Science and units
Unit converter
Length, mass, time, temperature, speed, area, volume, energy and pressure, with exact factors.
Class 6 to 12, diploma, makers
Science and units
Electricity units and cost calculator
One "unit" on an electricity bill is one kilowatt-hour. List your appliances with their watts and hours of use, add the rate from your own bill, and see the units and the cost for a day or a month.
- Made forClass 6 to 12, families, makers
- Shows the workingEvery step, ready for your practical file
- Instant answersResults change as you type
Units and electricity cost
List each appliance with its wattage (on its label or box), the hours it runs each day and how many you have. Empty rows are skipped.
Electricity used in 30 days43.08 units (kWh)Cost at the ₹8 per unit you entered: ₹344.64
- Units per day
- 1.436 kWh
- Units in 30 days
- 43.08 kWh
- Cost
- ₹344.64at ₹8/unit
Working
- Units (kWh) = watts × hours × how many ÷ 1,000.
- Ceiling fan: 60 W × 8 h × 2 ÷ 1,000 = 0.96 kWh a day.
- LED bulb: 9 W × 6 h × 4 ÷ 1,000 = 0.216 kWh a day.
- Laptop charger: 65 W × 4 h × 1 ÷ 1,000 = 0.26 kWh a day.
- All together: 1.436 kWh a day × 30 days = 43.08 units.
- Cost = 43.08 units × ₹8 = ₹344.64.
- Note: A real bill also has fixed charges, taxes and slab rates (the rate per unit often rises with use), so it will not match this exactly. Use it to compare appliances and habits.
- Tip: The label wattage is the maximum. A fan on low speed or a fridge that switches on and off uses less on average.
Each appliance
| Appliance | Watts | Hours/day | How many | kWh/day | kWh in 30 days | Cost |
|---|---|---|---|---|---|---|
| Ceiling fan | 60 | 8 | 2 | 0.96 | 28.8 | ₹230.40 |
| LED bulb | 9 | 6 | 4 | 0.216 | 6.48 | ₹51.84 |
| Laptop charger | 65 | 4 | 1 | 0.26 | 7.8 | ₹62.40 |
What it works out
Understanding electricity units and cost
Power, in watts, is how fast an appliance uses energy, and energy is power multiplied by time. Electricity is billed by energy in units, and one unit is one kilowatt-hour (kWh): 1,000 watts for one hour.
Working it out makes a strong school project and a useful habit at home: it shows which appliances really cost money (anything that heats or cools for hours) and which barely matter.
Formula and worked example
Working out electricity units and cost by hand
The calculator uses exactly these formulas. Write them in your notebook and check one answer by hand: it is the surest way to remember them.
The formula
- Units (kWh) = watts × hours × number of appliances ÷ 1,000
- Cost (₹) = units × rate (₹ per unit)
- watts
- from the appliance's label or rating plate
- hours
- hours of use per day, times the number of days
- rate
- the per-unit charge on your own electricity bill
Worked example
A 1,500 W water heater for 30 minutes a day
- Units a day = 1,500 × 0.5 ÷ 1,000 = 0.75 kWh.
- Units a month = 0.75 × 30 = 22.5 units.
- At ₹7 a unit (an example rate; use the rate on your own bill), the cost is 22.5 × 7 = ₹157.50.
- Halve the time to 15 minutes and the units and cost halve too.
Answer22.5 units a month; ₹157.50 at ₹7 a unit
In projects and exams
How students use it
Watts and hours to units (kWh) and cost, at the per-unit rate on your own electricity bill. Here is where it earns its place in a school year.
Energy audit project
List every appliance at home and show which ones use the most units in a month.
Comparing bulbs
Put a 60 W bulb and a 9 W LED bulb side by side for the same hours and see the difference over a year.
Solar model sizing
Find the daily units your solar model has to produce for the load it runs.
Data for maths and EVS
Turn your home's usage into tables, bar graphs and pie charts.
Check your work
Common mistakes to avoid
These are the slips that cost marks in exams and burn out parts on the bench.
Forgetting to divide by 1,000
Watts × hours gives watt-hours. Divide by 1,000 for kWh, the unit on the bill.
Using the label wattage for every hour
Fridges, irons and air conditioners switch on and off, so they use less than label watts × hours.
Confusing kW and kWh
kW is power (how fast); kWh is energy (how much). A 2 kW heater for 3 hours uses 6 kWh.
Expecting an exact match with the bill
Bills add fixed charges and taxes, and often charge more per unit at higher use.
Questions
Questions about electricity units and cost
Stuck on a project or a sum? Ask the institute on WhatsApp.
Ask on WhatsAppWhat is one unit of electricity?
One kilowatt-hour (kWh): the energy used by a 1,000 W appliance running for one hour, or a 100 W appliance for ten hours.
Where do I find an appliance's wattage?
On its label, rating plate, box or manual. A plug-in energy meter measures what it actually uses.
Why does my bill not match this?
Bills include fixed charges, duties or taxes, and slab rates that change with how much you use, and the meter is read on particular dates. Use the calculator to compare appliances and habits.
Does a fan on low speed use less electricity?
With an electronic regulator, yes. Older resistor-type regulators waste part of the saving as heat in the regulator itself.
Free ebooks
Read more about electricity units and cost
Free PDF ebooks in easy English: download, print and share them.
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