How to Size a Home Solar System in Punjab

The usual advice is to work out your daily use and divide by the hours of sun. That works in most of the world, but in Punjab it leads to oversized systems, because it ignores the 300 free units. The method below allows for them.

1. Read the right figures from your bill

• Units consumed: the current reading minus the previous one. PSPCL home bills usually cover two months, so halve the figure to get a monthly number.
• Sanctioned load, in kW. Your system can’t 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 for each.

Collect a year of bills if you can, since use varies a lot between July and January.

2. Check whether solar can save you anything

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

3. Aim to get under the line

For a general-category home over the line, every unit is charged, and bringing net units back under 300 removes the whole bill. You don’t need to generate everything you use, only 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. The sizes on offer depend on the system type you choose in step 5: on-grid starts at 3 kW, while hybrid and off-grid both go down to 1 kW.

Take a home using 500 units a month: (500 − 250) ÷ 102 ≈ 2.5, rounded up to 3 kW. On-grid or hybrid both work, and both are 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 the smallest on-grid size, but on hybrid it can be a 1 kW system, still fully subsidised and still enough to take the bill to zero. Covering all 500 units of use 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, and 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, goes down to 1 kW, and keeps essentials running in a power cut. Off-grid only makes sense where the grid is unreliable or missing, and it gets no 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.