Time of Day Tariff Calculator

The gap is the point, not the peak rate

A Time of Day tariff charges different rates at different hours — typically a surcharge of 15–25 % during morning and evening peaks, the base rate through the day, and a rebate of a similar size overnight.

Most reactions to a ToD tariff focus on the peak surcharge, which is the wrong number. What matters is the difference between the peak and off-peak rates, because that difference is what a shifted unit earns. Everything else is just the bill.

If a unit costs 20 % more at peak and 20 % less overnight, moving it saves 40 % of the base rate — not 20 %. That is the entire economics of load shifting in one line.

Who is being paid, and for what

A ToD tariff is not a penalty. It is the utility passing on its own cost structure: evening peaks are met by the most expensive generation on the system, and overnight there is surplus capacity nobody is using. You are being offered a discount for rescheduling.

Two consequences follow. A plant that genuinely cannot move anything is simply paying more, and should say so when the tariff is negotiated. A plant with any flexible load is being handed a saving that costs nothing but a change to a timer.

What can actually shift

The honest input to this calculator is not “what fraction of our load is in the peak band” but “what fraction could we move without anybody noticing”. On most plants that is 10–30 % of the peak, and it is worth measuring rather than guessing — the method is in turning meter data into savings.

Shifting does not touch the demand charge

This is the trap. A ToD saving is an energy saving: you consume the same units, later. The demand charge is billed on the highest recorded kVA in the month, and moving units between bands does nothing to it unless the peak demand itself falls.

Worse, careless shifting can make demand worse. Move three batch processes to the same off-peak hour and you have built a new peak overnight — one that sets your demand charge for the month while saving a few hundred rupees on energy.

Stagger what you shift. And if the demand charge is the larger line on your bill, the priority is cutting the peak, not moving units — which usually means power factor correction first, because that reduces kVA without touching production at all. Work out what it is worth with the power factor calculator.

Where storage changes the arithmetic

A battery turns a ToD tariff into an arbitrage: charge at the off-peak rate, discharge during the peak. The margin per unit is the rate gap, less round-trip losses of 10–20 %.

Run the numbers before believing it. A gap of ₹2 per unit on a 100 kWh bank cycled daily is a few thousand rupees a month against a capital cost in lakhs — payback measured in years, and the bank wearing out on daily cycling. Storage for ToD arbitrage alone rarely pays; storage that also provides backup, or that soaks up surplus solar, often does. Size it with the battery bank calculator, which applies the depth-of-discharge and rate effects that make nameplate Ah misleading.

Before you act on the number

• Get the actual band timings from your tariff order. They differ by state and by season, and some tariffs have two peak windows a day.
• Get the band split from interval data, not from an estimate. Your meter records it; the DISCOM portal usually exposes it.
• Check whether the demand charge is also time-differentiated. Some tariffs bill peak-hour demand separately, which changes the priority entirely.
• Confirm the meter supports ToD registers before designing around it — see choosing an energy meter.

The meter has to support it before any of this is real

Time-of-day billing needs a meter that records consumption against a clock, which is not what every connection has. A standard single-rate meter accumulates one register, and no amount of load shifting behind it changes the bill by a paisa.

A TOD-capable meter keeps separate registers per time block and switches between them on an internal real-time clock. That clock is the part worth being sceptical about. It drifts, it has to survive supply outages on a backup cell, and the block boundaries are set by the utility and occasionally revised. A meter whose clock has drifted by twenty minutes bills a portion of your peak consumption at the off-peak rate, or the reverse, and neither party will notice until a dispute brings it up.

Which makes two things worth doing before restructuring a shift around the numbers on this page. Confirm the connection actually has a TOD meter and that the registers are being read separately, because a TOD meter billed on total consumption is common enough to check for. And ask for the block timings in writing, since the seasonal variation applied in several states means the schedule you were quoted may not be the one in force.

Seasons, and the day the arithmetic changes

Most Indian TOD schedules are not constant through the year. The peak window follows the state load curve, and that curve moves — evening peaks lengthen in winter as lighting load arrives earlier, while summer peaks broaden into the afternoon as cooling load dominates.

Some states publish two or three seasonal schedules with different block boundaries and different multipliers. Others keep the timings fixed and vary the rates. A few have introduced a solar-hours block in the middle of the day at a discount, reflecting surplus generation — which inverts the traditional advice, because the cheapest electricity of the day may now be at noon rather than at night.

The practical consequence is that a saving calculated once is a snapshot. A process rescheduled to exploit a night rate is exposed if the tariff order revises that block, and tariff orders are issued annually. Where the change being contemplated is expensive or hard to reverse — a shift pattern, a thermal store, a battery — it is worth checking how stable the schedule has been across the last few orders rather than assuming the current one is permanent.