Solar panel calculator: size, panels, roof area and payback
This solar panel calculator starts from your electricity bill — units (kWh) per month — and works out the rooftop system you need: capacity in kW, the number of panels of a given wattage, the roof area, yearly generation, and the money side — installed cost, subsidy, year-one and lifetime savings, and simple payback.
The sizing depends on where you are. A table of average daily peak sun hours covers every major Indian state (from 4.1 h in the North-East to 5.6 h in Rajasthan and Gujarat) plus the Gulf, the US, Europe, Australia, Africa and South-East Asia. A performance ratio of 70–85% allows for inverter, wiring, temperature, dust and shading losses, so the kW figure is what an installer would quote.
Cost and subsidy are inputs because they change every year: enter the installer’s price per kW and, in India, the PM Surya Ghar subsidy (up to ₹78,000) or, in the US, the federal tax credit. Results are estimates, not financial advice.
How to use the Solar Panel Calculator
- Enter your average monthly consumption in units (kWh) and the share of it you want solar to cover (100% for full net-metering, less if your roof is small).
- Pick your region to load the peak sun hours, or type a value from a solar map; choose a performance ratio that matches your roof and climate.
- Choose the panel wattage (330–590 W) or enter a custom watt and area, and a spacing factor — 1.1 for panels flush on a pitched roof, 1.3–1.6 for tilted rows on a flat Indian roof.
- Enter the installed cost per kW, any subsidy or tax credit, your tariff, expected tariff increases, panel degradation and system life.
- Read the system size, panels, roof area, generation, savings and payback; copy or share the summary to compare quotes.
Solar Panel Calculator features
Peak sun hours by region
Thirty presets including ten Indian regions, with the value editable for your own solar data.
Panels and roof area
Whole panels at your chosen wattage, plus the real roof area including row spacing and access.
Generation and savings
Yearly and monthly units, savings capped at what you actually consume, with tariff escalation and 0.5%/yr panel degradation.
Subsidy-aware payback
Net cost after subsidy or tax credit and a simple payback in years, plus lifetime savings over the system life.
Indian and global pricing
Cost per kW and tariff in ₹, $, £, €, A$ and more.
CO₂ estimate
Tonnes of CO₂ avoided per year at a grid factor of 0.7 kg per kWh.
How the solar system size is calculated
- Daily target = monthly units × share ÷ 30.4. For 300 units a month: 9.86 kWh per day.
- System size (kW) = daily target ÷ (peak sun hours × performance ratio) = 9.86 ÷ (5 × 0.8) = 2.47 kW.
- Panels = kW × 1,000 ÷ panel watts, rounded up: 2,470 ÷ 540 = 4.6 → 5 panels = 2.7 kW installed.
- Roof area = panels × panel area × spacing factor = 5 × 2.58 m² × 1.3 ≈ 17 m² (180 ft²).
- Generation = installed kW × sun hours × performance ratio × 365 = 2.7 × 5 × 0.8 × 365 ≈ 3,940 kWh a year — about 1,460 units per kW, which matches the 1,400–1,600 units per kW per year usually quoted for most of India.
- Payback = net cost ÷ yearly savings. ₹1,62,000 − ₹78,000 subsidy = ₹84,000, ÷ ₹28,800 a year ≈ 2.9 years.
Peak sun hours used for Indian states
These are annual averages of global horizontal irradiance; a south-facing tilt at the latitude angle adds 5–10%. Monsoon months in the west and north-east can halve the output, which is why net metering that banks summer surplus matters.
| Region | Peak sun hours / day | Units per kW per year (PR 0.8) |
|---|---|---|
| Rajasthan, Gujarat | 5.6 | 1,635 |
| Karnataka, Telangana, Andhra Pradesh | 5.3 | 1,548 |
| Maharashtra, Madhya Pradesh, Tamil Nadu | 5.2 | 1,518 |
| Delhi, Haryana, Punjab, Uttar Pradesh | 5.0 | 1,460 |
| Bihar, Jharkhand, Odisha, Himachal, Uttarakhand | 4.8 | 1,402 |
| Kerala, West Bengal | 4.5–4.6 | 1,314–1,343 |
| Assam and the North-East | 4.1 | 1,197 |
Before you sign a quote
- Match the system to your sanctioned load and the DISCOM’s net-metering limit (often 1–10 kW for homes); a system larger than your consumption earns little for the surplus.
- Check the roof: it needs unshaded space from about 10 am to 3 pm and, for a flat RCC roof, room for tilted rows — the spacing factor covers this. The roofing calculator gives the true roof area.
- Inverters last 10–12 years, so budget one replacement; panels carry 25-year warranties with about 0.5% degradation per year.
- Pair daytime loads with the array: an AC sized with the AC tonnage calculator running at midday uses solar power directly. The electricity bill calculator shows what the system displaces.
- Sizing an irrigation pump? Use the solar pump calculator.
Solar Panel Calculator FAQ
How many solar panels do I need for 300 units a month?
About 2.5 kW of panels in most of India — five 540 W panels (2.7 kW) — needing roughly 17 m² (180 ft²) of flat roof. In a cloudier climate with 3.8 sun hours you would need about 3.3 kW. The solar panel calculator shows the figure for your region.
How many units does a 1 kW solar system generate per day?
Roughly 4 units a day in India (3.5–4.5 depending on the state and season), about 1,400–1,600 units a year: peak sun hours × performance ratio = 5 × 0.8 = 4 kWh per kW per day.
How much roof area does a 3 kW system need?
Six 540 W panels cover 15.5 m²; with row spacing on a flat roof allow 20–25 m² (220–270 ft²). On a pitched roof with flush panels, 17–18 m² is enough.
What is the PM Surya Ghar subsidy and how do I enter it?
The 2024 scheme pays ₹30,000 per kW for the first 2 kW and ₹18,000 for the third kW, so a 3 kW system gets the maximum ₹78,000. Type the total subsidy for your size in the subsidy field; it is subtracted from the gross cost before payback.
What is a performance ratio?
The share of the panels’ rated output that reaches your meter after inverter, wiring, temperature, dust and shading losses. 80% is typical; use 70–75% for hot, dusty or partly shaded roofs.
Is the payback calculation reliable?
It is a simple estimate: net cost divided by savings, limited to the units you consume and adjusted for tariff rises and degradation. It ignores maintenance, inverter replacement, financing and feed-in rates, so treat it as a comparison tool rather than financial advice.