Study guide: Energy use over time and useful service
Integrate changing power records, account for outputs, and compare energy for a stated task.
Ideas to explain in your own words
- Distinguish watts from watt-hours and convert to kilowatt-hours.
- Calculate total energy when power changes between stated intervals.
- Compute an output fraction and complete an energy ledger.
- Compare plans that meet a requirement rather than only comparing watts.
Watch your reasoning
- 12 × 5 = 60 Wh, then 60 ÷ 1,000 = 0.06 kWh. Multiplying power by time gives energy; the conversion is required to express it in kWh.
- 30×4 = 120 Wh and 5×20 = 100 Wh, totaling 220 Wh = 0.22 kWh.
- 150/500 × 100 = 30%, and 500 − 150 = 350 J thermal under the supplied complete-output model.
- C meets both constraints and uses less energy than A. B is excluded by the minimum light score.
A worked review example
Calculate energy from power and time: 60 W × 4 h = 240 Wh = 0.24 kWh. The device’s modelled power is still 60 W; 0.24 kWh is the energy for that four-hour period. Add Energy across Power Intervals: 40×3 + 10×2 + 0×19 = 140 Wh = 0.14 kWh. Total energy depends on duration in each mode; the sum of mode power levels is not a daily energy amount. Distinguish Useful Efficiency from Conservation: Lighting efficiency is 25%. Thermal transfer plus storage account for the other 225 J, so the low useful fraction does not imply energy destruction. Choose an Energy Plan for a Defined Service: C provides score 620 for 4 h and uses 15×4 = 60 Wh. A uses 80 Wh for the same required duration. B uses only 48 Wh but its 500 score fails the stated service.
Make a practice plan
- Try the supplied lesson problems without looking at the answers.
- Compare your method with the worked examples and explain any correction.
- Use the new review cases, then complete the transfer task on paper or in your own document.