Two houses, same 5 kW system, same sunshine. One saves Rs 45,000 a year, the other Rs 28,000. The difference is not the equipment. It is what their states do with the extra units they send back to the grid.
The three systems
Net metering
Your extra units are banked at the same rate you pay. Generate 100 extra units in April, and in July you can draw 100 units free. Your meter effectively runs backwards.
This is the best deal for you. Building a slightly bigger system makes sense.
Net billing
Your extra units are bought at a lower rate than you pay, often half or less. You buy at Rs 8, you sell at Rs 3.
Build only what you will use. Exporting is a poor deal here.
Gross metering
Everything you generate is sold to the DISCOM at a fixed rate. You keep buying all your own electricity at retail. There is no self consumption at all.
Only worth it if the fixed rate is generous, which it usually is not for homes.
What it does to system sizing
Take a home using 400 units a month:
| System | Net metering | Net billing |
|---|---|---|
| 3 kW (matches usage) | Good | Good |
| 5 kW (some export) | Still good | Poor value |
| 8 kW (heavy export) | Reasonable | Bad idea |
Under net metering, a bigger system is a reasonable bet. Under net billing, everything above your own usage earns you very little. This is the single most important thing to check before choosing a size, and most installers will not raise it.
Banked units expire
Under net metering your banked units usually get settled at the end of the financial year, often at a low rate. They do not carry forward forever.
So even with net metering there is no point sizing a system to bank huge amounts. Match it to what you use across a year.
What to ask your DISCOM
Before you sign anything with an installer, get these answered:
- Is it net metering, net billing or gross metering?
- What rate are exported units credited at?
- Is there a cap on system size relative to my sanctioned load?
- When are banked units settled, and at what rate?
- How long does net meter installation usually take?
The short version
Net metering: you can size a little above your usage.
Net billing: size to your usage, not above.
Gross metering: do the maths carefully, it often does not work for homes.
Find out which one applies to you first. Everything else about your solar decision follows from it.