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Energy-Saving Plan: Audit Your Home and Save kWh

An energy-saving plan has five steps: list the appliances, measure their energy with E = P × t (in kWh), find the biggest user, change habits (use for fewer hours), and change devices (use more efficient ones such as LEDs). Then check the new total and the cost saved.

🎬 Step-by-step story

  1. Step 1: list what a home uses. Each box is one appliance.
  2. Step 2: measure. Energy = power × time. Taller bar means more kWh per day.
  3. Step 3: find the biggest user. Here it is the red one, the cooler or AC. Save there first.
  4. Step 4: change a habit. Run the cooler fewer hours and the bar drops.
  5. Step 5: change a device. Swap old lamps for LEDs. Use both controls and read the new total.

Tip: drag the 3D scene to turn it. Use two fingers to zoom.

🤔 Common doubts, cleared

Why do we use kWh and not joules?

A joule is very small. A bulb uses thousands of joules each second. 1 kWh is 3.6 million joules, a handy size for bills.

Why start with the biggest bar?

The same percentage cut gives more kWh on a big bar. Cutting the red bar saves more than all the small ones together.

Does less AC time really help?

Yes. Energy = power × time. Power stays the same, so fewer hours means fewer kWh. Drag the slider and watch the red bar.

If LED uses less power, is it as bright?

Yes. LEDs turn more of their energy into light and less into heat, so a 9 W LED can give the light of a 60 W old bulb.

Is the plan finished after step 5?

No. Check the next month's meter reading to see if the saving is real, then fix the plan if needed.

Step 1 and 2: list and measure

An energy audit means finding out where energy goes. Write down each appliance, its power in watts (W) (it is printed on the label) and the hours it runs each day.

Energy = power × time. If power is in kilowatts and time in hours, energy is in kilowatt-hours (kWh). One kWh is one "unit" on the bill. 1 kW = 1000 W.

Example: a 1500 W cooler for 4 hours uses 1.5 × 4 = 6 kWh.

Step 3: find the biggest user

Add up all the kWh. The appliance with the tallest bar is where a small change gives the biggest saving. Heaters, coolers, geysers and air conditioners usually use the most, because their power is large. A fridge has small power but runs all day, so it also adds up.

Step 4 and 5: cut by habit and by device

Habit changes cost nothing: switch off lights and fans in empty rooms, use natural light, run a washing machine with a full load, unplug chargers (standby power wastes energy).

Device changes cost money now and save later: LED bulbs (9 W give about the same light as a 60 W old bulb), star-rated appliances, better insulation, solar water heater.

An efficient device turns more of its input energy into useful output. Efficiency = useful energy out ÷ energy in × 100%.

Check the plan: cost and payback

New total kWh tells how much you saved. Cost saved = kWh saved × price per unit. For a device change, payback time = cost of the new device ÷ money saved per day. A good plan has a short payback time. Write the plan as a table: appliance, old kWh, change, new kWh, saving. Then check it again after a month with your meter reading.

Key formulas and definitions

Worked examples

1. A 60 W bulb glows for 5 hours. How many kWh does it use?

P = 0.06 kW. E = 0.06 × 5 = 0.3 kWh.

2. An LED of 9 W replaces the 60 W bulb (5 hours a day). How much energy is saved per day?

Old: 0.3 kWh. New: 0.009 × 5 = 0.045 kWh. Saving = 0.255 kWh per day.

3. A home uses 12 kWh per day. The price is 8 per unit. What is the bill for 30 days?

Energy = 12 × 30 = 360 kWh. Bill = 360 × 8 = 2880.

4. An LED costs 120 and saves 2 per day. What is its payback time?

Payback = 120 / 2 = 60 days.

Common mistakes

Practice quiz

1. The unit of electricity on a bill is:
2. A 2 kW heater runs for 3 hours. Energy used:
3. Where should an energy plan start?
4. Which is a habit change?
5. Payback time of a device is:

Practice: answer these yourself

Type or choose your answer, then press Check. Use a hint if you are stuck; the full solution appears after you answer.

Frequently asked questions

How do I calculate the electricity used by an appliance?

Energy in kWh = power in kW × hours used. Divide watts by 1000 to get kW.

What is a unit of electricity?

One unit is one kilowatt-hour (1 kWh) = 3.6 × 10^6 joules.

What is the quickest way to cut the electricity bill?

Find the biggest user and use it less or replace it with an efficient one.

Where this is taught

China九年级（初三）Ch.22 Energy and sustainability

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