How much does it cost to operate a 28-W porch light for 24 h if electric energy costs 8¢/ kWh .

Answer 1

28 * 24 = ? Wh

8¢/ kWh = .008¢/ Wh

cost = Wh * .008¢/ Wh

Answer 2

0.028kW * 24h = 0.672 kWh.
Cost = 0.672kWh * 0.08/kWh = 0.054 = 5.4 cents.