For a home, the yearly units on the bill are most of the story. For a factory on a time-of-day tariff they're only the start. When you use power matters as much as how much.
The three blocks on your bill
Many commercial and industrial connections in West Bengal are billed in three time-of-day blocks:
- Normal: 6 am to 5 pm
- Peak: 5 pm to 11 pm, the most expensive
- Off-peak: 11 pm to 6 am, the cheapest
Your bill shows the units used in each block. If yours doesn't, ask us and we'll help you get the break-up.
How solar units are set off
The 2025 WBERC rooftop solar regulations spell out the order. Units generated in a block are first set off against units consumed in the same block. If there's surplus, it moves to the next lower-priced block, and so on. Anything left carries forward to the next bill.
Solar generates during the normal block. So surplus can flow down into your off-peak units, but it can never reach the peak block, which costs more. Physically that makes sense too: the sun has set by the time peak hours start.
What this means for plant size
Size the plant to your normal-block consumption first. That's where every solar unit is worth the full normal rate. Spill-over into off-peak units still saves money, but each of those units is worth less.
In one factory we studied recently, roughly a quarter of all units were used in peak hours. Those units were out of solar's reach entirely, so the useful plant size came down by about a fifth compared with sizing to total consumption. The smaller plant also paid for itself faster than the bigger one would have.
If you're thinking of moving off the time-of-day category, do it before you apply for net metering. Afterwards the rules lock you in.
Can a battery help?
A battery is the only way to get solar into peak hours: charge it in the day, run the evening shift from it. That can make sense when the evening load is steady, but it should be sized to the actual evening load rather than to the yearly peak units. We look at it separately so it's judged on its own merits.
The approvals that come with size
- Net metering in West Bengal runs up to 500 kW, or your contract demand, whichever is lower.
- 100 kW and above needs approval from the Chief Electrical Inspector (CEIG) before the plant is switched on.
- Plants up to about 170 kW can usually connect at low tension. Bigger plants connect at 11 kV, with the net meter on the HT side.
We file all of this, but the timelines are set by WBSEDCL and the inspectorate, so build them into your plans.
Diesel generators
Most factories in Bengal keep a DG set for outages. If solar keeps running while the DG is on, it can push power back into the generator and damage it. A DG synchronisation controller prevents that by holding solar output below the generator's safe minimum load. We include one on every site with a DG.
Seasonal factories
Tea factories, rice mills and cold stores often run hard for part of the year and ease off for the rest. Because banked units reset on 1 April, the plant should be sized against the whole year, not the busy season. We model it month by month so the quiet months don't quietly waste generation.
What we need from you
Twelve months of bills (or as many as you have), your contract demand, a rough idea of shift timings, and whether you run a DG. With that we can tell you the right size, how much of each block solar can reach, and what the plant will do month by month. Send them over. The wider rules are in net metering in West Bengal.
Quick answers
What size of solar plant does a factory need?
Size it to your normal-block (6 am to 5 pm) consumption first. Under West Bengal's set-off rules, solar cannot reach peak-hour units without a battery.
Is CEIG approval required for rooftop solar in West Bengal?
Yes, for plants of 100 kW and above, before the plant is energised.
Can a solar plant run alongside a diesel generator?
Yes, with a DG synchronisation controller that stops solar from back-feeding the generator.