Solar System Size Calculator for Pakistan

How the sizing works

The calculator works backwards from the electricity you already use.
Your monthly units divided by 30 gives your daily consumption. That figure is divided by the number of productive hours your city gets each day – the peak sun hours – and then divided again by the system’s real-world efficiency, because no array delivers its nameplate rating in the field.
System size (kW) = (monthly units ÷ 30) ÷ (peak sun hours × 0.78)
A house in Lahore using 600 units a month needs about 20 units a day. At 4.9 peak sun hours and 78% efficiency, that works out to roughly 5.2 kW – around nine 585W panels.
Switch the calculator to “By units” if you have your bill in front of you. It is the more accurate of the two modes, because converting a rupee figure back into units requires assuming a tariff, and your tariff depends on which slab you land in.

Peak sun hours, city by city

Peak sun hours are not daylight hours. One peak sun hour means one hour of irradiance at 1,000 watts per square metre – the standard test condition panels are rated at. A twelve-hour day in Lahore delivers roughly five peak sun hours once you account for the sun’s angle through the morning and evening.
This is why an identical system generates measurably more in Quetta than in Islamabad.

The spread between Quetta and Islamabad is nearly 19%. The same 10 kW array produces about 1,330 units a month in Quetta against 1,120 in Islamabad – a difference of roughly 2,500 units a year, which is not a rounding error when you are calculating payback.

Why 78% and not 100%

A 585W panel produces 585W in a laboratory at 25°C with clean glass and perfect irradiance. It does not produce 585W on a roof in Multan in June.
The performance ratio accounts for everything between the nameplate and the meter:
Heat. Panel output falls as cell temperature rises – typically around 0.3% per degree above 25°C. On a Pakistani summer roof, cells routinely run 25–30°C above ambient, and that alone costs 8–12%.
Dust and soiling. This matters more here than in most markets. An uncleaned array in a dusty month can lose 5–15% before anything else is counted.
Inverter conversion. Turning DC into usable AC costs 2–3% even on a good unit.
Wiring, mismatch and connections. Another 2–4% between the array and the distribution board.
Added together, 78% is a realistic working figure for a well-installed system that gets cleaned. A neglected array in a dusty area will do worse; a clean array in a mild month will do better.

Where the cost estimate comes from

Most solar calculators quote a flat rupees-per-watt figure that someone typed in once and never revisited. Ours is assembled from three parts, and two of them are live.
Panels are priced from the actual panels in our catalogue, at their actual rupees-per-watt. When a price changes, the calculator changes with it.
The inverter is priced from real units in our catalogue at the rating your system needs. Ratings within 30% of your system size are pooled together, so one unusually expensive model cannot distort the band. If your system is larger than anything listed, it is costed as multiple units – which is how large installations are actually built.
The balance of system – mounting structure, DC and AC cabling, protection devices, earthing and labour – is added at 35% of equipment cost. This is the one part still carried as an assumption rather than read from the catalogue.
The band is wide on purpose. The low end is a budget on-grid build; the high end is a premium hybrid with a battery-capable inverter, which can cost several times more per watt. The fine print underneath your result names the exact figures used, so you can see which end of the band your build is likely to sit at.

What this calculator cannot know

Sizing from a bill gets you a defensible starting number. It does not get you a design. Four things can move the answer, and only a site visit settles them.
Roof area and orientation. If your usable roof is smaller, the system shrinks regardless of what your bill says.
Shading. A single water tank, parapet or neighbouring wall shading one panel can drag down a whole string. Panel-level optimisers or microinverters solve it, at a cost.
Your supply phase. Single-phase and three-phase connections take different inverters at different prices. Larger systems generally require three-phase.
What you actually want solar to do. Cutting your bill and staying on during load shedding are two different systems at two different prices. This calculator sizes for the first.

How big a system will your DISCO approve?

Sizing for your consumption and sizing for grid approval are two different questions, and the second one has a hard limit that catches people out.
Approval is tied to your sanctioned load – the connection capacity your DISCO has allocated to you, not how much electricity you use. It is printed on your bill. A household with a modest sanctioned load cannot get a large system approved regardless of what its consumption justifies.
So before you commit to a size, do two things.
Find your sanctioned load on your bill. It is usually shown in kilowatts near your connection details, and it is often lower than people expect.
Check the system you want against it. Under Regulation 3(2) of the Prosumer Regulations 2026 your system cannot exceed your sanctioned load, cut from 1.5 times under the old rules. A 5kW sanctioned load means a 5kW system, however much your consumption would justify. At 25kW and below your DISCO approves the application itself and NEPRA is not involved.
If the system you need is larger than your sanctioned load allows, the route is to apply for a load enhancement with your DISCO first. That is a separate application with its own cost and timeline, and it is far better discovered now than after you have paid for panels.

How many panels, and how much roof?

Panel count falls out of the system size directly: divide the kilowatts by the wattage of the panels you are buying. A 5 kW system built from 620W panels is eight panels. The same 5 kW from 580W panels is nine. The calculator above uses the median wattage of the panels currently in our catalogue, so the count reflects what is actually being sold rather than a figure someone typed in once.
Roof space is the constraint that more often decides the answer.
A modern 580–620W panel measures roughly 2,280 × 1,130 mm – about 28 square feet of panel. But panel area is not roof area, and the gap between them depends entirely on your roof.
On a sloped roof, panels are flush-mounted following the existing pitch. Allow around 30 to 35 square feet per panel once you account for edge clearances and walkways.
On a flat roof, panels sit on tilt frames, and each row casts a shadow that the next row has to clear. That inter-row spacing roughly doubles the footprint – allow 50 to 60 square feet per panel.
So a 5 kW system of eight panels needs about 250 square feet of sloped roof, or closer to 450 square feet of flat roof. If your usable area is smaller than that, the honest answer is a smaller system or higher-wattage panels, not a tighter layout.
Two things that shrink usable roof more than people expect: water tanks and their shadows, and parapet walls on the southern side. Both are worth measuring before you size anything.

Next step

Knowing the size is half the question. The other half is what it saves you and when it pays for itself – and the honest answer there is usually lower than what you will be quoted.