Year 8 Science | Victorian Curriculum 2.0
Household energy & energy audits
Topic 08 | Physical sciences | Answer key

Year 8 answers

Fluency

Watts, kW and kWh

    1. (a) 0.80.8 kW, (b) 24002400 W, (c) 1.51.5 kW.
    2. 0.1×10=10.1 \times 10 = 1 kWh.
    3. 2×3=62 \times 3 = 6 kWh.
    4. 0.05×8=0.40.05 \times 8 = 0.4 kWh.
    5. 1.5×2×0.30=0.901.5 \times 2 \times 0.30 = 0.90 dollars.
Fluency

Reading a bill

    1. 10 859−10 234=62510\,859 - 10\,234 = 625 kWh.
    2. 720×0.28=201.60720 \times 0.28 = 201.60 dollars.
    3. 91×1.10=100.1091 \times 1.10 = 100.10 dollars.
    4. 440×1.10=484.00440 \times 1.10 = 484.00 dollars.
    5. Any three of: meter readings (start and end), kWh used, tariff/price per kWh, supply charge, total cost, GST, billing period.
Fluency

Efficiency labels

    1. A: 400×0.30=120400 \times 0.30 = 120 dollars/yr. B: 500×0.30=150500 \times 0.30 = 150 dollars/yr.
    2. The 4-star fridge — more stars means more efficient, so less energy per year.
    3. Watts only tell you power; “kWh per year” builds in how much and how often the appliance actually runs, which is what you actually pay for.
    4. Difference =300= 300 kWh/yr. Annual saving =300×0.30=90= 300 \times 0.30 = 90 dollars. Over 55 years, total saving =450= 450 dollars.
Reasoning

Audit thinking

    1. Replace or reduce use of the heater. It draws ∼33×\sim 33\times more power than the LED and probably runs for hours each winter night, so it dominates the bill. LED lights are already low power.
    2. Many appliances use several watts continuously, 24 hours a day. Across a whole home, standby can add up to 50-100 kWh/year — not huge individually, but meaningful and easy to cut.
    3. Estimate the annual energy saved (lower heating/cooling kWh), multiply by the electricity tariff, then compare to the insulation cost. Payback time = cost / annual saving. If less than the life of the house, it is worthwhile.
    4. Heating a large amount of water from cold to ∼60\sim 60°C takes a lot of energy, and hot water is used every day year-round. Compared with a fridge (small steady draw) or lighting, hot water typically dominates.
Problem solving

Applied contexts

    1. 0.20.2 kW ×12\times 12 h =2.4= 2.4 kWh/night. ×200=480\times 200 = 480 kWh/yr. Cost =480×0.30=144= 480 \times 0.30 = 144 dollars/yr.
    2. 3.5×5×60=10503.5 \times 5 \times 60 = 1050 kWh. Cost =1050×0.30=315= 1050 \times 0.30 = 315 dollars.
    3. 5000×0.30=15005000 \times 0.30 = 1500 dollars avoided in year 1 (if all solar output replaces grid energy).
    4. Winter is higher because of heating (and often hot water usage). Two effective actions: improve insulation/draughtproofing; replace old resistive heaters with a reverse-cycle heat pump, or add solar panels to offset grid use.
Reasoning

Challenge

    1. Daily energy =3.6×3=10.8= 3.6 \times 3 = 10.8 kWh. Annual =10.8×365=3942= 10.8 \times 365 = 3942 kWh. Cost ≈1183\approx 1183 dollars/yr. Cheaper alternative: a heat-pump hot water system (roughly 3×3\times more efficient) or solar hot water — could cut the bill by 6060-80%80\%.
    2. Any three of: better insulation; smaller or more efficient appliances; fewer people or hours at home; use of renewable energy (solar); lower thermostat settings; more natural light; turning off standby.
    3. Total annual saving =(400+1800)×0.30=660= (400 + 1800) \times 0.30 = 660 dollars/yr. Payback =6000/660≈9.1= 6000 / 660 \approx 9.1 years.
    4. An audit targets the biggest energy users and waste. Replacing or switching them off cuts kWh, which lowers bills directly and, because grid electricity in Australia still includes a large share of fossil fuels, also cuts CO2_2 emitted at power stations. If done across many homes, the total emission reduction is significant.
Year 8 Science study companion | Answer key