Solar Panel Size Calculator — kW & Panel Count
System sizing is the first decision in any solar purchase and the one vendors most often inflate. The honest method starts from your electricity bill: monthly units divided by per-kW generation gives the kW you need, and everything else — panel count, roof area, inverter rating — falls out of that number. This tool runs the method with Pakistani generation averages.
Should I size for my summer bill or the annual average?
Average the last twelve months’ units — sizing to the August AC peak buys panels that idle for eight months, while sizing to a winter month leaves the summer bill largely intact. Most households land between the extremes; pull the unit history from the bill’s back page or the DISCO app.
Why does the tool assume 580 W panels?
That’s the mainstream tier-1 panel size shipping into Pakistan in 2025–26 — the market moved from 540–555 W rapidly. Older 450 W stock still circulates at discounts; if you’re quoted those, the panel count rises about a quarter for the same kW.
How firm is the roof-area estimate?
The ~26 sq ft per panel figure includes mounting and walkway spacing on a flat Pakistani roof. Sloped or obstructed roofs need site survey; the number here tells you whether your roof is plausibly sufficient before you invite the surveyor.
Solar Panel Size Calculator
The sizing method, step by step
Take twelve months of units from your bills and average them. Divide by 120 — the realistic monthly kWh each installed kW yields across a Pakistani year — and you have the system kW that offsets your consumption. Divide the kW (in watts) by the panel wattage for the count, multiply panels by ~26 sq ft for roof feasibility, and round the inverter up to the next commercial size. Five minutes of arithmetic, and every vendor proposal can now be audited against it.
Where households deliberately deviate from the formula
Net-metering optimists size above consumption to sell surplus — defensible where the export approval exists, speculative where it’s pending. Load-shedding-weary households size for a future battery, which mainly affects inverter choice (hybrid now versus grid-tied replaced later). And budget-bound buyers size below the formula to a round number of panels, which works fine: solar economics are linear enough that 70 percent of the system delivers 70 percent of the saving. The formula is the reference point, not a mandate — but know which deviation you’re choosing and why.
The shading audit before any purchase
No sizing formula survives a shaded roof. Before committing, stand on the roof at 10 AM, noon, and 3 PM on one clear day and note what casts shadows where — water tanks, parapet walls, the neighbour’s upper storey. Panels in shade for two midday hours lose far more than two hours’ proportion of output, since string wiring drags whole rows down with the shaded panel. If shade is unavoidable, specify optimisers or microinverters and accept the cost; if it covers most of the roof, the honest answer is a smaller system than the formula suggests.
More questions answered
Panels are rated at peak DC output that real conditions rarely deliver; inverters are sized to the AC output you’ll actually see. A modest DC-to-AC oversize (panels 10–25 percent above inverter rating) is standard practice and improves morning/evening yield — your installer proposing it is a good sign, not a trick.
South-facing at roughly latitude tilt is the Pakistani optimum the generation average assumes. East/west splits sacrifice 10–15 percent annual yield — compensate by sizing up half a kW per 3 kW of array if your roof forces the orientation.
Yes, if the inverter and structure anticipate it — expansion onto a maxed-out inverter means replacing it. Specify the target size now, install the inverter and mounting for it, and populate panels in stages. The incremental panel cost is the cheap part of any expansion.
A proposal more than 20 percent above this tool’s output needs a stated reason — planned consumption growth, poor orientation, heavy shading. "Headroom" without specifics is margin-stuffing. Below the tool’s number, ask which loads the system is deliberately not covering.