MDI in your electricity bill: what it means and how solar affects it

If you run a shop, clinic, office or small factory, your electricity bill probably shows a figure labelled MDI, or "maximum demand". Many people ignore it because it is not counted in units. But MDI can quietly add to your bill through demand charges, and in some cases penalties. It also affects how much rooftop solar can really save you.

This short guide explains what MDI means, why it matters, and what solar can and cannot do about it.

What MDI means

MDI stands for Maximum Demand Indicator. It records the highest average load your premises drew from the grid during the billing period, measured over short intervals (typically 15 or 30 minutes, depending on the meter and tariff rules).

It is shown in kW or kVA, not in units (kWh). The difference matters:

• Units (kWh) measure how much energy you used over the month, like the distance a car travels.
• MDI (kW or kVA) measures your peak rate of use during the month, like the car's top speed.

Two businesses can use the same number of units in a month but have very different MDI, if one switches everything on at once and the other spreads its use through the day.

Why MDI matters on your bill

For many commercial and industrial connections, the bill has two parts:

1. Energy charges: based on units consumed.
2. Demand (fixed) charges: based on your contracted or recorded maximum demand.

Depending on your tariff category and DISCOM rules, demand charges may be calculated on your recorded MDI, your contracted (sanctioned) demand, or a minimum share of it. If your MDI goes above your contracted demand, many tariffs charge extra, sometimes at a higher rate, as a penalty for exceeding the limit.

The exact rules differ between DISCOMs (TGSPDCL and TGNPDCL in Telangana; APSPDCL, APEPDCL and APCPDCL in Andhra Pradesh) and between tariff categories, and they change over time. Check your bill's tariff details or ask your DISCOM for the current rules that apply to your connection.

kW or kVA? Power factor in brief

Some meters record MDI in kVA rather than kW. kVA includes the effect of power factor: how efficiently your equipment uses current. Motors, compressors and some older equipment can have a poor power factor, which raises kVA demand for the same useful power. Improving power factor, usually with capacitor banks, can reduce kVA-based MDI. An electrician can advise on this.

What causes a high MDI

MDI is driven by moments when many large loads run together:

• several air conditioners starting at the same time;
• motors, pumps and compressors starting together;
• kitchen equipment, heaters or ovens switching on at once;
• start-up of machinery at the beginning of a shift.

Even a short spike, if it lasts the length of the measuring interval, can set the month's MDI.

How to reduce MDI without solar

Before thinking about solar, there are simple ways to lower MDI:

• Stagger start-ups. Do not switch on all ACs, pumps and motors at once.
• Schedule heavy loads so they do not overlap.
• Improve power factor (for kVA-based meters).
• Maintain equipment. Worn motors and dirty AC coils draw more.
• Review your contracted demand. If your real peak is well below it, or regularly above it, ask about adjusting it to match your use.

Does rooftop solar reduce MDI?

This is where people are often surprised. Solar always reduces the units you buy, but it does not always reduce your MDI.

When solar can help

If your peak demand happens during sunny daytime hours, solar supplies part of that load directly, so the grid sees less. A clinic or office whose peak is around midday, with ACs and equipment running, can see a lower recorded MDI.

When solar may not help

• If your peak is in the evening or at night, after solar output has fallen, MDI is unchanged.
• If a cloudy spell coincides with a peak, solar output drops and the grid supplies the load. Because MDI records the single highest interval in the month, one cloudy, busy interval can set it.
• If start-up spikes happen early in the morning, before solar ramps up.

This is why we never promise that solar will cut demand charges. The reliable saving is on energy charges: every unit your system produces is a unit you do not buy. Any MDI reduction is a bonus that depends on your load pattern.

Solar plus load management

The best results come from combining solar with smart use:

• Shift heavy loads into the middle of the day, when solar is strongest, but stagger them so they do not all start together.
• Pre-cool buildings in the late morning and early afternoon.
• Run pumps and batch processes in sunny hours.

Batteries can, in principle, shave peaks by supplying power during spikes, but for most small businesses the cost is hard to justify purely for MDI. Our solar battery guide explains the trade-offs.

What to check before going solar

1. Look at 12 months of bills and note your MDI each month.
2. Find out when your peaks happen. Your meter or DISCOM may provide interval data, and your facilities team will often know.
3. Compare your MDI with your contracted demand.
4. Size the solar system from daytime energy use, not from MDI.
5. Ask your installer to show savings separately for energy charges and demand charges, and to be clear about which are certain and which depend on your load pattern.

Our commercial rooftop solar guide covers sizing and savings in detail, and our commercial solar ROI calculator estimates energy savings. For planning, our calculators assume ₹10 per unit for commercial consumers.

A worked example

Imagine a small clinic with a contracted demand of 25 kW:

• Most days, its demand peaks around 18 to 20 kW in the early afternoon, when ACs and equipment run together.
• One day, at the start of the morning shift, all the ACs, a sterilizer and the water pump switch on within a few minutes. Demand reaches 27 kW for one measuring interval.
• That single interval sets the month's MDI at 27 kW, above the contracted 25 kW. Depending on the tariff, that can mean extra demand charges for the month.

What would help:

• Staggering the morning start-up by a few minutes would have kept MDI below 25 kW.
• A solar system would reduce the afternoon peak, when panels are producing strongly. It would not have helped with the early-morning spike, before solar output builds.

This is why load management and solar work best together.

Where to find interval data

To understand your peaks, you need to know when they happen. Options:

• some meters store load profiles that the DISCOM can share;
• some businesses install a simple energy monitor on the main supply;
• facilities staff often know when heavy equipment starts.

Even a rough picture of when your peaks happen tells you whether solar is likely to affect them.

How Origian helps

When we survey a business, we look at both units and demand on your bills, ask when your peaks occur, and size the system to your daytime consumption. We show you the energy savings clearly, and we will tell you honestly if solar is unlikely to change your demand charges.

To review your bill with us, book a free site survey.

MDI and contract demand: keep them in step

On many commercial and industrial connections, the bill compares your recorded maximum demand with your contract demand (the load you have agreed with the DISCOM). If your MDI regularly exceeds the contract demand, penalties may apply. If your contract demand is much higher than you ever use, you may be paying fixed charges for capacity you do not need. Review both with your bills from the last 12 months, and speak to the DISCOM about revising the contract demand if it is badly out of step.

Frequently asked questions

What is MDI in an electricity bill? MDI stands for Maximum Demand Indicator. It records the highest average power your connection drew over a short interval during the billing period. On commercial and industrial bills it often sets the demand charge, so one short spike, such as several motors starting together, can raise the whole month's bill.

Does rooftop solar reduce MDI on an electricity bill? Sometimes. Rooftop solar reduces MDI only if your peak demand occurs in strong daylight, when the panels are producing. If peaks happen in the evening, on cloudy days, or when motors start together, solar may not change the recorded maximum. Check your interval data before expecting solar to cut demand charges.

How can I reduce MDI on my electricity bill, with or without solar? Reduce MDI by spreading heavy loads: stagger motor and compressor starts, avoid running pumps, ACs and machines together, use soft starters where suitable, and improve power factor. Rooftop solar can help further if your peak falls in daylight. Review your interval data to see what causes the peak.

Is MDI the same as sanctioned load? No. The sanctioned or contract load is the capacity you have agreed with the DISCOM. MDI is the highest demand your meter actually recorded in a billing period. If MDI regularly exceeds the contract demand, penalties may apply; if it is far below, you may be paying for capacity you never use.

Why is MDI important for commercial solar sizing? MDI shows when and how sharply your business draws power. For commercial rooftop solar, sizing from consumption alone can miss demand charges: if peaks occur at night, solar cuts units but not MDI. Reviewing MDI and interval data helps decide the system size, and whether load management should come first.

What is the difference between kW and kVA in MDI? kW is real power that does useful work; kVA is apparent power, which also includes the reactive power drawn by motors and similar loads. Where demand is billed in kVA, a poor power factor raises the recorded MDI. Improving the power factor can lower kVA demand without changing the work actually done.