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Your Electricity Bill, Decoded: Watts × Hours × Rate
Everyday · 6 min read · Last updated September 2026
The formula
Cost = kWh × price per unit
Worked example — a 300 W fridge compressor actually runs ~8 h/day: 300 × 8 ÷ 1,000 = 2.4 kWh/day → 72 kWh/month → 576/month at 8 per unit. A 1.5 kW AC at 8 h/day: 1.5 × 8 × 30 = 360 kWh → 2,880/month. Same formula, five-fold difference — appliance wattage × runtime is where bills are decided.
Tiered slabs change the marginal cost
Many utilities price in slabs: the first 100 kWh cheap, the next band pricier, and so on. Your average rate is not the rate for the next kWh — crossing into a higher slab makes every additional AC hour more expensive than your bill average suggests. Check your bill's slab structure before estimating savings; shaving consumption below a slab boundary saves at the higher marginal rate.
Finding the big consumers
Wattage × runtime ranks them honestly: ACs, water heaters and dryers dominate; fridges run constantly but at low power; phone chargers and idle electronics are rounding errors despite folklore. Measure suspects with the appliance's nameplate wattage and your usage pattern in the Electricity Bill Calculator — and compare appliance upgrades with the Appliance Running Cost tool.
Try the calculators
Worked example: Electricity Bill Calculator at its default inputs
Take the default setup on Electricity Bill Calculator: no fields changed, nothing tuned to a nice round number. Run in the site's own engine, those values produce the answer below.
| Input | Default |
|---|---|
| Power (W) | 100 |
| Hours/Day | 8 |
| Days | 30 |
| Rate ($/kWh) | 0.12 |
Result: Cost: $2.88
The steps behind that answer
- Step 1: Energy = Power × Time = 100W × 8h × 30d / 1000 = 24.0 kWh
- Step 2: Cost = 24.0 kWh × $0.12 = $2.88
What Power (W) does to the answer
Hold every other field still and move only power (w).
| Power (W) | Result |
|---|---|
| 50 | Cost: $1.44 |
| 100 | Cost: $2.88 |
| 150 | Cost: $4.32 |
If the rows barely differ, power (w) is not your lever — change a different input. If they swing widely, get that number right before trusting the total.
Related guides
Units, slabs and the marginal rate
A unit is one kilowatt-hour: a 1,000-watt appliance running for one hour. Slab tariffs charge a different rate per unit depending on how much you use, so the bill is a sum of bands rather than units multiplied by one price.
That structure makes the rate you pay for the last unit higher than your average rate. Adding a large appliance changes the last band, which is why the extra cost is often bigger than a simple per-unit estimate predicts.
Fixed charges and taxes
Most bills include a fixed monthly charge, a meter charge and a duty or tax on the total. These do not scale with usage, so a low-usage month still carries a floor, and they explain why halving consumption rarely halves the bill.
Read the bill structure once and write down the fixed portion. It is the number to compare when deciding whether a small solar install or a tariff change is worth it.
Estimating an appliance honestly
Watts times hours gives watt-hours, then divide by 1,000 for units. The catch is that nameplate watts are a maximum: an air conditioner on inverter control, a fridge cycling on and off, or a laptop idling all draw less than the label.
For anything with a duty cycle, use a plug-in energy meter for a week rather than the label. For small electronics, the label is close enough and the numbers are too small to matter.
Standby load is the other hidden line: routers, televisions and chargers left plugged in can add 5-15 units a month across a household. Small per device, paid every month, and removable with a switched extension.
Frequently asked questions
How do I calculate my electricity bill?
For each appliance: watts × daily hours ÷ 1,000 = daily kWh. Multiply by 30 for monthly kWh, sum across appliances, then multiply by your rate (respecting slab tiers if your utility uses them).
How much does it cost to run an AC?
Wattage × hours × days × rate. A 1.5 kW split AC at 8 h/day and 8/unit: 1.5 × 8 × 30 = 360 kWh → 2,880 a month. Higher rates or longer runtimes scale it linearly.
What uses the most electricity in a home?
Usually heating and cooling (AC, heaters), water heating and dryers — high wattage times long runtime. Always-on low-wattage devices like fridges matter, but chargers and idle electronics barely register.
Why is my bill higher than my kWh math?
Slab pricing (later units cost more), fixed charges, taxes and fuel-adjustment surcharges sit on top of the energy line. Check the bill’s non-energy lines before blaming the appliances.
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