Sizing a system from the customer's electricity bill

The fastest way to tell a serious rooftop salesperson from an order taker is what they ask for first.

The order taker asks how much roof there is. The serious one asks for twelve months of bills, because the bill decides the system and the roof only decides whether it fits.

Six numbers, not one

Most people read the units consumed and stop. A bill carries at least six things that change your answer.

1. Units consumed, month by month. One month is not enough. You need the shape of the year, because a household that uses 200 units in December and 600 in May is a different design problem from one that sits flat at 400.
2. Sanctioned load. Most DISCOMs will not approve a system larger than the sanctioned load, and several tie the net metering approval to it. A customer wanting 8 kW on a 5 kW sanctioned load needs a load enhancement application first, which adds weeks and sometimes a deposit.
3. Connection category. Domestic, commercial, industrial or agricultural. This sets the tariff, the applicable net metering rules and whether the customer can claim input tax credit.
4. The tariff slabs. Indian domestic tariffs are usually telescopic, so the units you displace come off the top slab first. A customer paying Rs 8 for their last hundred units and Rs 4 for their first hundred saves at Rs 8, and quoting an average rate understates the saving.
5. Fixed and demand charges. These do not go away when you install solar. A proposal that shows the bill going to zero when Rs 900 of fixed charges remain is a complaint waiting to happen.
6. Phase and meter type. Single phase or three phase decides the inverter. Whether the meter is already bidirectional decides part of the timeline.

The arithmetic, in the order it should be done

Start from annual units rather than from a rule of thumb per kW of roof.

Take the annual consumption. Divide by the annual yield you actually model for that location, which in much of India sits somewhere between 1,300 and 1,500 units per kW per year once losses are applied. Use your own modelled figure rather than a national average, because coastal humidity, dust and shading move it.

A household using 400 units a month uses 4,800 a year. At 1,400 units per kW, that is about 3.4 kW of array to cover the whole bill.

Now apply the constraints, in this order:

• Sanctioned load. If it is 3 kW, the conversation changes to load enhancement before it changes to panels.
• Subsidy shape. Central assistance for a household caps at 3 kW. Going to 3.4 kW is fine, but the customer should understand that the last 0.4 kW carries no subsidy.
• Roof area and shade. Roughly 100 square feet per kW is a reasonable first pass for a fixed tilt array, before you subtract the water tank, the walkway and the shaded strip. Shading analysis without guesswork covers the part that takes bites out of this.
• Export economics. This is the newest constraint and the one most sizing habits have not caught up with.

Why 100 percent offset is no longer the automatic answer

The old default was to size the system to wipe out the annual bill and let the meter handle the rest.

That assumed an exported unit and an imported unit were worth roughly the same. Across a widening set of states and consumer categories, they are not. Where export is credited at less than the retail tariff, or where a charge is levied on generation, every unit the customer self consumes is worth more than every unit they export, and the optimal system is smaller than the one that zeroes the bill on paper.

Which means the consumption shape from point one becomes a design input rather than background. A house that is empty from nine to five exports most of what it makes. A house with someone at home, or a shop, or a small factory, consumes it. Those two should not get the same system even with identical annual units. The direction of travel on export rules is in net metering is being rewritten.

What to ask when the customer has no bills

It happens constantly, especially with new construction and with customers who pay through an app and never look.

Three fallbacks, in order of quality:

1. The DISCOM portal. Most now show twelve months of consumption history against the consumer number, which is on any old bill or the meter.
2. An appliance count. Air conditioners, pumps, geysers, refrigeration, with rough run hours. Crude, but it catches the big loads.
3. The neighbour. For a similar house on the same street, the bill is a reasonable proxy for a first pass, clearly labelled as an estimate.

Never size from the roof alone. A 10 kW system on a house that uses 2 kW worth of electricity is how an EPC gets a bad review after doing technically competent work.

The short version

• Read six things off the bill: monthly units across a year, sanctioned load, connection category, tariff slabs, fixed and demand charges, and phase.
• Size from annual units divided by your modelled yield per kW, not from roof area.
• Apply constraints in order: sanctioned load, subsidy cap, roof and shade, export economics.
• Telescopic tariffs mean displaced units come off the top slab. Averaging the rate understates the saving.
• Fixed charges survive the installation. Show them in the post solar bill.
• Where export is worth less than import, the right system is smaller than the one that zeroes the annual bill.