Energy calculation: How many watt-hours correspond to using a 65 W load continuously for 18 h (assume constant power)?

Introduction / Context:
Utility billing and battery sizing both rely on energy rather than instantaneous power. Energy in watt-hours is power (in watts) multiplied by time (in hours). This problem reinforces that core relationship for steady loads.

Given Data / Assumptions:

• Power P = 65 W (constant).
• Time t = 18 h.
• We ignore power factor and assume purely resistive or already RMS/real power.

Concept / Approach:

Energy E (Wh) = P (W) * t (h). This linear scaling directly gives energy consumption over time for a steady load.

Step-by-Step Solution:

Verification / Alternative check:

Convert to kWh: 1,170 Wh / 1000 = 1.17 kWh. For a 65 W device, 1.17 kWh over 18 h is reasonable (about the energy of a small appliance run for part of a day).

Why Other Options Are Wrong:

11.7 Wh is off by a factor of 100. 11,700 Wh is 10× too large. 117,000 Wh is 100× too large. 117 Wh is 10× too small.

Common Pitfalls:

Mixing Wh and kWh, or converting hours to seconds unnecessarily and introducing errors. Always keep consistent units.

Final Answer:

1,170 Wh