4 m² of collectors (Flat-plate, selective absorber) cover 61% of the 3,411 kWh/yr hot-water load: 91% in the best month, 23% in the darkest. Against your current water heater (Electric immersion heater) that saves about 507 €/yr and pays back in 6.9 years.
Annual solar fraction
61%
2,097 kWh of 3,411 kWh
Solar heat delivered
2,097kWh/yr
524 kWh per m² of collector
Hot-water heat demand
3,411kWh/yr
incl. 631 kWh tank standby losses
Backup energy still needed
1,314kWh/yr
Electric immersion heater, efficiency/COP 1
Annual saving
€507
after 68 kWh pump electricity
Payback
6 yr 11 mo
IRR 14.4% · NPV 3,663 €
CO₂ avoided
0.81t/yr
16.2 t over 20 years
Storage per m² collector
50L/m²
50–75 L/m² is the sweet spot
Months at ~100% solar
2
Jul, Aug: plan for stagnation
Daily hot water
200L at 45 °C
1 tank volumes per day
Monthly f-chart results
| Month | Sun on collector (kWh/m²/day) | Mains °C | Load (kWh) | Solar fraction | Solar (kWh) |
|---|---|---|---|---|---|
| Jan | 2.37 | 6.4 | 332 | 28% | 94 |
| Feb | 3.34 | 6.6 | 299 | 46% | 137 |
| Mar | 4.3 | 8.4 | 318 | 62% | 198 |
| Apr | 5.08 | 10.8 | 290 | 75% | 219 |
| May | 5.42 | 13.4 | 281 | 83% | 233 |
| Jun | 5.51 | 15.8 | 256 | 87% | 221 |
| Jul | 5.77 | 17.4 | 253 | 91% | 230 |
| Aug | 5.81 | 18 | 248 | 91% | 227 |
| Sep | 5.02 | 16.7 | 249 | 80% | 199 |
| Oct | 3.97 | 13.9 | 278 | 61% | 171 |
| Nov | 2.5 | 11 | 289 | 34% | 97 |
| Dec | 2.04 | 8.2 | 319 | 23% | 72 |
Sizing options (tank ≥ 50 L/m²)
| Collector | Tank (L) | Solar fraction | kWh/yr | kWh per m² |
|---|---|---|---|---|
| 1 m² | 200 | 23% | 785 | 785 |
| 2 m² | 200 | 40% | 1,365 | 683 |
| 3 m² | 200 | 53% | 1,792 | 597 |
| 4 m² ◀ | 200 | 61% | 2,097 | 524 |
| 5 m² | 250 | 68% | 2,312 | 462 |
| 6 m² | 300 | 72% | 2,451 | 408 |
| 8 m² | 400 | 77% | 2,618 | 327 |
| 10 m² | 500 | 80% | 2,731 | 273 |
| 12 m² | 600 | 82% | 2,813 | 234 |
- Climate: NASA POWER long-term monthly irradiance and temperature at 44.43, 26.1 (embedded default-site data for pre-rendering), transposed to 45° / 180° (Hay–Davies). Cold-water temperature follows the damped air temperature with a one-month lag.
- Method: monthly f-chart for liquid systems with storage-size and DHW-temperature corrections; collector F_R(τα) = 0.693, F_R·U_L = 3.92 W/m²K.
- f-chart was derived for flat-plate collectors. For evacuated tubes it is a reasonable approximation (typically within ±5 percentage points).