Electric power and the energy a home uses

Answer Key - Grade 11 Science Worksheet - Challenge Me

Circle the best answer, or write your answer on the line.

1. 1.

   A TV draws 120 W for 5 h a day and 2 W the other hours. Over 30 days at $0.15 per kWh, what does it cost Ari?

   $2.87
2. 2.

   A desktop draws 200 W for 6 h a day and 5 W the other hours. Over 28 days at $0.14 per kWh, what does it cost Esme?

   $5.06
3. 3.

   Jun's 1,800 W kettle is 90% efficient. Water needs 4,186 J per kg per °C. How long to heat 1.0 kg from 20 °C to 100 °C?

   • About 207 s
   • About 258 s, using 100 °C as the temperature rise
   • About 167 s, from 334,880 × 0.90 ÷ 1,800
   • About 186 s, ignoring the 90% efficiency
4. 4.

   Electricity costs $0.15 per kWh. For 10 kWh of heat, a resistive heater needs 10 kWh; a heat pump giving 3 kWh of heat per kWh needs less. Compare, Kira.

   • The heat pump, which uses 3.3 kWh costing $5.00
   • The heat pump uses 3.3 kWh, costing $0.50 against $1.50
   • They cost the same, since both give 10 kWh of heat
   • The heater, since no device beats 100% efficiency
5. 5.

   A plant turns fuel into electricity at 36% efficiency, and 94% reaches the home. How much fuel energy gives 1.0 kWh at the home, Luz?

   • About 2.78 kWh, leaving out the transmission loss
   • About 3.33 kWh, subtracting 6 points from 36%
   • About 0.34 kWh, multiplying 1.0 × 0.36 × 0.94
   • About 2.96 kWh
6. 6.

   A home needs 12,000 kWh of heat a year for 15 years. A resistive heater costs $800; a heat pump (COP 3) costs $4,000. Electricity costs $0.15 per kWh. Which is cheaper, Milo?

   • The heat pump, by $14,800 over 15 years
   • The heat pump, by $14,000, leaving out the heater price
   • The heat pump, by $18,000: the energy saving alone
   • The resistive heater, since it costs $3,200 less to buy
7. 7.

   A 20 A, 120 V circuit may carry 80% of its rating for hours. Which set of appliances can run together for hours, Pia?

   • Microwave 1,200 W and toaster 1,100 W: 2,300 W is under 2,400 W
   • Heater 1,500 W, lamp 100 W, TV 150 W and laptop 65 W: 1,815 W
   • Kettle 1,500 W and toaster 1,100 W: 2,600 W is about 22 A, near enough
   • Two 1,500 W heaters: 3,000 W is fine on a 20 A circuit

8. Read this first

   Meter readings at 8 am (example data)

   day	reading (kWh)
   Mon	60000
   Tue	60036
   Wed	60071
   Thu	60108
   Fri	60143

9. 8.

   Using the meter table, what does the data imply about the home's average power, and what can it not show, Ren?

   • Average about 1.5 kW; it cannot show when power peaks
   • Average about 1.5 kW; it shows that peak power is 1.5 kW
   • Average about 36 kW, from 36 kWh in a day
   • Average about 1.5 kW; the load was steady throughout each day

10. Night meter readings (example data)

    time	reading (kWh)
    11 pm	61480.0
    6 am	61482.1

11. 9.

    Only the refrigerator and standby loads run overnight. From the meter table, what is the base load, and about how many kWh a year, Tala?

    • About 0.30 kW, or 2,100 kWh a year
    • About 0.30 kW, or 260 kWh a year
    • About 2.1 kW, from 2.1 kWh overnight
    • About 0.30 kW, or 2,600 kWh a year
12. 10.

    An EV uses 0.30 kWh per km at the battery through a 90%-efficient charger. Electricity is $0.15 per kWh. A gas car uses 7.0 L per 100 km at $1.00 per L. Compare costs, Zee.

    • Per 100 km the EV costs $5.00 and the gas car $7.00
    • The EV costs $5.00 and the gas car $70
    • The EV costs $4.05 and the gas car $7.00
    • The EV costs $4.50 and the gas car $7.00, leaving out the charger loss