How to Estimate Your Electricity Bill from Units Used: A Worked Example

Introduction

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The bill lands, you open it, and the number is nearly double last month’s. Nobody bought anything new. Nobody, as far as you know, did anything different. So where did it go?

We have all had that month. Here is the thing nobody explains at the meter: you are not paying for “electricity” as one vague thing. You pay for units, counted one by one, priced in steps that get dearer the more you use. Count them yourself, and a bill like that becomes something you can predict, question, and bring down on purpose.

One honest note: rates differ by supplier and they change, so the prices below are round example numbers. Drop in the rates from your own last bill and the steps work exactly the same.

What Is One Unit of Electricity?

Electricity is measured in kilowatt-hours (kWh) — and one kWh is what bills call one unit.

A kilowatt is 1,000 watts — how fast an appliance uses power. Run a 1,000-watt (1 kW) appliance for one hour and you have used exactly 1 unit. A 100-watt bulb takes 10 hours to get there; a 2,000-watt heater takes just 30 minutes. The formula for any appliance:

Units = (Watts × Hours used) ÷ 1,000

That is why a small appliance left on all day can cost more than a powerful one used briefly: you pay for watts multiplied by time, not watts alone.

How Slab Rates Work

Most home tariffs do not charge one flat rate. They use slabs (also called blocks): the first batch of units is cheap, the next costs more, heavy use costs most. Basic needs stay affordable; heavy consumption pays.

An example slab structure might look like this:

Units in the month	Example rate per unit
First 100 units	3.00
Next 100 units (101–200)	5.00
Above 200 units	7.00

Now the point that confuses almost every household the first time — it certainly confused us. Crossing into a higher slab does not re-price everything you used. Only the units inside that slab pay the higher rate. Use 150 units and the first 100 still cost the cheap rate; just the extra 50 cost more. Think staircase, not cliff edge. Plenty of people panic-cut their usage mid-month over this misunderstanding, so it is worth getting straight.

A real bill also adds a fixed charge for the connection, taxes or duties, and sometimes a fuel adjustment. Add those after the slab calculation.

Let’s price a real-ish month

Take a family of four in a hot month — fans running, fridge working hard, the air conditioner on at night. Their meter says 260 units. Hardly outrageous. Watch what the slabs do to it. The tariff is the example above (3.00 / 5.00 / 7.00), plus a fixed charge of 50 and tax at 5% on the energy charge.

Step 1: Split the units into slabs

First 100 units
Next 100 units (101–200)
Remaining: 260 − 200 = 60 units in the top slab

Step 2: Charge each slab at its own rate

100 × 3.00 = 300
100 × 5.00 = 500
60 × 7.00 = 420

Energy charge = 300 + 500 + 420 = 1,220

Step 3: Add the fixed charge

1,220 + 50 = 1,270

Step 4: Add tax

Tax is 5% of the energy charge: 1,220 × 0.05 = 61

Estimated bill = 1,270 + 61 = 1,331

So 260 units comes to about 1,331. Quick sense-check: the average cost per unit, 1,331 ÷ 260 ≈ 5.12, sits between the lowest and highest slab rates. If yours lands outside that range, a step has gone wrong.

Mid-month, you can project: today’s meter reading minus the previous one, divided by days passed, times the days in the billing month. Run that total through the slabs — if it lands you just inside an expensive one, you still have time to trim.

Which Appliances Use the Most?

Roughly: an air conditioner (around 1,500 W) uses about 1.5 units per hour, a water heater (2,000 W) about 2 units per hour, an iron (1,100 W) just over 1 unit per hour — while an LED bulb (9 W) needs over 100 hours for a single unit. Heating and cooling dominate most bills; if yours jumps in summer or winter, that is almost always why.

What actually brings it down

Forget switching off one phone charger and feeling virtuous — the savings live in the big, hot, motor-driven things. In rough order of impact:

Attack the big loads first. Shorter air-conditioner and heater hours save more than a dozen tiny savings elsewhere. Each degree you raise the AC temperature trims its cost noticeably.
Kill standby use. Chargers, TVs and set-top boxes drawing power all night add up across a month; switch them off at the socket.
Wash and heat in fuller loads. A machine heating a full load uses barely more than a half load — fewer, fuller cycles win.
Keep filters clean. A clogged AC filter makes the motor work longer for the same result.
Watch the slab line. Shifting one heavy task into the next billing month can keep you in a cheaper slab.

Frequently Asked Questions

“Same units as last month, but the bill is higher. How?”

Happens more often than suppliers admit. Check: has the rate been revised, has a fuel adjustment or tax gone up, and — the sneaky one — how many days does this bill cover? A “month” of 34 days costs more than one of 29, at exactly the same usage.

“I think my meter reading is just wrong. What do I actually do?”

Photograph the meter today, reading and date visible, and compare it with the readings on the bill. If they do not line up, contact the supplier straight away and keep that photo. And if you have run your own estimate, say so — a customer who arrives with numbers gets a very different hearing.

“Is there one number I can track month to month?”

Yes: total bill ÷ units used — your average cost per unit. It creeps up in heavy months, because more units fell into the expensive slabs; that is the system working as designed. But a sudden jump with no change in your habits? Worth a phone call.

Conclusion

Next time, don’t open the bill cold. Read your meter a few days before it is due, run the slab steps on a scrap of paper, and have your own number waiting. Nine times out of ten it lands close to theirs. The tenth time, you caught the mistake — worth the ten minutes.