08:00 | 1.4 | 0.9 | 0.4 | 0.0 |
| 09:00 | 2.2 | 0.3 | 1.8 | 0.0 |
| 10:00 | 2.9 | 0.3 | 2.5 | 0.1 |
| 11:00 | 3.5 | 0.3 | 0.0 | 3.1 |
| 12:00 | 3.9 | 0.6 | 0.0 | 3.3 |
| 13:00 | 4.0 | 0.6 | 0.0 | 3.5 |
| 14:00 | 3.9 | 0.3 | 0.0 | 3.5 |
| 15:00 | 3.5 | 0.3 | 0.0 | 3.1 |
| 16:00 | 2.9 | 0.3 | 0.0 | 2.6 |
| 17:00 | 2.2 | 0.3 | 0.0 | 1.8 |
| 18:00 | 1.4 | 1.3 | 0.0 | 0.1 |
| 19:00 | 0.6 | 0.6 | -0.7 | 0.0 |
| 20:00 | 0.0 | 0.0 | -1.3 | 0.0 |
Battery: positive is charging, negative is discharging. Grid: positive is exported, negative is imported.
Fictional company. Every figure on this site is an illustrative example.
Why the sun and the house disagree
- Panels make the most power around solar noon. Homes use the most in the morning and evening.
- A battery moves some of the midday surplus to the evening. What is left goes to the grid.
- That mismatch, not the panel count, is what decides how much you use yourself.
Same roof, your numbers
Run your own roof through the calculator
Or read a worked scenario. None of them is an installation.
- Terrace house with a south-facing roofIllustrative: 59% of the electricity met on site, EUR 520 to EUR 820 a year in example tariffs.
- Workshop in a converted barnIllustrative: 16% of the electricity met on site, EUR 330 to EUR 530 a year in example tariffs.
- Small flat with a shared roof shareIllustrative: 30% of the electricity met on site, EUR 250 to EUR 410 a year in example tariffs.
All concept scenariosPlain answers about kilowatts, batteries and billsTalk it through, if you want to