How to Read Our Numbers
What This Calculator Shows (and Doesn't Show)
Our calculator is designed to give you honest, transparent numbers — not the optimistic figures you often see from installers. Here's how to interpret every key metric.
Payback Time
What it means: How many years until your cumulative lifetime savings equal your upfront cost.
How we calculate it:
- We simulate every year of your system's lifetime (default 25 years)
- Each year: solar savings + battery savings + export income − maintenance costs − any replacements
- Panel output degrades by 0.5% per year
- Inverter and battery replacements happen at their end of life (priced at 50% of original cost)
- We track the running total until cumulative net savings equal your upfront investment
- This is dynamic payback — more honest than the simple "cost ÷ year-1 savings" formula most installers use
Why ours differs from installer quotes:
- We assume flat electricity prices by default (no growth)
- We include inverter replacement at year 12
- We include maintenance (cleaning, insurance, monitoring)
- We model your actual load profile (heat pump, when you're home, etc.)
Rule of thumb: If an installer quotes < 5 years without subsidies, ask what assumptions they're using. Our flat-price paybacks are typically 7–15 years for most of Europe.
Self-Consumption
What it means: The percentage of your solar production that you use directly in your home (instead of exporting to the grid).
Why it matters:
- Self-consumed kWh saves you the retail price (€0.10–0.40)
- Exported kWh earns only the feed-in tariff (€0.01–0.12)
- A 1% increase in self-consumption can improve payback by 3–6 months
Realistic ranges:
| Home Type | Self-Consumption | Why |
|---|---|---|
| Permanent, someone home | 50–65% | Daytime load matches solar |
| Permanent, everyone at work | 30–45% | Most solar exported |
| Weekend home | 20–35% | Solar generated Mon–Fri, you there Sat–Sun |
| With heat pump | 30–50% | High winter demand, but also high total consumption |
| With battery | +10–20% | Battery stores midday surplus for evening use |
Winter reality: In December, self-consumption is often 80–100% (all solar used at home). In June, it can drop to 20–40% (most solar exported). The annual average is what matters for payback.
Net Present Value (NPV)
What it means: Would you be better off putting your money in a savings account at your discount rate, or spending it on solar?
How to read it:
- NPV > 0: Solar beats the bank. Good investment.
- NPV < 0: The bank wins. Solar is not financially optimal (though you may still value independence or environment).
- NPV = 0: Break-even with the bank.
Important: Our NPV compares your solar setup against an efficient heating baseline (heat pump, SCOP 4.6). If you select resistive electric heating, NPV will be worse because resistive heating is inefficient — the baseline assumes you'd use a heat pump instead. This prevents the calculator from showing "solar + resistive heating" as a good investment when "solar + heat pump" is clearly better.
Why we use 6% discount rate:
- Inflation-adjusted stock market long-term return: ~5–7%
- Safe government bonds: ~2–3%
- 6% is a conservative middle ground for household capital
Battery Economics
What we show: Battery cost is included in the total system cost for payback. We show one honest payback for the entire system.
Why this matters:
- Solar + battery together might pay back in 12 years
- If you later wonder "was the battery worth it?", run the calculator with and without the battery and compare the two paybacks
- This avoids the misleading "blended average" that makes poor battery investments look good
When batteries make sense:
- High electricity price spread (peak vs off-peak)
- Low feed-in tariff (so exported solar is nearly worthless)
- You're home in the evening to use stored power
- You value blackout protection (not modeled in our financials)
When batteries don't make sense:
- Flat-rate tariff (no peak/off-peak spread)
- High feed-in tariff (export is almost as valuable as self-consumption)
- Weekend homes (battery sits empty most of the week)
Feed-In Tariff
What it means: What the grid pays you for excess solar you export.
Why it's critical:
- In Spain: ~€0.03/kWh (low — self-consumption matters a lot)
- In Hungary: ~€0.014/kWh (very low — batteries look better)
- In Germany: ~€0.07–0.13/kWh (medium — depends on system size and date)
- In Bulgaria: ~€0.108/kWh (high — batteries rarely pay back)
Trend: Feed-in tariffs have fallen dramatically across Europe. Self-consumption is now more important than export income for most households.
Wasted Energy
What it means: Solar production that couldn't be used, stored, or exported due to export limits.
Why it happens:
- Export limit: Some countries cap how much you can export (e.g., Hungary 2.5 kW/phase, Germany 4.6 kW/phase)
- No battery: Surplus solar on sunny summer afternoons has nowhere to go
- Battery full: If the battery is already full, further surplus is wasted
How to reduce it:
- Add a battery (stores surplus)
- Increase self-consumption (run dishwasher, washing machine, EV charging at midday)
- Upgrade to 3-phase connection (raises export limit)
Heating Covered by Solar
What it means: What percentage of your heat pump's electricity demand is met directly by solar (or battery discharge to heating).
Reality check:
- December: 10–25% of heating covered by solar (weak sun, high heating need)
- June: 80–100% of heating covered (strong sun, no heating need)
- Annual average: 20–40% for most of Europe
Important: This is NOT the same as "solar runs my heat pump." In winter, the grid supplies 75–90% of heating. Solar helps most in spring and autumn.
H-Tarifa Loss (Hungary Only)
What it means: If you have a discounted heat-pump electricity tariff (H-tarifa) in Hungary, installing solar cancels that discount.
How we model it:
- Before solar: you pay the discounted H-tarifa rate for heat pump electricity
- After solar: any grid-imported heat pump electricity is charged at the standard A1 rate
- Self-consumed heat pump electricity costs nothing (a saving compared to before)
Impact: Can add 1–3 years to payback for heat-pump households. Always factor this in.
Sensitivity: Try Different Scenarios
The default calculation assumes flat electricity prices forever. This is intentionally conservative. Try these scenarios to see the range:
| Scenario | Price Growth | Typical Effect on Payback |
|---|---|---|
| Flat prices (default) | 0% | Baseline — most honest starting point |
| Historical average | 3%/year | Payback improves by 1–2 years |
| Optimistic | 5%/year | Payback improves by 2–4 years |
| Crisis | 10%/year for 3 years, then 3% | Payback improves by 3–5 years |
Our recommendation: Start with flat prices. If solar makes sense at flat prices, any price growth is a bonus. If it only makes sense with 5% growth, you're gambling on the future.
Known Limitations
- Electricity only: We do not model gas, wood, or PHEV fuel savings. Solar payback should never be blended with heating fuel savings.
- NPV heating baseline: NPV uses the minimum of your actual heating electricity and the heat-pump-efficient equivalent (
heatingDemand / 4.6). Resistive electric heating will show worse NPV because the baseline caps it at the efficient alternative. - No shading: We assume unshaded panels. Real-world shading from trees, chimneys, or neighboring buildings can reduce output by 10–30%.
- No subsidies in base case: Some countries have generous subsidies (Italy Ecobonus 50–65%, Ireland SEAI grants). Check our country guides for subsidy details.
- Average weather: We use typical-year weather data. An unusually cloudy year can reduce production by 10–15%.
- No income tax modeling: Some countries tax solar income differently. We assume net-of-tax figures.
Bottom Line
If you remember one thing: Always ask for the flat-price payback first. If an installer can't show you a reasonable payback at today's prices, their growth assumptions are doing the work — not the solar panels.
Use our calculator to:
- Get a conservative baseline
- Test sensitivity to price growth
- Compare battery vs no-battery separately
- Understand seasonal variation
Then use our country guides for:
- Local subsidy details
- Regulatory quirks (H-tarifa, net metering, export caps)
- Realistic equipment costs in your market
Last updated: May 2026