Watts say how fast you are using energy. Watt-hours say how much you used.
Two different quantities with almost the same name, and the bill is in one of them while the nameplate is in the other.
A 100 watt lamp left on for 3 hours: 100 x 3 = 300 watt-hours. Divide by a thousand to put it in kilowatt-hours -- 300 Wh = 0.3 kWh. The lamp's rating never changed; it was 100 watts the whole time. What accumulated was the energy.
Check yourself: A 100 W lamp runs for 3 hours. How much energy did it use, in kWh?
Answer: 0.3 kWh. Power times time is 100 x 3 = 300 Wh, and 300 Wh divided by 1000 is 0.3 kWh.
Power multiplied by time gives energy. Watts alone cannot tell you how much was used, because they say nothing about how long.
No card. No account needed to start. Score by topic against the 70% mark.
Written from the owner's poster for this lesson. Nothing added beyond what the poster states.
Written for Texas apprentices as free study material. The Texas apprentice licence is an application, not an exam (Occupations Code 1305.161), and nothing here counts as apprenticeship hours or continuing-education credit. VoltMark is not affiliated with, endorsed by, or sponsored by NFPA, PSI, TDLR, or any state or local licensing authority. Always follow your employer's safety programme and your supervising electrician.
Electrician licensing exam prep: practice questions, timed exam simulations, and step-by-step help finding every answer in the NEC.