Energy per unit of output is your total monthly energy (grid plus DG) divided by what you made that month, in kWh and in rupees per piece or per kg. With 60,000 kWh from the grid at ₹7 a unit plus ₹75,000 of demand and fixed charges, 3,000 kWh from a DG that burned ₹90,000 of diesel, and 45,000 pieces made, each piece carries 1.40 kWh and ₹13.00 of energy. If idle machines use 15% of the grid units and you cut that idle energy by half, the saving is ₹0.70 a piece, or ₹3.78 lakh a year. This calculator works it out from your own bill, your DG log and your output.
How the calculator works
The calculator takes eight inputs, all for the same month:
- Grid kWh a month. Units consumed, from the electricity bill. If your HT bill is in kVAh, use the kVAh figure and enter the rate per kVAh below; the grid part of the result is then in kVAh, which is what you actually pay for. The DG figure stays in kWh from its meter.
- Energy charge per unit (₹/kWh). The variable part of the bill divided by units: energy charge, plus fuel and power purchase adjustment, plus electricity duty, less any rebates. Leave out the demand charge here; it goes in the next field.
- Demand and fixed charges a month (₹). The demand charge on your contract demand (kVA) plus customer or meter charges. This part of the bill does not move when you use fewer units in the month.
- DG kWh a month. From the DG set's energy meter or log.
- Diesel cost a month (₹). What you paid for the diesel the DG burned that month.
- Output a month, in pieces or kg. Good output only. Pick the unit from the list. Counting rejects inflates output and hides the energy they used.
- Share of grid kWh used by idle machines (%). The part of your units drawn by machines that are switched on but not producing. The idle machine energy cost calculator gives idle kWh a year; divide that by 12, then by your monthly grid units.
- Cut in idle-machine energy (%). How much of that idle energy you expect to remove, for example by switching off during long waits or unloading compressors.
The formulas:
Grid cost = grid kWh × energy charge + demand and fixed charges
All-in grid cost per kWh = grid cost ÷ grid kWh
DG cost per kWh = diesel cost ÷ DG kWh
kWh per unit = (grid kWh + DG kWh) ÷ output
₹ per unit = (grid cost + diesel cost) ÷ output
kWh saved = grid kWh × idle share × cut
Saving per month = kWh saved × energy charge (yearly = × 12)
The saving is priced at the energy charge only, not at the all-in rate. Switching idle machines off removes units from the bill, but the demand charge on your contract demand usually stays where it was, because idling rarely sets your peak. Pricing the saving at the all-in rate would overstate it. The saving is also counted on grid units only; idle machines running through a power cut on DG burn diesel too, so the real saving can be a little higher.
Worked example (India)
A component plant on an HT connection with a DG set for cuts. All volume and cost inputs are assumptions for illustration and are the calculator's defaults, except where a source is given.
About the tariff. Each state commission sets industrial HT tariffs in its own order. As one example, the Odisha Electricity Regulatory Commission's retail supply tariff for FY 2026-27, effective 1 April 2026, sets HT Large Industry at a demand charge of ₹250 per kVA per month and an energy charge of 585 paise per kVAh up to 60% load factor and 475 paise above it, with electricity duty charged on top (OERC tariff notification for FY 2026-27). Rates, slabs and billing units differ by state, so take yours from your own bill.
| Input | Value | Basis |
|---|---|---|
| Grid kWh a month | 60,000 kWh | assume |
| Energy charge per unit | ₹7.00/kWh | assume, all-in with FPPA and duty |
| Demand and fixed charges a month | ₹75,000 | assume 300 kVA billing demand × ₹250/kVA (Odisha HT Large Industry rate above); the ₹250 a month customer service charge is left out as too small to matter |
| DG kWh a month | 3,000 kWh | assume |
| Diesel cost a month | ₹90,000 | assume |
| Output a month | 45,000 pieces | assume |
| Idle share of grid kWh | 15% | assume |
| Cut in idle energy | 50% | assume |
| Output | Working | Result |
|---|---|---|
| Grid cost a month | 60,000 × ₹7 + ₹75,000 | ₹4,95,000 |
| All-in grid cost per kWh | ₹4,95,000 ÷ 60,000 | ₹8.25 |
| DG cost per kWh | ₹90,000 ÷ 3,000 | ₹30.00 |
| Energy cost a month | ₹4,95,000 + ₹90,000 | ₹5,85,000 |
| kWh per piece | 63,000 ÷ 45,000 | 1.40 kWh |
| Energy cost per piece | ₹5,85,000 ÷ 45,000 | ₹13.00 |
| kWh saved a month | 60,000 × 15% × 50% | 4,500 kWh |
| Saving a month | 4,500 × ₹7 | ₹31,500 |
| Saving per piece | ₹31,500 ÷ 45,000 | ₹0.70 |
| Saving a year | ₹31,500 × 12 | ₹3.78 lakh |
Three things stand out.
The DG is the expensive kWh. In this example the DG supplies under 5% of the units (3,000 of 63,000) but about 15% of the energy cost (₹90,000 of ₹5,85,000), because a DG unit costs ₹30 against ₹8.25 from the grid. A month with more power cuts raises ₹ per piece even if the plant uses no more energy. The power cut and DG downtime cost calculator shows the other half of that cost, the output lost while lines stand.
The demand charge is fixed per month, not per piece. ₹75,000 spread over 45,000 pieces is ₹1.67 a piece. In a slow month with 30,000 pieces it becomes ₹2.50 a piece. Low output makes every piece carry more of the fixed charges.
Idle energy is a production question. The ₹3.78 lakh comes from switching off machines that are not making anything. Before you cut, find out why they idle: waiting for material, for an operator, for a first-piece approval. How to see machine idle hours from your phone covers how to get those hours without sensors.
Using the number
- Track it monthly, per product family. One plant-wide figure mixes heavy and light products. If your output mix changes, kWh per piece moves even when nothing got better or worse. Run the calculator for each line or product family where you can split the meter.
- Compare like with like. Use kWh per kg for process industries such as spinning, castings or extrusion, and kWh per piece for assembly and machining. The guide on where a spinning mill loses money uses power per kg as one of its core numbers.
- Separate base load from production load. Lights, compressors, chillers and idle machines draw power whether or not you make anything. Idle machines, lights and compressors: the energy bill cameras can explain shows where that base load hides.
What this number does not tell you
- Why energy per unit changed. A higher figure can come from more power cuts, more idle time, a heavier product mix, more rejects or a lower output month spreading fixed charges thinner. The calculator gives the figure; the reason needs the meter data and the production record side by side.
- Time-of-Day effects. A flat energy charge averages peak and off-peak rates. If your idle hours fall in peak slabs, each idle unit costs more than the average.
- Load factor and power factor rebates. Some tariffs pay a lower rate above a load factor threshold, and kVAh billing charges you for poor power factor. Both change your real rate per unit; the bill shows which apply.
- What a camera cannot see. A camera can show when each machine was running, idle or stopped, which is the honest source for the idle share. It cannot read your energy meter, your DG fuel use or your tariff. Those come from the bill, the DG log and the meter.
Related guides
- Idle machine energy cost calculator
- Power cut and DG downtime cost calculator
- Power Cuts and DG Set Limits in 2026: What Idle Hours Cost NCR and Punjab Factories (₹)
- Idle Machines, Lights and Compressors: The Energy Bill Cameras Can Explain
- Machine Hour Rate in India: Count the Hours You Never Sold
- OEE calculator
FAQ
How do I calculate energy per unit of production? Add the month's grid units and DG units, then divide by good output for the same month. For cost, add the full electricity bill (energy charges plus demand and fixed charges) and the month's diesel cost, then divide by the same output. With the default inputs that is 63,000 kWh ÷ 45,000 pieces = 1.40 kWh and ₹5,85,000 ÷ 45,000 = ₹13.00 a piece.
Should demand charges be included in energy cost per piece? Yes, for the cost per piece, because you pay them every month. No, for the saving from switching off idle machines, because the demand charge depends on your contract demand and peak, not on the units you save. That is why the calculator prices the saving at the energy charge only.
Why does DG power cost so much more than grid power? A DG unit is paid for in diesel, and in this example diesel works out to ₹30 per kWh against ₹8.25 all-in from the grid. Divide your monthly diesel spend by the DG's kWh for your own figure.
How much can cutting idle-machine energy save? It depends on how much of your energy goes to idle machines. In the example, idle machines take 15% of grid units; halving that saves 4,500 kWh and ₹31,500 a month, ₹0.70 a piece and ₹3.78 lakh a year. Measure the idle share first with the idle machine energy cost calculator.
Mama reads a factory's existing cameras and each morning tells the owner where machine hours went idle the day before, which is the input this calculator needs for the idle share. Mama can draw a camera plan for the machines that idle longest. Leave an email or phone in the form below.