There is a natural experiment from the recent switch away from DST in Indiana. Matthew Kotchen and Laura Grant report:
Our main finding is that–contrary to the policy’s intent–DST increases residential electricity demand. Estimates of the overall increase range from 1 to 4 percent, but we find that the effect is not constant throughout the DST period. There is some evidence of electricity savings during the spring, but the effect lessens, changes sign, and appears to cause the greatest increase in consumption near the end of the DST period in the fall. These findings are consistent with simulation results that point to a tradeoff between reducing demand for lighting and increasing demand for heating and cooling. Based on the dates of DST practice before the 2007 extensions, we estimate a cost of increased electricity bills to Indiana households of $8.6 million per year. We also estimate social costs of increased pollution emissions that range from $1.6 to $5.3 million per year.
In other words, with DST less is spent on light but more is spent on air conditioning. Here is a summary article on the work, from today’s WSJ. Do note this:
There may also be social benefits to daylight-saving time that weren’t covered in the research. When the extension of daylight-saving time was proposed by Mr. Markey, he cited studies that noted "less crime, fewer traffic fatalities, more recreation time and increased economic activity" with the extra sunlight in the evening.