How to read your electricity bill and size a system
Every honest solar quotation starts from your electricity bill, because three numbers on it decide what capacity makes sense and what your DISCOM will actually permit. Anyone who quotes you a system size before seeing those numbers is guessing - and you are the one who lives with the guess for twenty-five years.
The three numbers that matter
1. Units consumed per month
Printed as units, kWh, or consumption. This is how much electricity you actually used in the billing period, and it is the single most important input to sizing. One month is not enough on its own - a March bill and a May bill can differ by half in a Gujarat household, because of air conditioning. Take an average across as many months as you have, ideally a full year.
2. Sanctioned load
Printed in kW or sometimes as a connected-load figure. This is the maximum load your connection is officially rated for, and it caps what the DISCOM will approve. A connection sanctioned for 3 kW will generally not be permitted a much larger plant without a load enhancement application, which carries its own charges and delay. Check this before you fall in love with a bigger system.
3. Tariff category
Residential, commercial, or an industrial or HT category. It determines the rate per unit you are paying, and therefore what each solar unit is worth to you. It also determines whether you are eligible for the PM Surya Ghar subsidy at all - the scheme covers residential connections only.
While you have the bill in hand, note your consumer number too. Every application, from net metering to the subsidy claim, is filed against it.
The sizing arithmetic
In Ahmedabad's irradiation conditions, a well-installed, shadow-free rooftop plant generates roughly four units per kilowatt per day across the year - about 120 units a month per kW. Actual output varies with weather, dust, orientation, tilt and grid availability, but this is a reliable planning figure.
The rule of thumb:
Monthly units ÷ 120 = approximate kW required
A worked example
Say your bills average 300 units a month.
- 300 ÷ 120 = 2.5 kW to offset roughly all of your consumption.
- At about 10 square metres of shadow-free area per kW, you need roughly 25 square metres of clear roof.
- Your sanctioned load needs to support it - if it is 2 kW, that is your practical ceiling until you enhance it.
- Modules come in fixed wattages, so the real system lands on a nearby size. A plant built from three 620 Wp modules, for instance, is 1.86 kW - not a round number, and that is completely normal.
Check the roof, not just the bill
| What to check | Why it matters |
|---|---|
| Shadow-free area | Roughly 10 sq.m. per kW. Shade from a water tank, staircase room, parapet or neighbouring building can disproportionately cut output from an entire string, not just the shaded module. |
| Roof type and condition | RCC slab, metal shed or tiled roof each need a different mounting approach. An old roof due for waterproofing should be dealt with before a plant goes on top of it. |
| Orientation | South-facing arrays generate best in India. East or west facing still works, with somewhat lower output. |
| Distance to the meter | Long cable runs mean voltage drop and thicker, costlier cable. Most quotations include evacuation up to a stated distance and charge beyond it. |
| Future plans | An additional floor, a new tank, or a tree that will grow - all of these change the shadow map in a few years. |
Why bigger is not automatically better
It is tempting to size up. Three things argue against it for a household:
- The subsidy is capped. Under PM Surya Ghar the household ceiling is reached at 3 kW. Capacity beyond that is entirely at your own cost.
- Exported units are usually worth less than the units you offset by consuming your own generation. A plant that massively overshoots your consumption exports a lot at a lower value.
- Your sanctioned load and the DISCOM's feasibility assessment may not permit it anyway.
For a business the calculation is different - higher tariffs and daytime load usually mean the constraint is roof area rather than economics. That is covered on the business and industry page.
What this arithmetic will not tell you
It gives you a sensible ballpark and a way to sanity-check any quotation you are handed. What it cannot account for is your actual shading, roof structure, meter position, phase configuration and DISCOM feasibility - all of which need someone standing on the roof with a tape measure. That is the difference between an estimate and an assessment.
Our free assessment does the second part. Send your bill on WhatsApp and we will work through the consumption, sanctioned load and tariff, tell you the capacity that actually fits, and put it in writing - including where we think you should install less than you were hoping for.