Armed with a consumption profile (Chapter 3) and understanding of solar yield (Chapter 4), we can now size a solar PV system for a specific household and goal.
Solar PV sizing is not a single calculation — it depends on your objective:
These objectives lead to different system sizes. This chapter focuses on the most common residential goal: maximize self-consumption while covering a meaningful fraction of annual consumption.
The self-consumption fraction (SCF) is:
SCF = Energy consumed directly from solar / Total solar production
If you produce 5,000 kWh and consume 3,500 kWh of it directly (the rest is exported), SCF = 70%.
The self-sufficiency ratio (SSR) is:
SSR = Energy from solar (used directly + from battery) / Total household consumption
These two metrics trade off against each other: oversizing the solar array increases SSR but decreases SCF (more surplus must be exported at low value).
Without storage, the SCF decreases as system size increases past the “consumption-matched” point:
| System size vs. consumption | Approximate SCF (no battery) |
|---|---|
| Small (covers 30% of needs) | 80–90% |
| Medium (covers 50–60% of needs) | 60–75% |
| Large (covers 80–90% of needs) | 35–55% |
| Oversized | 25–40% |
From Chapter 2 or your utility bills:
Example: 4-person household, no electric heating → 5,000 kWh/year
For a grid-connected system without storage, a practical target is 50–70% SSR (covering half to two-thirds of annual needs from solar). Higher targets require oversizing, which wastes production in summer.
Target: 60% SSR → need to cover 3,000 kWh/year from solar.
From the table in Chapter 4, select the specific yield for your location and roof orientation.
Location: Lyon, France, south-facing 35° → 1,150 kWh/kWp/year
Required kWp = Target solar production (kWh/yr) / Specific yield (kWh/kWp/yr)
But your solar production target should be larger than your coverage goal because not all production will be self-consumed (some will be exported). Apply an estimated SCF correction:
If you expect SCF ≈ 70%:
Required production = 3,000 / 0.70 = 4,286 kWh
Required kWp = 4,286 / 1,150 = 3.7 kWp
Round up to a practical system: 4 kWp (typically 9–10 standard panels).
For 420 Wp panels at ~21% efficiency (≈ 1.95 m² each):
Number of panels = 4,000 Wp / 420 Wp = ~10 panels
Roof area = 10 × 1.95 m² = 19.5 m²
A typical accessible south roof of a 120 m² house has 30–60 m² of usable roof area, so 10 panels is easily accommodated.
Household: 4 persons, 120 m², central France (Lyon) Annual consumption: 5,200 kWh/year Heating: Gas boiler. Hot water: resistance tank (included in consumption). Goal: 60% SSR
| Parameter | Value |
|---|---|
| Target solar coverage | 3,120 kWh/yr |
| Location specific yield | 1,150 kWh/kWp/yr |
| Estimated SCF | 70% |
| Required production | 4,457 kWh/yr |
| Required kWp | 3.9 kWp → 4 kWp |
| Number of panels (420 Wp) | 10 panels |
| Roof area | ~20 m² |
| Estimated actual SSR | ~55–65% |
| Estimated annual grid import savings | ~3,000 kWh → €600/yr at €0.20/kWh |
| Month | Solar prod. (kWh) | Consumption (kWh) | Surplus (+) / Deficit (−) |
|---|---|---|---|
| January | 160 | 500 | −340 |
| February | 230 | 450 | −220 |
| March | 400 | 430 | −30 |
| April | 480 | 380 | +100 |
| May | 560 | 350 | +210 |
| June | 600 | 320 | +280 |
| July | 620 | 300 | +320 |
| August | 570 | 310 | +260 |
| September | 450 | 380 | +70 |
| October | 300 | 440 | −140 |
| November | 175 | 480 | −305 |
| December | 140 | 510 | −370 |
| Annual | 4,685 kWh | 4,850 kWh | −165 kWh net |
The system nearly covers annual needs — but heavy reliance on grid in winter (Oct–Feb). Without storage, most summer surplus is exported.
Adding a heat pump to the same household changes everything:
Annual consumption: 5,200 (base) + 4,500 (heat pump) = 9,700 kWh/year
The heat pump runs mostly in winter, precisely when solar production is lowest. Sizing implications:
| Strategy | System size | SSR | Notes |
|---|---|---|---|
| Match daytime loads only | 3–4 kWp | 25–35% | Minimal investment, best payback |
| Cover non-heating consumption | 4–5 kWp | 35–45% | Good balance |
| Cover all loads optimally | 8–10 kWp | 55–65% | High upfront, more export |
| Oversized for self-sufficiency | 12–16 kWp | 70–80% | Needs large battery for value |
For most heat pump households in central Europe, 5–7 kWp with 10–15 kWh battery is the sweet spot: covers most spring/summer/autumn loads plus some winter assist via stored daytime solar.
| Month | Solar (kWh) | Consumption (kWh) | Grid import (kWh) | Self-use (kWh) |
|---|---|---|---|---|
| Jan | 235 | 1,200 | 1,010 | 190 |
| Feb | 340 | 1,050 | 760 | 290 |
| Mar | 600 | 900 | 390 | 510 |
| Apr | 720 | 650 | 130 | 520 |
| May | 840 | 480 | 0 | 480 |
| Jun | 900 | 380 | 0 | 380 |
| Jul | 930 | 350 | 0 | 350 |
| Aug | 855 | 370 | 0 | 370 |
| Sep | 675 | 500 | 0 | 500 |
| Oct | 450 | 750 | 390 | 360 |
| Nov | 263 | 1,000 | 790 | 210 |
| Dec | 210 | 1,270 | 1,090 | 180 |
| Total | 7,018 | 8,900 | 4,560 | 4,340 |
SSR = 4,340 / 8,900 = 49%. Grid imports are concentrated in Nov–Feb.
The inverter must handle:
Common rule: inverter nominal AC power ≥ 80% of installed kWp (slight undersizing is acceptable and saves cost; peak irradiance hours are brief).
For a 6 kWp system: a 5 kW or 6 kW hybrid inverter is appropriate.
When your solar produces more than you consume, the surplus flows to the grid. Most jurisdictions handle this one of three ways:
| Model | Mechanism | Export value |
|---|---|---|
| Net metering | Surplus “runs meter backwards” | = retail electricity price |
| Feed-in tariff (FIT) | Fixed payment per kWh exported | €0.04–0.12/kWh (varies) |
| Self-consumption only | Export blocked or curtailed | Zero |
Net metering is the most favorable but is being phased out in many countries as solar penetration increases. Where FITs are low (< €0.08/kWh vs retail at €0.20–0.30/kWh), maximizing self-consumption is far more valuable than exporting.
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