Solar Panel Calculator PRO

FAQ

How long do solar panels take to pay for themselves?

Typical payback periods range from 6–12 years depending on system cost, electricity prices, sunlight availability, and how much of the solar power you consume directly. After payback you enjoy essentially free electricity for the remaining 13–19 years of system life.

What affects solar panel ROI the most?

The four biggest factors are: (1) electricity prices — higher prices mean faster payback; (2) sunlight hours at your location; (3) system installation cost; (4) available government incentives or rebates. Self-consumption rate also plays a major role when export compensation is low.

How long do solar panels last?

Quality solar panels typically last 25–30 years with minimal degradation — around 0.5% per year. Most manufacturers offer 25-year performance warranties. Inverters are the weak link and usually need replacement after 10–15 years at roughly 15% of the original system cost.

What is self-consumption and why does it matter?

Self-consumption is the percentage of your solar production that you use directly in your home rather than exporting to the grid. It matters because many utilities pay only 5–30% of retail price for exported energy. High self-consumption — achieved through battery storage or adjusting appliance schedules — maximises financial return.

What is the difference between self-consumption and self-sufficiency?

Self-consumption = percentage of production you use yourself. Self-sufficiency = percentage of your total electricity demand covered by solar. A small system on a large home may have 80% self-consumption but only 20% self-sufficiency. Both metrics matter for different reasons.

What are NPV and IRR in a solar context?

NPV (Net Present Value) expresses all future energy savings in today's money after accounting for inflation and opportunity cost of capital. A positive NPV means the investment beats your discount rate. IRR (Internal Rate of Return) is the effective annual return — compare it directly to a savings account or index fund to decide if solar is worthwhile.

Is adding battery storage worth it?

Batteries pay off best when: (1) your export compensation is very low (e.g. 5–8 ct/kWh in Germany), (2) you generate significantly more than you consume during daylight hours, or (3) you have time-of-use tariffs with expensive evening rates. Typical battery payback is 10–15 years, so they are optional for most homeowners today.

Why does the calculator include inverter replacement cost?

String inverters and hybrid inverters typically need replacement after 10–15 years at roughly 10–20% of original system cost. An honest 25-year financial model must include this cost — otherwise NPV and IRR are overstated by thousands of euros.

What is net metering?

Net metering allows you to sell excess solar energy back to the grid, with the exported kWh offsetting grid purchases at or near retail price. It is common in the US and some EU countries. Where net metering is available, self-consumption is less critical because export is compensated at full retail price.

How do I choose the right solar system size?

A common rule of thumb: size your system to cover 50–80% of your annual electricity consumption. Oversizing beyond your roof space or local grid limits can reduce per-kW value. The PRO mode calculator shows a warning if your system is significantly oversized or undersized relative to your consumption.

What specific yield (kWh/kWp) should I enter for my location?

Specific yield depends heavily on location: Southern Spain/Portugal 1,400–1,800 kWh/kWp, Central Europe (Germany, France) 900–1,100 kWh/kWp, UK 850–950 kWh/kWp, Poland 950–1,050 kWh/kWp. Use the PVGIS tool from the EU Joint Research Centre for an accurate site-specific estimate.

What are typical system losses and should I change the default?

The default 12% system losses include inverter conversion losses (~4%), wiring losses (~2%), soiling (~2%), shading (~2%), and temperature derating (~2%). Leave it at 12% unless you have a shading-free roof and microinverters, in which case 8–10% is reasonable.

How does PV degradation affect long-term returns?

Solar panels lose about 0.5% of their rated output per year due to light-induced degradation and cell aging. After 25 years a panel producing 1,000 kWh in year 1 will produce around 882 kWh — an 11.8% reduction. The calculator models this exponential decline in every projection year.

What subsidies and tax incentives are available for solar?

Incentives vary by country and region. Common schemes include: US Federal Investment Tax Credit (ITC, currently 30%), German KfW loans, French Ma Prime Rénov, and Polish "Mój Prąd" grants. Enter the net subsidy amount in the calculator to see its impact on payback period and IRR.

How do I calculate the cost of electricity I save?

The calculator multiplies your self-consumed production by your electricity purchase price, plus exported production by the export/feed-in rate. It also compounds electricity price growth year by year so that savings increase over time alongside retail tariffs.

What is a good IRR for a solar investment?

A solar IRR of 6–10% is typically considered attractive versus bank savings rates of 2–4%. IRR above 10% is excellent and indicates a very short payback period. An IRR below 4% means you might be better off putting the capital in a fixed-income product instead.

Does the calculator account for electricity price increases?

Yes — the PRO mode applies an annual electricity price growth rate (default 5%) compounded over the full analysis period. You can adjust this in the Advanced section. Higher assumed price growth increases NPV and IRR; setting it to 0% gives you the conservative case.

How much CO₂ does a typical solar system save?

A 5 kWp system producing 5,000 kWh/year saves roughly 1.8–3.5 tonnes of CO₂ per year depending on your country's grid emission factor (50–700 gCO₂/kWh). Over 25 years that is 45–88 tonnes — equivalent to removing a car from the road for 4–8 years.

What is the difference between kWp and kWh?

kWp (kilowatt-peak) is the rated power of your solar panels under standard test conditions — it describes the size of your system. kWh (kilowatt-hour) is the energy your system actually produces over time. Multiply kWp by the specific yield for your location to estimate annual kWh production.

Do solar panels work on cloudy days?

Yes — solar panels still generate electricity on cloudy days, typically at 10–25% of their rated capacity depending on cloud cover. Most UK, German, and Polish households still achieve 900–1,050 kWh/kWp per year despite frequent overcast conditions. The calculator uses annual averages that already account for weather.

Should I orient panels south or can I use an east-west split?

South-facing panels at 30–35° tilt maximise annual yield. East-west split installations (common on flat roofs) produce 10–15% less total energy but spread generation more evenly across the day, increasing self-consumption. For the calculator, adjust specific yield downward by 10–15% if using east-west configuration.

What is the O&M cost for a solar system?

Operations and maintenance (O&M) costs are typically 0.5–1.5% of installed system cost per year and include cleaning, monitoring, minor repairs, and insurance. The calculator defaults to 1% annually, inflation-adjusted. You can customise this in the Advanced section of PRO mode.

Can I include a heat pump or EV charging in the analysis?

Yes — if you add an EV charger or heat pump, your annual consumption increases significantly, which boosts self-consumption and makes solar more valuable. Enter your estimated total annual consumption including the new appliance in the Household Consumption field. The "House + EV" and "House + Heat Pump" presets cover these scenarios.

What happens after the 25-year analysis period?

Most quality solar panels continue producing electricity beyond 25 years, though at slightly lower efficiency. The calculator conservatively ends at year 25, which aligns with typical manufacturer performance warranties. Any production after year 25 represents additional return not captured in the displayed IRR.

Is the Basic or PRO mode more accurate?

PRO mode is significantly more accurate for real-world decisions. Basic mode uses simple annual production × electricity price — it ignores self-consumption ratio, battery storage, electricity price inflation, inverter replacement, and discount rates. PRO mode's IRR and NPV are suitable for making a €5,000–€25,000 investment decision.