When LED is marketed, one of its common selling point is that it will save you money by saving energy. This may be true in a few circumstances, but if you do the math then you'll find that the devil really is in the details. To show you why, let's do a calculation:

We be making some assumptions here, but I have made them fairly realistic.

To replace a HID streetlight with LED, it costs $200 upfront.

The old fixture uses 100 watts and the LED one uses 50. (A typical replacement wattage)

This LED fixture will last 8 years of 8 hours of use per night (23360 hours)

The price of electricity per kilowatt is $.10. (This is slightly lower than the USA average, but green energy will surely lower the price)

To figure out how much it's going to save, we need to find the saved watthours per day. Assuming 50 watts are saved when running, that's 400 watthours saved per night. If we combine this with the price of electricity, we now get a whopping 4 cents saved per day. (.40 kWh X $.10)

To pay off that 200 dollars, the fixture has to run for 5000 days, or over 13 years! That's only for initial payback, and this calculation didn't factor in early failures.

Actually, it gets a little worse. Some utility companies offer a deeply-discounted rate specifically for streetlights. Plugging in these lower costs makes the payback time even longer. And if your power provider doesn't offer the special streetlight rate, others offer time-of-use programs. Commonly, TOU makes nighttime power usage cheaper than daytime, and we all know that streetlights are supposed to come on at night.

How's that for energy saving? ;)