What size solar system do I need for my house?

Take three recent electricity bills, average the monthly units, and divide by roughly 120 to get the kilowatts needed in this region. A household averaging 600 units a month lands at about 5 kW. Roof area only becomes a constraint after that number exists.

Is it better to oversize a solar system?

Only if you have net metering approved, so surplus becomes credit rather than waste. Without it, an oversized system generates electricity nobody uses and you have paid for panels doing nothing after midday.

How much roof do I need per kilowatt?

Roughly 100 square feet per kilowatt once frame spacing and walking access are allowed for. So a 5 kW array needs about 500 square feet of genuinely unshaded roof, not 500 square feet of total roof.

What size solar system do I need?

It comes from your bill, not your roof. Three months of statements, one division, and you have the number. Below is the arithmetic, then three real households in this district worked through end to end.

The whole method

Step 1
Average three bills
Read the units consumed, not the rupee total. Tariffs move; units describe you.
610 units / month
Step 2
Divide by 120
A rough regional figure covering annual sun hours, losses and winter shortfall.
610 ÷ 120 = 5.1
Step 3
That is your kilowatts
Round to the nearest common inverter size. Now, and only now, check the roof.
5 kW system

The divisor is a regional rule of thumb, not physics. It is close enough to argue with a salesman and not precise enough to order equipment on — that is what the survey is for.

Three households, worked through

Same method, three very different answers. Notice that the deciding factor is not house size — it is when the electricity is used.

Three worked solar sizing examples for households and a shop in the Talagang and Chakwal area
Premises	Two-bedroom home, TalagangLights, fans, fridge, TV. No air conditioning.	Family home, ChakwalTwo air conditioners run through summer afternoons. Four fans, pump, fridge.	General store, BalkassarChillers running continuously, lighting, ceiling fans, open 10 hours.
Monthly units	220	610	1,150
When it is used	Evening	Afternoon	All daylight hours
System size	2 kW	5 kW	10 kW
Installed cost	PKR 250,000 – 340,000	PKR 550,000 – 750,000	PKR 1,000,000 – 1,400,000
Our reading	On-grid only. Consumption is mostly after dark, so a battery would do more for comfort than the panels do for the bill.	The classic case. Consumption peaks exactly when the sun is up, so an on-grid system offsets most of it and net metering credits the rest.	Best payback of the three. Load runs while the sun is up, so almost nothing is exported and almost nothing is wasted.

Then, and only then, the roof

Roof area is a constraint, not an input. If the roof cannot hold the size your bills call for, that is worth knowing — but it does not change what you need, only what is possible.

Space per kilowatt: About 100 sq ft, after frame spacing and access
A 5 kW array: Roughly 500 sq ft of unshaded roof
Shading: A parapet or water tank can drag a whole string down
Structure: A standard concrete slab carries it comfortably

Three ways sizing goes wrong

Sized from the roof

The installer looked at the slab, counted how many panels fit, and quoted that. You end up paying for generation you cannot use, or being short because the roof is small.

Sized from a package

“5 kW package” is a product, not an answer. It happens to be right for a lot of families and wrong for anyone whose consumption sits outside the middle.

Sized on summer

A system that covers your July bill will fall well short in December, when days are short and the sun is low. Size on the annual average or plan to be disappointed at exactly the wrong time of year.

Questions on sizing

What size solar system do I need for my house?: Take three recent electricity bills, average the monthly units, and divide by roughly 120 to get the kilowatts needed in this region. A household averaging 600 units a month lands at about 5 kW. Roof area only becomes a constraint after that number exists.
Is it better to oversize a solar system?: Only if you have net metering approved, so surplus becomes credit rather than waste. Without it, an oversized system generates electricity nobody uses and you have paid for panels doing nothing after midday.
How much roof do I need per kilowatt?: Roughly 100 square feet per kilowatt once frame spacing and walking access are allowed for. So a 5 kW array needs about 500 square feet of genuinely unshaded roof, not 500 square feet of total roof.