How often a van's battery runs flat: a flat roof panel and a 100 Ah lithium bank, simulated day by day
Our own calculation, 2026-10-30, with the EU Joint Research Centre's PVGIS 5.2 off-grid tool (https://re.jrc.ec.europa.eu/pvg_tools/en/): it simulates a panel, a battery and a daily use over the 2005-2020 weather, day by day. Settings for every case: a panel flat on the roof (slope 0); a 1,280 Wh lithium battery (100 Ah at 12.8 V) that stops at a 40% discharge cut-off; PVGIS's default hourly use profile. "Flat" below means the battery hit its cut-off that day. Check: in all 20 cases the yearly share of flat days equals the day-weighted average of the 12 monthly shares (to 0.05 points).
Key results:
- Lisbon, 500 Wh a day: with a 200 W panel the battery runs flat on 100% of December days and 33.15% of all days in the year; with 400 W, on 14.92% of December days and 3.11% of the year.
- Lisbon, 300 Wh a day: 200 W runs flat on 25.2% of December days (4.86% of the year); 400 W on 0% of December days (0.1% of the year).
- Tarifa, 300 Wh a day: 200 W runs flat on 6.85% of December days and 1.2% of the year.
- Munich, 300 Wh a day: 200 W runs flat on 100% of December days (31.9% of the year); 400 W on 49.4% of December days (8.73%).
- Amsterdam, 300 Wh a day: even 400 W runs flat on 99.6% of December days (21.49% of the year). Edinburgh, 300 Wh a day, 400 W: 100% of December days (25.82% of the year).
- What the panel actually delivers per December day in Edinburgh: 54 Wh with 200 W, 107 Wh with 400 W; in Amsterdam 75 and 151 Wh.
- In July no case runs flat on more than 5.24% of days (Edinburgh, 200 W, 500 Wh).
| Place | Panel | Use per day | Flat days, year | Flat days, December | Delivered per December day |
|---|---|---|---|---|---|
| Tarifa | 200 W | 300 Wh | 1.2% | 6.85% | 296 Wh |
| Lisbon | 200 W | 300 Wh | 4.86% | 25.2% | 276 Wh |
| Lisbon | 200 W | 500 Wh | 33.15% | 100% | 282 Wh |
| Lisbon | 400 W | 500 Wh | 3.11% | 14.92% | 483 Wh |
| Munich | 200 W | 300 Wh | 31.9% | 100% | 128 Wh |
| Munich | 400 W | 300 Wh | 8.73% | 49.4% | 250 Wh |
| Amsterdam | 400 W | 300 Wh | 21.49% | 99.6% | 151 Wh |
| Edinburgh | 400 W | 300 Wh | 25.82% | 100% | 107 Wh |
What this cannot tell you: a tilted panel, a bigger battery, shore power or driving with a charger (all add energy); shade or snow; a use pattern other than PVGIS's default profile; a cold battery that will not charge below freezing.