Reading your industrial electricity bill before going solar

Before investing in solar, every industrial consumer should understand one document thoroughly: their electricity bill. It shows what you pay for, when, and why. That determines how much solar can actually save — and avoids the common mistake of assuming every unit of solar saves the full average tariff.

The main components

Industrial tariff structures are set by the state regulator and vary by consumer category, voltage and time. Your bill typically includes some or all of the following.

Energy charges

Charges per unit (kWh) of electricity consumed. This is usually the largest component and the one solar most directly reduces.

Demand charges

Charges based on the contracted demand or the maximum demand recorded in the billing period (in kVA or kW). These are charged regardless of how much energy you use.

Time-of-day (ToD) tariffs

Different energy rates at different times of day — for example, higher rates in peak hours and lower rates in off-peak hours. Whether and how ToD applies depends on your category and current tariff orders.

Power factor incentives or penalties

Adjustments based on how efficiently the facility uses electrical power. Poor power factor can attract penalties.

Taxes and duties

Electricity tax or duty as applicable.

Other charges

Meter rent, fuel cost adjustments, or other regulatory charges.

Check the current tariff order of the Tamil Nadu Electricity Regulatory Commission for the specific structure and rates applicable to your category.

What solar affects

Energy charges

Every unit of solar consumed on site displaces a unit bought from the grid. The saving per unit is the energy rate you would otherwise have paid at that time.

Demand charges

Maximum demand can occur when solar is low — early morning, evening, cloudy days. So solar may not reduce your recorded maximum demand. Demand charges are often unchanged after installing solar.

Time-of-day effects

Solar generates during the day. If peak tariff periods are in the morning or evening, solar may cover only part of them. Savings depend on how solar hours align with your tariff periods.

Using twelve months of bills

Gather at least a year of bills and record, month by month:

• Total units consumed
• Units in each ToD period, if applicable
• Maximum demand
• Energy charges, demand charges, other charges
• Power factor

This shows:

• Seasonal variation — consumption and solar generation both vary by season
• Effective energy rate — what you actually pay per unit of energy, excluding fixed charges
• Demand pattern — whether demand charges are a big share of the bill

Calculating realistic savings

A simple but realistic approach:

1. Estimate monthly solar generation from a site-specific study
2. Estimate how much is consumed on site at each time of day. See matching solar to your load profile.
3. Value each unit at the energy rate it displaces
4. Account for any export arrangements and their value. See net metering vs gross metering.
5. Don't assume demand charges fall

Using the average bill rate (total bill divided by total units) overstates savings, because it includes fixed charges solar doesn't reduce.

Interval data

Many industrial meters record consumption in short intervals. Interval data — consumption every 15 or 30 minutes — gives a much better picture than monthly bills. Ask your distribution company or check your metering system for access.

Beyond solar

Your bill may reveal other opportunities:

• Power factor correction to avoid penalties. See power quality at solar industrial sites.
• Load shifting to off-peak periods
• Demand management to reduce maximum demand
• Battery storage for peak shaving. See battery storage for industrial sites.
• Off-site renewables if your roof is too small. See open access vs group captive.

Common mistakes

• Using average bill rate to value solar
• Expecting demand charges to fall
• Ignoring ToD periods
• Using one month's bill instead of twelve
• Not checking current tariff orders

Preparing for a solar proposal

When you share bills with a solar provider, share all twelve months, interval data if available, your sanctioned and contracted demand, and any planned changes in operations. Ask the provider to show, line by line, which bill components their savings estimate assumes will change. A proposal that cannot explain this clearly is probably using the average bill rate, and overstating the benefit.

How we help

Through our C&I solar work with industrial consumers, we help analyse bills and consumption patterns to build realistic savings estimates, and coordinate EPC partners or developers for the right solution.