Solar Panel Calculator
Find the solar system size and number of panels for your monthly electricity units, the roof space needed, and how quickly it pays for itself.
From your bill. Use a summer month to cover the AC season.
About 5 to 6 across most of Pakistan and the Gulf, 2.5 to 3 in the UK.
Losses from heat, dust, wiring and the inverter. 75% to 80% is typical.
Optional: a quote divided by the system's watts, for the payback estimate.
Units you use while the sun is up, such as ACs and fans in the day. The rest goes to the grid.
Net billing (new connections in Pakistan since 9 February 2026) pays about Rs 8 to 11 a unit; check your DISCO's current rate. If you signed a net-metering agreement before 9 February 2026, enter your full tariff.
System size
5.27 kW
- Panels of 585 W
- 9
- Roof space
- 251 sq ft, about
- Units generated a year
- 7,687 kWh
- Estimated cost
- Rs 737,100
- Pays for itself in
- 2.9 years
To cover 600 units a month you need about 5.27 kW: 9 panels of 585 W, taking roughly 251 sq ft of roof. At Rs 140 a watt it costs about Rs 737,100 and pays for itself in 2.9 years.
How it's worked out
- Size needed
600 units ÷ (30 days × 5 h × 80%)5 kW - Panels
5,000 W ÷ 585 W, rounded up9 panels = 5.27 kW - Saving a year
3,843 units used × Rs 55 + 3,843 units exported × Rs 11Rs 253,668 - Simple payback
Rs 737,100 ÷ Rs 253,668 saved a year2.9 years
- An estimate for planning. Units you use as they're made save your full tariff; units sent to the grid earn only the export price. Batteries for backup cost extra. Get a site survey before you buy.
- NEPRA can change the buy-back rate during your agreement.
How it's worked out
System kW = monthly units ÷ (30 × peak sun hours × efficiency) · Panels = system watts ÷ panel watts, rounded up
Saving a year = units used as they're made × your tariff + units sent to the grid × the export price
Worked example
600 units a month ÷ (30 × 5 × 80%) = 5 kW. That's 8.5 panels of 585 W, so 9 panels: 5.27 kW making about 7,690 units a year. At Rs 140 a watt it costs about Rs 737,100. This assumes net billing, the terms for new connections since 9 February 2026: using half the solar in the day saves 3,843 units at Rs 55, and the other 3,843 units sent to the grid earn Rs 11 each, about Rs 253,700 a year. It pays for itself in about 2.9 years.
Questions people ask
What size solar system do I need for 600 units a month?
About 5 kW with 5 peak sun hours and 80% efficiency: nine 585 W panels, a 5.27 kW system.
How many units does a 1 kW system make?
About 120 units a month in most of Pakistan and the Gulf (1 kW × 5 sun hours × 80% × 30 days), less in winter and more in summer.
How much roof space does it need?
About 28 sq ft for a modern 585 W panel and about 7 sq ft for a small 100 W one, plus space to walk around them. Nine panels need roughly 250 sq ft.
How long does solar take to pay back?
Under net billing, which applies to new connections in Pakistan since 9 February 2026, often about 3 to 5 years. Units you use yourself save your full tariff, but units sent to the grid earn only about Rs 8 to 11. The more of your solar you use during the day, the faster it pays back. Households with net-metering agreements signed before 9 February 2026 keep the old terms until their agreement ends, which often means 2 to 3 years. Batteries add cost and lengthen the payback.
About this calculator
Formula and rules last checked 12 November 2026Written and tested by the CalculatorDex teamReport a problem
Sources: NEPRA (Prosumer) Regulations, 2026, S.R.O. 251(I)/2026 · NEPRA, amendment to Regulation 21 (existing net-metering agreements)