Why Installer Payback Claims Can Be Misleading
The 5 Common Assumptions Worth Checking
Pitfall #1: Blended Payback
What they show:
Solar cost: €7,000
Battery cost: €5,000
Total cost: €12,000
Annual savings: €1,500
Payback: 8 years
Why this is misleading: The battery drags down the solar payback, making both look better than they are.
The separated numbers:
Solar alone: €7,000 / €1,200 = 5.8 years
Battery alone: €5,000 / €300 = 16.7 years
The battery may not pay back on its own. But blended, the combined figure can look more reasonable than either alone.
Always ask for separate payback calculations. See our 8 Battery Myths guide for why blended payback is the most common sales tactic — and Myth #6 for a worked example.
Pitfall #2: Optimistic Self-Consumption
What they assume: 70–80% of solar is used in the home.
Self-consumption: The percentage of solar energy you use directly in your house instead of exporting to the grid. Every kWh you self-consume saves you the full retail electricity price (€0.10–0.40). Every kWh you export only earns the low feed-in tariff (€0.01–0.12). So higher self-consumption = much better economics.
The reality:
| Home Type | Self-Consumption |
|---|---|
| Permanent, someone home | 50–65% |
| Permanent, everyone at work | 30–45% |
| Weekend home | 20–35% |
| With battery | +10–20% |
Important: these are annual averages. In winter, self-consumption is higher (all solar consumed at home, little production). In summer, self-consumption is lower (lots of production, most exported). The battery boost is also seasonal — it helps in summer and spring, but in winter the battery often has nothing to charge (see our Solar and Heat Pumps guide).
For a weekend home: Most solar is generated Mon–Fri when you're not there.
- Solar generation: 60% on weekdays
- Weekend consumption: 30% of weekly total
- Result: 70%+ exports at low feed-in tariff
Real self-consumption for weekend home: 25–35%. Not 70%.
Pitfall #3: Ignoring Inverter Replacement
What they show: "25-year payback" with no equipment replacement.
The reality:
- Inverter lifespan: 10–15 years
- NREL data: 34% of residential inverters fail within 15 years
- Replacement cost: €1,000–2,500
- Timing: Usually year 10–14
Inverter: The box that converts DC electricity from your panels into AC electricity your house can use. It's like the alternator in a car — it works hard every day and eventually wears out.
Impact on payback:
- Add €1,500 at year 12
- At 6% discount rate: equivalent to €750 today
- Adds ~1 year to payback
Ask your installer: "Is inverter replacement included in the quote? Show me the replacement cost."
Pitfall #4: Assuming Massive Price Growth
What they show: "With 5% annual price growth, payback is 7 years."
The reality (Hungary):
- 2010: 25 Ft/kWh
- 2015: 27 Ft/kWh
- 2020: 30 Ft/kWh
- 2025: 36 Ft/kWh
- Average growth: ~2.5%/year
The reality (Germany):
- 2010: €0.23/kWh
- 2015: €0.29/kWh
- 2020: €0.32/kWh
- 2025: €0.38/kWh
- Average growth: ~3.5%/year
Installers often assume 5–10% growth. This is not based on history.
Ask for three scenarios:
- Flat prices (best case for honesty — "what if nothing changes?")
- 3% growth (historical average — "what if trends continue?")
- 5% growth (optimistic — "what if I'm lucky?")
Why flat prices matter: If an installer can't show you a payback at today's prices, they're asking you to gamble on the future. A honest payback calculation starts with "what if nothing changes?" and then shows how price growth would improve it.
See our full analysis in What If Energy Prices Keep Climbing? — we modelled 5 scenarios across 6 countries with historical data for 9 EU countries. The difference between "flat forever" and a 2022-style crisis is the difference between an 11-year payback and an 8-year payback (Germany).
Pitfall #5: Hiding Maintenance Costs
What they show: "Zero running costs."
The reality:
| Cost | Annual | 25-Year Total |
|---|---|---|
| Cleaning (2×/year) | €100 | €2,500 |
| Inspection | €50 | €1,250 |
| Inverter replacement | — | €1,500 |
| Insurance increase | €50 | €1,250 |
| Monitoring subscription | €30 | €750 |
| Total | €230 | €7,250 |
Maintenance adds 10–15% to total cost of ownership.
How to Calculate Honest Payback Yourself
Step 1: Get Real Numbers
Annual solar generation: ____ kWh (from PVGIS)
Self-consumption rate: ____ % (realistic for your lifestyle)
Export rate: ____ %
Electricity price: ____ €/kWh
Feed-in tariff: ____ €/kWh
System cost: ____ €
Battery cost (if any): ____ €
Step 2: Calculate Annual Savings
Self-consumed value = Generation × Self-consumption % × Electricity price
Export value = Generation × Export % × Feed-in tariff
Gross saving = Self-consumed value + Export value
Net saving = Gross saving − Maintenance (€200/year)
Limitation: This simplified formula assumes a flat self-consumption percentage. In reality, self-consumption varies by month — higher in winter (all solar used at home) and lower in summer (lots exported). Our calculator uses a 365-day hourly simulation for more accuracy, particularly for heat pump homes where winter self-consumption is critical.
Important seasonal note: Most solar savings come from summer and shoulder seasons, not winter. A system that pays back in 10 years on paper may save almost nothing in December–January. See our Solar and Heat Pumps guide for the monthly breakdown.
Step 3: Add Hidden Costs
Upfront cost = System cost + Battery cost
Year 12 cost = Inverter replacement (50% of original inverter cost)
Total cost = Upfront + Year 12 cost
Step 4: Calculate Payback
Payback = Total cost / Net saving (years)
NPV @ 6% = Use spreadsheet or calculator
NPV (Net Present Value): Would you rather have €7,000 in the bank earning 6%, or spend it on solar? NPV adds up all your solar savings over 25 years and subtracts what the bank would have given you. If positive, solar wins. If negative, keep the cash.
Step 5: Check Sensitivity
If prices stay flat: Payback = ___ years
If prices grow 3%/yr: Payback = ___ years
If prices grow 5%/yr: Payback = ___ years
Honest Payback Benchmarks
Important: These benchmarks assume a household without a heat pump (typical 3,000–5,000 kWh/yr consumption). Heat pump homes have 2–3× higher consumption, which changes self-consumption and payback significantly — see our Solar and Heat Pumps guide for those numbers. These are also rough estimates; use our calculator for your specific situation.
| Country | Solar Only | Solar + Battery | Notes |
|---|---|---|---|
| Spain | 5–7 years | 7–10 years | High sun, good prices |
| Italy | 10–14 years | 12–16 years | Ecobonus 50–65% only |
| Germany | 7–9 years | 9–12 years | High prices, medium sun |
| France | 8–10 years | 10–14 years | Moderate prices |
| Poland | 14–18 years | 9–16 years with G12 TOU | Low retail prices; G12 time-of-use makes batteries viable |
| Hungary | 12–18 years | >20 years | Low prices, flat tariff |
| Ireland | 5–7 years | 7–9 years | High prices, grants |
Feed-in tariff: What the grid pays you for excess solar. It's fallen dramatically in most countries — in the UK it went from ~£0.43/kWh in 2011 to effectively zero for new systems today. This is why self-consumption matters more than ever.
The Waste You're Also Buying
These payback numbers show only the financial side. Every solar installation also creates a future waste liability:
| System Size | Panels | Mounting + Cabling | Inverters | Battery (opt) | Total Hardware |
|---|---|---|---|---|---|
| 3 kWp | 132 kg | 48 kg | 30 kg (2×) | — | ~210 kg |
| 5 kWp | 220 kg | 80 kg | 30 kg (2×) | — | ~330 kg |
| 8 kWp | 352 kg | 128 kg | 30 kg (2×) | — | ~510 kg |
| 10 kWp | 440 kg | 160 kg | 45 kg (3×) | — | ~645 kg |
| +10 kWh battery | +0 | +0 | +0 | +120 kg | +120 kg |
Inverter count note: String inverters last ~12 years. A 25-year system needs 2 inverters. Larger systems (>8 kWp) may need 3. Each inverter adds ~15 kg of e-waste.
Per €1,000 of upfront cost: A 5 kWp system costs ~€7,000 and creates ~330 kg of waste → 47 kg of waste per €1,000 spent. This is before considering the carbon footprint — see our Environmental Lifecycle Guide for the full picture.
What this means: When comparing the €7,000 (plus 330 kg of future waste) against 25 years of energy savings, you're deciding whether the trade-off is worth it. Solar is not "zero impact" — it concentrates environmental cost into hardware manufacturing and end-of-life disposal, in exchange for lower operational emissions. The Lifecycle Calculator shows both sides of this trade-off.
If an installer quotes < 5 years without subsidies, it's worth asking which assumptions they used.
The One Question That Clarifies the Numbers
"If electricity prices stay flat forever, what is my payback?"
An installer who gives a straight answer:
- "12–15 years" (for most of Europe)
- "It might not pay back financially, but here's why people still buy it..."
An installer who focuses only on growth scenarios:
- "But prices never stay flat!"
- "You have to factor in inflation!"
- "The government will raise prices!"
- "Let's use a conservative 5% growth..."
If the payback relies entirely on price growth, the assumption is doing the work — not the solar panels. See our price crisis analysis for how much difference that growth rate makes.
The honest approach: Always start with the flat scenario. Then show how growth improves it. We recommend presenting a flat-price payback as the primary number, with growth scenarios as sensitivities — not the other way around.
Last updated: May 2026