It is worthwhile to look at the payback times for simple energy efficiency-upgrades (negawatts), such as changing out incandescent lights for more energy-efficient compact fluorescent lights (CFLs). Ten 60 W incandescent light bulbs can be replaced with ten 15 W CFL bulbs for $30 and still produce about the same amount of light. Each bulb saves 45 watt-hours (60 W – 15 W = 45 W) per hour of operation. Assuming an average operating time of 5 hours per day, the energy savings for all ten bulbs is 2,250 watt-hours or 2.25 KWH per day (450 W x 5 hrs. per day)—about 821 KWH per year. These bulbs will last about 10,000 hours (about 5 1/2 years), compared to the 30 years of example solar systems, but the ROI shown is 4.3 months at the most.