How to Size a Home Solar System in Punjab

The usual advice, “work out your daily use and divide by sun hours”, is fine in most of the world. In Punjab it leads to oversized systems, because it ignores the 300 free units. Here is a method that does not.

1. Read the right figures from your bill

• Units consumed. Current reading minus previous reading. PSPCL home bills usually cover two months: halve the figure for a monthly number.
• Sanctioned load, in kW. Your system cannot be larger than this.
• Category. DS is domestic supply. Check whether you are general category, or SC, BC, non-SC BPL or Freedom Fighter, because the free units work differently.

Collect a year of bills if you can. Use varies a lot between a July and a January.

2. Check whether solar can save you anything

If your monthly use is 300 units or less, your bill is already zero. Solar has nothing to save. Stop here unless you want backup for power cuts; see hybrid systems.

3. Aim for the line, not for zero units

For a general-category home over the line, every unit is charged. Bring the net units back under 300 and the whole bill goes. So the goal is not to generate everything you use; it is to generate enough to get under the line, with some headroom for the hot months.

We aim for a net of about 250 units a month:

System size (kW) ≈ (your monthly units − 250) ÷ 102

102 units per kW a month is a conservative figure for Punjab after heat, dust and wiring losses. Round the result up to the next available size. Which sizes are available depends on the system type you choose in step 5: on-grid starts at 3 kW, but hybrid and off-grid both go down to 1 kW.

Example. A home using 500 units a month: (500 − 250) ÷ 102 ≈ 2.5, rounded up to 3 kW — on-grid or hybrid, both work, both subsidised. A home at 320 units works out to less than 1 kW on the same formula. On on-grid that still means a 3 kW system, because that is on-grid’s floor; on hybrid it can genuinely be a 1 kW system, still fully subsidised and still enough to take the bill to zero. Covering all 500 units of use, rather than just clearing the line, would need about 5 kW, and the extra capacity would mostly export surplus to PSPCL for little in return.

4. Check the limits

• Sanctioned load. If the size from step 3 is bigger, either install up to your sanctioned load, or apply to PSPCL for load enhancement first. Applying for more than your sanctioned load gets subsidy applications rejected.
• Roof area. Allow about 100 sq ft of shade-free roof per kW. Water tanks, stair rooms and a neighbour’s taller building all cast shade; panels in shade produce far less.

5. Choose the system type

On-grid is the cheapest per kW and gets the PM Surya Ghar subsidy, but only from 3 kW upwards. Hybrid gets the same subsidy and also works below that — down to 1 kW — plus it keeps essentials running in a power cut. Off-grid, only where the grid is unreliable or absent; it does not receive the subsidy, at any size. See how the three differ.

Mistakes we see

• Sizing from one summer bill, or one winter bill, instead of a year.
• Sizing to cover 100% of use when the free units make most of that capacity pointless.
• Ignoring sanctioned load until the subsidy application is rejected.
• Forgetting that a two-month bill needs halving.

The calculator runs all five steps on your own figures in a few seconds.