Why start from your own bill
The most dependable estimate uses your own last bill rather than a published rate table. Your bill already reflects your tariff category, the consumption band you were in and the adjustments and taxes that apply to your connection. Rates are revised during the year; this method keeps working when they change.
- Your most recent bill, on paper or as the duplicate from this site.
- Today's reading from the meter display, read from outside the meter box.
- The reading date printed on the bill, to count the days since.
- A calculator — the one on your phone is fine.
The method, step by step
The worked example uses the same made-up bill as the diagrams in the guide to reading a bill: 232 units cost Rs 6,960 in cost of electricity, and the current charges came to Rs 8,762 before any arrears. Eighteen days after the reading date, the meter shows 12,820.
- On the last bill, divide the cost of electricity by the units billed. That is your average rate per unit.
- Divide the bill's total current charges, before arrears, by the cost of electricity. This ratio carries the adjustments, duty and taxes.
- Subtract the present reading printed on the last bill from today's meter reading to get the units used so far.
- Divide those units by the days since the reading date, then multiply by 30 for a full month.
- Multiply the monthly units by the average rate, then by the ratio. The result is your estimate.
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When the estimate will be off
The method assumes next month is billed the way last month was. These are the changes that break that assumption, and which way they push the real bill.
| What changed | Effect on the bill |
|---|---|
| You move into a higher consumption band | The average rate rises, so the bill is higher than the estimate. |
| A protected consumer goes over 200 units | Protected rates can be lost, and the bill can jump well past the estimate. |
| A new fuel price or quarterly adjustment | The per-unit adjustment changes, up or down, whatever your usage. |
| A billing period longer or shorter than 30 days | Units, and the bill, scale with the days between readings. |
| Arrears, instalments or credits | They are added to or taken off the total separately. |
Watch the limits
If your estimate lands close to a band edge — 100, 200, 300 units and so on — a few days of heavier use can take the bill into a higher band. Your daily average makes this easy to track: in the example, 178 units in 18 days is about 9.9 units a day, or roughly 297 units over 30 days, just under 300. Checking the meter once a week in summer shows whether you are heading over. The high-bill guide lists how much common appliances use, so you can see which ones move the daily figure.
Common questions
How do I calculate my electricity bill from units?
Divide the cost of electricity on your last bill by the units billed to get your average rate, multiply your expected units by that rate, then multiply by the ratio of total current charges to cost of electricity on the same bill. The result is an estimate; the issued bill is what you pay.
Can I work out my units before the meter reader comes?
Yes, approximately. Read your meter, subtract the present reading printed on your last bill, divide by the days since that reading date and multiply by 30. That gives your expected units for the month.
Why is my bill higher than my estimate?
The usual reasons are moving into a higher consumption band, losing protected status after going over 200 units, a new fuel price or quarterly adjustment, a longer billing period, or arrears added to the total.
Something inaccurate? Report a correction. Do not include private bill details.
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