Step 1 find the number of INC bulbs needed over this bulb's lifetime

25000 / 750 = 33.3333

Step 2 determine the cost of replacement INC bulbs and the difference with this bulb's cost

33.3333 * $0.50 = $16.67 cost of replacing the INC bulb

$16.67 - $5.25 = $11.42 replacement savings

Step 3 determine energy usage over the life of this bulb

100 watts * 25000 hours / 1000 watts per kW = 2500 kWh

13 watts * 25000 hours / 1000 watts per kW = 325 kWh

Step 4 subtract to find the difference in energy usage between the bulbs

2500 kWh - 325 kWh = 2175 kWh lifetime savings

Step 5 find the lifetime energy cost savings

2175 kWh * $0.118 = $256.65

Step 6 combine energy savings with replacement cost savings to find total lifetime savings

$256.65 + $11.42 = $268.07 total lifetime savings

Step 7 find savings per year

5 hours per day * 365 days per year = 1825 hours per year

1825 hours per year / 25000 lifetime hours = 7.30 % of bulb life used per year

7.30 % * 2175 kWh lifetime savings = 158.775 kWh yearly savings

7.30 % * $268.07 lifetime savings = $19.57 yearly savings