Solar · sizing
What size solar system do I need?
Average the units on your last three IESCO bills and divide by about 120. An F-sector house on 650 units a month lands at 5 kW; a G-11 apartment on 250 units at 2 kW; a Blue Area office or Bahria Town villa around 1,200 units at 10 kW. Roof area is a constraint checked afterwards, not the input.
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The whole method
Three bills, one division
- Step 1
Average three bills
Read the units-consumed line, never the rupee total. Tariffs and taxes wander month to month; your units describe you.
650 units / month
- Step 2
Divide by 120
One installed kilowatt yields roughly 120 units a month averaged across the Islamabad year — winter haze and system losses included.
650 ÷ 120 = 5.4
- Step 3
Round to an inverter size
Common sizes run 2, 3, 5 and 10 kW. Take the nearest, then — and only then — check whether the roof holds it.
→ 5 kW system
The divisor is a rule of thumb for this latitude — sharp enough to test a quote against, too blunt to order equipment with. That last step is what the free survey is for.
Three bills from this city, worked through
Same method, three very different answers. The deciding factor is not the size of the building — it is when the electricity gets used. A Bahria Town villa with pool pumps and ACs running all day lands in the same 10 kW column as the office.
| Premises | Apartment, G-11 MarkazLights, fans, fridge, TV; one inverter AC used sparingly. | House, F-11Two inverter ACs through summer afternoons, water pump, two fridges. | Office, Blue AreaACs, thirty workstations and a server room, nine to six, six days. |
|---|---|---|---|
| Monthly units | 250 | 650 | 1,200 |
| When it is used | Evenings | Afternoons | Working hours |
| System size | 2 kW | 5 kW | 10 kW |
| Installed cost | PKR 350,000 – 430,000 | PKR 550,000 – 750,000 | PKR 1,000,000 – 1,400,000 |
| Our reading | Marginal for on-grid alone, because most of the load runs after sunset. Net metering credit — or a small battery — is what makes this one add up. | The textbook case. The ACs pull hardest exactly when the array produces most, so the panels eat the expensive units directly. | The quickest payback on this page. Nearly every unit is consumed the hour it is generated, so nothing waits on export credit. |
Then, and only then, the roof
The roof can veto the size; it cannot choose it. If the slab cannot carry what the bills call for, that is worth knowing early — it changes what is possible, not what you need.
- Per kilowatt
- About 100 sq ft of clear, unshaded slab
- A 5 kW array
- Roughly 500 sq ft after walkways and frame gaps
- Shade
- A mumty, tank or the taller plaza next door drags a whole string down
- Plazas and rentals
- Landlord permission before any drilling — standard in markaz buildings
Three ways sizing goes wrong
Sized off the roof
Counting how many panels fit the slab answers the wrong question. You either pay for generation you cannot use, or discover too late that the roof was never the limit.
Sized off a package
A 5 kW complete package is stock, not advice. It happens to fit the median family house and quietly fails the G-11 apartment and the Blue Area office at either end.
Sized off June
A system matched to a summer bill runs short from November to February, when Islamabad days are short and often hazed. Size on the annual average and let summer be the bonus.
Questions on sizing
What size solar system does an Islamabad house need?
Average the units on three recent IESCO bills and divide by roughly 120. A house averaging 650 units a month lands at about 5.4, so a 5 kW system; 250 units points to 2 kW; 1,200 units to 10 kW. The roof only enters the conversation after that number exists.
Should I oversize the system to be safe?
Only with IESCO net metering approved, so midday surplus becomes credit instead of waste. Without the bidirectional meter, an oversized array spends every afternoon generating units that nobody uses and nobody pays for.
How much roof does a system take?
Plan on about 100 square feet per kilowatt once frame spacing and walking room are counted. A 5 kW array wants roughly 500 square feet of genuinely unshaded slab — shade from a mumty, a water tank or the plaza next door counts against it.
Does winter change the size?
It changes the output, which is why the sizing uses an annual average. December generation in Islamabad can fall to well under half of June's — short days plus haze. A system sized on a summer bill disappoints from November to February, on schedule.