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Life · Guided lesson

Calculate use over a stated period

About 10 minutes · 3 reading + 7 practice and checks
Time includes reading, practice, and questions. Extra videos or further study are optional.
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A desk-based household energy comparison with checked kWh estimates, usage assumptions and a safe no-installation conservation recommendation.

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What to include

  • Calculate power-time energy with explicit units.
  • Complete a quantity-aware 30-day device table.
  • Separate ratings, measurements and standardized estimates.
  • Compare matched lighting output and calculate differences.
  • Recommend an existing-control action preserving essential services.
  • Recalculate the final comparison and retain evidence limits.

Review and revise

Use these criteria to check your work and choose what to improve.

  • Method: applies the course-specific rules to the supplied cases with visible intermediate work.
  • Accuracy: results agree with the stated facts, units, boundaries and independent checks.
  • Reasoning: explains why the result follows and identifies a limitation or unresolved question.
  • Transfer: completes changed cases and revises the final project when a relevant assumption changes.
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Before you begin

The idea in everyday life

Fictional 30-day estimate: lamp A 60 W, 5 h/day, one lamp; lamp C 12 W, 5 h/day, two lamps; device D 1,500 W, 18 minutes/day, one device. Operating power is assumed constant for this exercise.

The idea

For a constant-power estimate, kWh=(watts÷1,000)×hours per day×days. Include quantity when more than one identical device runs. Convert minutes to hours by dividing by 60. Name the period; daily and monthly totals cannot be compared as if they covered the same time.

Follow along

Calculate use over a stated period — worked practice

For a constant-power estimate, kWh=(watts÷1,000)×hours per day×days. Include quantity when more than one identical device runs. Convert minutes to hours by dividing by 60. Name the period; daily and monthly totals cannot be compared as if they covered the same time.

Fictional 30-day estimate: lamp A 60 W, 5 h/day, one lamp; lamp C 12 W, 5 h/day, two lamps; device D 1,500 W, 18 minutes/day, one device. Operating power is assumed constant for this exercise.

  1. 1. 1. Convert

    Convert D’s daily duration to hours.

    You should see: 18÷60=0.3 h/day.

  2. 2. 2. Estimate

    Calculate the 30-day energy for A and C, including quantity.

    You should see: A: 0.06×5×30=9 kWh; C: 0.012×5×30×2=3.6 kWh.

  3. 3. 3. Complete

    Calculate D’s energy and the combined total.

    You should see: D: 1.5×0.3×30=13.5 kWh; total 26.1 kWh.

  4. 4. 4. Audit

    Check all entries use the same 30-day boundary.

    You should see: No day/year mix; C includes two devices.

Now change one thing

D’s duration changes to 12 minutes/day for 30 days at the same 1,500 W. Enter D’s estimated kWh.

Your work

What to finish

A checked three-row 30-day kWh table and combined total.

Common mistake

Watch for this

The total covers only the three listed groups over 30 days under constant-power assumptions, not every household device or a utility bill.

How can I tell I understand it?

Recalculate a changed duration while preserving power, quantity and period.

Try 3 questions

Quick knowledge check

Answer all 3 questions. Get at least 2 right and the lesson completes automatically.

Not yet passed

You’ll get an explanation for each answer. Signed in? Your results save to your account.

1What is 15 minutes in hours?
2Two 10 W lamps each run 4 h/day for 20 days. What is total energy?
3What boundary must accompany 26.1 kWh?
0 of 3 answeredApply the lesson to a short question, then review the explanation.
2
Finish this lesson

Pass the check. We’ll save the progress.

Answer all 3 questions above and get at least 2 right. You can review the explanations and try again.

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Sources and further reading

Explore the references behind this lesson.

Virginia Cooperative Extension: power-time energy estimation
Source & access notes · checked 2026-10-01

Web open/search showed the official page, its topic heading and relevant concept description. This is a content/source check, not a provider endorsement.

Access: Official page retrieved with the web reader on 2026-10-01; interactive tools and videos were not certified by retrieval.

Reuse: Original fictional teaching cases, calculations, questions and examples; link and short factual paraphrase only. No copied source exercises or illustrations.

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