Solar Panel Calculator

What your result means

System size is the total rated power of the panels, in kW, that makes the electricity you want to cover over a year, given your location's sunshine and normal losses. It's rounded up to whole panels. Output and savings are yearly averages: summer months make more and winter months less, and the weakest month shows how deep that dip goes.

How to use the Solar Panel Calculator

1. Step 1, Location: pick the nearest of 197 cities in 69 countries for its NASA sun hours, or choose "Somewhere else" and type your own. This also sets your currency.
2. Step 2, Electricity use: enter the units (kWh) on your bill for its billing period (a month, two months, a quarter or a year), your bill amount and price per unit, or your appliances with the + and − buttons.
3. Step 3, System type: grid-tied, hybrid with battery backup (choose the hours and the share of your home to keep running) or off-grid (choose the cloudy days to cover). Off-grid sizes for the darkest month.
4. Step 4, Panels & roof: set your panel's watt rating and, if you like, your clear roof area. Losses and efficiency sit under More options.
5. Step 5, Cost & Savings: the cost starts at a typical installed price for the US, Pakistan or India (dated estimates). Switch to your own prices (panels, inverter, battery, labour) or a real quote for your figure; payback and 25-year savings follow. The result and the month-by-month table update at every step.

Formula and worked example

monthly kWh          = units on your bill ÷ days in the billing period × 30.42,                       or bill amount ÷ price per unit (same period),                       or Σ appliances (qty × watts × hours a day) × 30.42 days ÷ 1,000daily need (kWh)     = monthly kWh × 12 ÷ 365 × share to coverkWh per kW per day   = peak sun hours × (1 − losses)system size (kW)     = daily need ÷ kWh per kW per daypanels               = size × 1,000 ÷ panel watts, rounded upyearly output (kWh)  = panels × panel watts ÷ 1,000 × kWh per kW per day × 365panel area (m²)      = system kW ÷ panel efficiencypanels on your roof  = roof area ÷ (panel watts ÷ 1,000 ÷ efficiency), rounded downdarkest-month sizing = daily need ÷ (the lowest monthly sun hours × (1 − losses))battery, backup      = daily use × share kept running × backup hours ÷ 24 ÷ efficiency ÷ depth of dischargebattery, off-grid    = daily use × cloudy days ÷ efficiency ÷ depth of discharge, in whole modulesinverter (kW)        = system kW ÷ 1.2, or your peak load if largeryearly savings       = kWh you use × price + kWh exported × export credittypical cost         = system watts × typical price per watt (+ battery kWh × typical price per kWh, US only)your own prices      = watts × (panels + mounting and labour per watt) + inverter kW × price per kW + battery kWh × price per kWh + otherpayback (years)      = installed cost ÷ yearly savings25-year savings      = Σ yearly savings × (1 − 0.5%)^year, at today's prices

Worked example (the defaults): a home using 900 kWh (units) a month needs 29.59 kWh a day. At 4.73 peak sun hours (the US average) and 20% losses, each kW of panels makes 3.78 kWh a day, so the home needs 29.59 ÷ 3.78 = 7.82 kW. With 400 W panels that rounds up to 20 panels, a 8.00 kW system making about 11,049 kWh a year. December is the weakest month at about 615 kWh. The panels cover roughly 38 m² at 21% efficiency, and at the US average price of 18.31¢ per kWh they save about $1,977 a year. The same steps work in any country: only the units, sun hours and your price change.

What a solar system costs: typical prices, your prices or a quote

The cost starts at a typical installed price where a public figure exists: the US, about $2.60 a watt installed, before incentives (EnergySage Marketplace data, 2026); India, the government (MNRE) benchmark of ₹50,000 a kW for the first 2 kW and ₹45,000 a kW after that, from February 2024 (MNRE rooftop solar benchmark cost, via pv-magazine India, 2024); Pakistan, PKR 78 a watt installed, the middle of a published PKR 66–90 range for a system without batteries (Zell.pk solar price guide, October 2026). These are dated estimates, not quotes; prices change often and vary by installer, brand and roof.

Choose "My prices" to price each part in your currency: panels and installation per watt, the inverter per kW (the result shows the size it needs), the battery per kWh and any other costs. Or choose "A quote" and enter the installer’s total. Payback and 25-year savings use whichever cost you pick.

Units, kWh and billing periods

A "unit" of electricity on a bill is one kilowatt-hour (kWh): 1,000 watts running for one hour. Bills show it as kWh, units or consumption, whatever the country, so the calculator works anywhere.

Bills come every month, every 2 months, every quarter (3 months), every year. Pick the period your bill covers and the calculator converts it to an average month (30.42 days).

Peak sun hours are not hours of daylight

One peak sun hour is one hour of sunlight at full strength: 1,000 watts on each square meter of panel. A summer day with 14 hours of daylight might deliver 6 or 7 peak sun hours, because the sun is weak in the morning and evening.

Our figures are NASA POWER averages for a panel facing the equator (south in the northern hemisphere, north in the southern) and tilted at the local latitude, a common fixed-roof layout. A flat roof, an east- or west-facing roof or shade from trees and buildings will lower the real number.

Off-grid and backup: size for the darkest month

A yearly average hides winter. Grid-tied homes do not mind, because the grid fills the gap, but an off-grid home has to get through its darkest month on solar and a battery. Choosing Off-grid sizes the panels for the month with the least sun, and the month-by-month table under the result shows output against use for every month.

The battery is sized from what it must store: for backup, your daily use × the share you keep running × the hours of backup; off-grid, your daily use × the cloudy days in a row it must cover. That is divided by the round-trip efficiency (95% by default) and the usable depth of discharge (90% for lithium iron phosphate), then rounded up to whole modules.

Which way should solar panels face, and at what angle?

Face fixed panels toward the equator: south in the northern hemisphere, north in the southern. For the angle, the calculator shows NASA's best fixed tilt for your city, the angle that collects the most sun over a year, plus the best angle for each month if your mount can be adjusted. Across our 197 cities the best fixed tilt is on average 8° flatter than the old rule of thumb "tilt = latitude".

Net metering, net billing and what solar saves

With full net metering, every kWh your panels make offsets a kWh you would have bought, so savings are simply output × your electricity price (up to what you use). Leave "Solar you use directly" at 100% for that case.

Many utilities instead credit exported power at a lower rate than they charge for imports (net billing, or a feed-in tariff). To model it, set the share of solar you use as it is made (more if you are home during the day or have a battery) and the export credit your utility pays.

Solar system size by monthly use and sun hours

400 W panels, 20% losses, covering 100% of use. About 3 sun hours suits northern Europe; 4 to 5 most of the US, southern Europe and South Asia; 6 or more desert climates.

Typical appliance use per month

kWh a month = watts × hours a day × 30.42 days ÷ 1,000. Cycling appliances use an average draw. The calculator's default appliance list (10 types) adds up to 259 kWh a month.

Peak sun hours in major cities worldwide

Source: NASA POWER climatology, January 2001 - December 2025 (25-year climatology), panel tilted at latitude facing the equator (retrieved 2026-10-17). Output assumes 20% losses. NASA's solar grid is about 1° wide, so neighbouring cities can share a value.

Peak sun hours by US state

Source: NASA POWER climatology, January 2001 - December 2025 (25-year climatology) (retrieved 2026-10-17). One representative city per state; output assumes 20% losses. The data was obtained from National Aeronautics and Space Administration (NASA) Langley Research Center's Prediction Of Worldwide Energy Resources (POWER) project funded through the NASA Earth Science Division.

Assumptions and limits

• Sun hours are 25-year NASA averages (January 2001 - December 2025 (25-year climatology)) for a panel facing the equator, tilted at the site's latitude. Any single year can be several percent higher or lower.
• The 20% loss default is a generic planning allowance for soiling, wiring, mismatch, heat and the inverter, not a measured figure. Change it to your installer's estimate.
• Shading, roof direction and tilt are not modelled. An installer's site survey gives a site-specific figure.
• Panel output slowly declines with age. The output figures ignore this; the 25-year savings use a generic 0.5% a year, and your panel's warranty states its own rate. Payback is simple: it leaves out financing, maintenance, inverter replacement and future price changes.
• Prices: US states load the EIA residential average. Everywhere else, enter your own price per unit (your bill ÷ the units on it), because tariffs, slabs, taxes and fixed charges vary by country and supplier.
• Appliance watts are generic estimates of running power, not nameplate maximums: a fridge, freezer or inverter AC cycles, so it is given an average draw over its hours. Your appliance labels give the real figures.
• Battery defaults are generic planning figures for lithium iron phosphate: 90% usable depth of discharge, 95% round-trip efficiency, 5 kWh modules. Check your battery's data sheet. The inverter rule (panels ÷ 1.2) is a generic sizing ratio; backup and off-grid inverters must also cover the largest load running at once.
• The 25-year savings assume today's electricity price and 0.5% less output each year; real prices usually change.
• Roof check units: Square feet, Square meters, Marla (225 sq ft), Marla (272.25 sq ft). A marla's size differs between areas and schemes, so pick the one your plot documents use. It counts panel area only; walkways, edges, tank rooms and shade need extra space, and an installer's layout is the real test.
• City locations are approximate, and the sun data is averaged over a wide area, so a nearby city gives much the same figure.

Frequently asked questions

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