Here's what you need to know to solve this problem:

A Watt is a unit of power, which is a rate of doing work. One Watt equals one Joule/second. The energy consumed by a light bulb is its wattage multiplied by the number of seconds it's lit. You can use dimensional analysis to convert from hours to seconds and then to Joules:

**5 hours x (60 minutes/1 hour)(60 seconds/1 minute)(60 Joules/sec)**

**= 1.08x10^6 Joules per light bulb**

**2 light bulbs x 1.08x10^6 Joules = 2.16x10^6 Joules**

A conversion factor is a mathematical expression of the relationship between two units. When using dimensional analysis, use conversion factors that result in the unit(s) you started with canceling out and the unit(s) you want to end up with being left. For example, I used 60 min/1 hour to cancel out hours then 60 sec/1 minute to cancel out minutes and end up with seconds. Seconds then canceled out when multiplied by 1 Joule/sec.

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