How to Size Your Solar System
A Common Misconception
"Fill the roof! More panels = more savings!"
This is the advice most installers give. It sounds logical. It is wrong for most households.
Here's why it sounds right: Bigger feels safer. If 5 kWp saves €500/year, surely 10 kWp saves €1,000/year? That's how most things work — double the input, double the output.
Here's why it's wrong: Solar panels don't save you money when they generate electricity. They save you money when you use that electricity while it's being made. Every kilowatt-hour you can't use gets exported to the grid — and the grid pays you a fraction of what it costs you to buy electricity.
The Analogy That Makes It Click
Imagine your house is a garden, and solar panels are rain barrels that collect free rainwater.
An installer looks at your roof and says: "You have space for ten barrels! Let's fill the whole roof!"
But your garden only needs five barrels of water per day. The other five barrels overflow. That overflow runs down the drain. The water company buys your overflow — but they pay you pennies per barrel, while they charge you euros per barrel for tap water.
More barrels doesn't mean more savings. It means more overflow. And overflow is worth almost nothing.
The smart move isn't ten barrels. It's five barrels, used well. Maybe add a small tank (battery) to store extra water for the evening. But ten barrels? You're just paying for overflow.
A key point: Solar economics aren't about how much you generate. They're about how much you generate while you're using electricity. The gap between generation and consumption is where money goes to die.
What Oversizing Actually Costs You
Example: Family Home, No Heat Pump (Germany)
- 4 people, some home during day
- Annual consumption: 4,200 kWh
- Roof: 35 m², south-facing
- Electricity: €0.38/kWh, feed-in: €0.09/kWh
| Size | Production | Self-use | Export | Payback | Waste Created |
|---|---|---|---|---|---|
| 4 kWp | 4,000 kWh | 55% | 45% | 10 yr | ~265 kg |
| 6 kWp | 6,000 kWh | 42% | 58% | 12 yr | ~395 kg |
| 8 kWp | 8,000 kWh | 35% | 65% | 14 yr | ~510 kg |
Notice the pattern: Every extra kWp generates more electricity, but less of it is used in your home. The surplus gets exported at €0.09/kWh instead of saving you €0.38/kWh. The extra panels cost thousands but earn pennies — and add hundreds of kilograms of hardware to your roof.
Best size: 4–5 kWp. Self-consumption drops fast as you go bigger.
Note: These paybacks are simplified estimates using flat self-consumption percentages. Our calculator uses a band-by-band hourly simulation for more accuracy. See our price crisis analysis for how energy price growth affects payback.
The Golden Ratio
Without a heat pump
| Annual consumption | Solar size | Production | Self-consumption | Payback | Waste |
|---|---|---|---|---|---|
| 3,000 kWh | 3 kWp | 3,200 kWh | ~55% | 6–9 yr | ~210 kg |
| 4,000 kWh | 4 kWp | 4,300 kWh | ~50% | 7–10 yr | ~265 kg |
| 5,000 kWh | 5 kWp | 5,400 kWh | ~45% | 7–11 yr | ~330 kg |
Rule of thumb: Size (kWp) ≈ Annual consumption (kWh) ÷ 1,000
This is approximate. Your actual self-consumption depends on when you're home, what you run, and whether you have a battery. But it's a much better starting point than "fill the roof."
With a heat pump
This changes everything. Your consumption doubles, but solar production doesn't.
| Annual consumption | Solar size | Production | Self-consumption | Payback | Waste |
|---|---|---|---|---|---|
| 8,000 kWh | 7 kWp | 7,600 kWh | ~32% | 10–14 yr | ~450 kg |
| 10,000 kWh | 8 kWp | 8,600 kWh | ~35% | 9–12 yr | ~510 kg |
| 12,000 kWh | 10 kWp | 10,800 kWh | ~38% | 9–12 yr | ~645 kg |
Rule of thumb with heat pump: Size (kWp) ≈ Annual consumption (kWh) ÷ 1,200
Notice: even with a 10 kWp system, you only self-consume ~38% because heating demand in winter dwarfs solar production. The heat pump needs energy in January; the sun delivers it in July. See our Can Solar Power Winter Heating? guide for a detailed analysis of solar + heat pump winter economics.
The Right Way to Size
Step 1: Know Your Consumption
Check your electricity bills for the last 12 months. Add them up.
If you have a heat pump: Your bill is probably 8,000–15,000 kWh/year. The heat pump itself uses ~2,000–4,000 kWh/year of electricity (for 8,000–15,000 kWh of heat at COP 4.6).
If you don't have a heat pump: Your bill is probably 2,500–5,000 kWh/year.
Important seasonal note: Sizing for annual average consumption misses the winter gap. A system that covers your annual demand perfectly may produce almost nothing useful in December — because solar production drops to 2–5% of annual per month, while heating demand peaks at 20%+ per month. See our Solar and Heat Pumps guide for the monthly breakdown.
Step 2: Account for Future Changes
| Future change | Add to solar size |
|---|---|
| Getting an EV (15,000 km/yr) | +3 kWp |
| Switching to heat pump | +4–6 kWp |
| Working from home more | +1–2 kWp |
| Kids leaving home | −1–2 kWp |
Don't size for everything at once. Size for what you have now, with 20% headroom. You can always add panels later. Adding 2 kWp in year 5 is cheaper than buying 10 kWp now and exporting 60% of it for 5 years.
Step 3: Check Your Roof
| Roof area | Max system size |
|---|---|
| 15 m² (~8 panels) | 3.2 kWp |
| 30 m² (~16 panels) | 6.4 kWp |
| 50 m² (~26 panels) | 10.4 kWp |
A typical panel is 1.7 m × 1.1 m = 1.87 m² and produces ~400 Wp.
Important: Shading kills output. A single chimney shadow can reduce a string of panels by 30%. South-facing, unshaded roofs are ideal. East-west roofs work at ~85% of south-facing yield.
When Bigger IS Better
There are cases where oversizing makes sense:
| Situation | Why bigger works |
|---|---|
| Very low feed-in tariff | You don't care about exports because they're worthless anyway |
| Time-of-use tariffs | You export at peak prices, import at off-peak |
| Net metering (1:1) | Every kWh exported = 1 kWh imported free |
| Planning to get an EV soon | Future-proofs your system |
| Limited roof access | Might as well use all the space you have |
Key distinction: Oversizing is smart when the overflow has value. In most of Europe, feed-in tariffs are so low that overflow is nearly worthless. In those markets, right-sizing wins.
When Smaller IS Better
| Situation | Why smaller works |
|---|---|
| Weekend home | You're not there when solar produces |
| Very high feed-in tariff | Export is almost as valuable as self-consumption |
| Budget constraint | 4 kWp now beats 8 kWp never |
| Tenant (not owner) | You can't take panels with you |
| North-facing or shaded roof | Output is so low it's not worth it |
The Battery Question
Should you add a battery?
Without a heat pump:
| Solar size | Consumption | Battery useful? |
|---|---|---|
| 4 kWp | 3,500 kWh | Maybe (small, 5 kWh) |
| 8 kWp | 4,000 kWh | Yes (10 kWh) |
| 10 kWp | 5,000 kWh | Yes (10–15 kWh) |
With a heat pump:
| Solar size | Consumption | Battery useful? |
|---|---|---|
| 5 kWp | 8,000 kWh | No — battery rarely charges in winter |
| 8 kWp | 10,000 kWh | Marginal — charges ~60% of days |
| 12 kWp | 12,000 kWh | Yes — fills most days, covers evening heating |
The hard truth: With a heat pump, your battery competes with heating for solar energy. In winter, heating wins. The battery sits empty. See our Battery Myths guide for why batteries often don't pay back.
Real Examples
Example 1: Family Home, No Heat Pump (Germany)
- 4 people, some home during day
- Annual consumption: 4,200 kWh
- Roof: 35 m², south-facing
- Electricity: €0.38/kWh, feed-in: €0.09/kWh
| Size | Production | Self-use | Export | Payback | Waste |
|---|---|---|---|---|---|
| 4 kWp | 4,000 kWh | 55% | 45% | 10 yr | ~265 kg |
| 6 kWp | 6,000 kWh | 42% | 58% | 12 yr | ~395 kg |
| 8 kWp | 8,000 kWh | 35% | 65% | 14 yr | ~510 kg |
Best size: 4–5 kWp. Self-consumption drops fast as you go bigger.
Example 2: Family Home, Heat Pump (Poland)
- 4 people, heat pump heating
- Annual consumption: 9,500 kWh (3,500 base + 6,000 heat pump)
- Roof: 45 m², south-facing
- Electricity: €0.28/kWh, feed-in: €0.12/kWh
| Size | Production | Self-use | Export | Payback | Waste |
|---|---|---|---|---|---|
| 5 kWp | 5,250 kWh | 30% | 70% | 14 yr | ~330 kg |
| 8 kWp | 8,400 kWh | 35% | 65% | 12 yr | ~510 kg |
| 10 kWp | 10,500 kWh | 38% | 62% | 11 yr | ~645 kg |
Best size: 8–10 kWp. With a heat pump you need the extra production, but self-consumption stays low. And every extra kWp adds ~65 kg of hardware to your roof.
Note: These are simplified estimates. See our calculator for band-by-band simulation with your specific numbers.
Example 3: Weekend Home, Hungary
- Used Sat–Sun only
- Annual consumption: 2,200 kWh
- Roof: 25 m², south-facing
- Electricity: 36 Ft/kWh, feed-in: 5 Ft/kWh
| Size | Production | Self-use | Export | Payback | Waste |
|---|---|---|---|---|---|
| 3 kWp | 3,150 kWh | 40% | 60% | ~46 yr | ~210 kg |
| 5 kWp | 5,250 kWh | 28% | 72% | ~58 yr | ~330 kg |
| 8 kWp | 8,400 kWh | 20% | 80% | ~73 yr | ~510 kg |
Best size: 3–4 kWp. Weekend homes have terrible self-consumption. See our Weekend Home Guide.
Quick Decision Table
| Your situation | Recommended size | Battery? | Waste created |
|---|---|---|---|
| Small home, no heating | 3–4 kWp | No | ~210–265 kg |
| Family home, no heating | 4–6 kWp | Small (5 kWh) | ~265–395 kg |
| Family home, heat pump | 8–12 kWp | Maybe (10 kWh) | ~510–770 kg |
| Family home, heat pump + EV | 10–15 kWp | Yes (10–15 kWh) | ~645–970 kg |
| Weekend home | 3–5 kWp | No | ~210–330 kg |
| Work from home | 5–8 kWp | Maybe (5–10 kWh) | ~330–510 kg |
Waste includes: panels, mounting/racking, cabling, and inverters (2× over 25 years). Battery adds ~12 kg per kWh. See our Lifecycle Calculator for your exact system's waste footprint.
The Honest Sizing Process
- Get your last 12 months of bills — know your real consumption
- Decide: are you getting a heat pump or EV in the next 5 years? — size for 80% of future demand, not 100%
- Measure your roof — unshaded, south-facing area in m²
- Check your feed-in tariff — if it's high (>€0.10/kWh), oversizing is less painful
- Run our calculator — try 3 sizes and compare payback
- Get 3 quotes — compare installer assumptions against our numbers
The key takeaway: The best solar system isn't the biggest one you can fit on your roof. It's the size where almost every kilowatt-hour you generate gets used in your home while it's being made. Everything else — every exported kilowatt-hour — is a sign you paid for capacity you didn't need.
Don't let an installer size your system. They have every incentive to sell you the biggest possible installation. Size it yourself, then verify.
Last updated: May 2026