Which energy efficient improvement to your home will save you the most money on your monthly utility bill? Take a wild guess…

According to a White Paper (PDF) released today by Westinghouse Solar (formerly Akeena Solar) and the California Solar Energy Industries Association (CALSEIA) entitled “Reducing Home Energy Costs by Combining Solar and Energy Efficiency,” solar power offers homeowners one of the surest ways to cut their monthly electricity bills.

The paper emphasizes that owners of a typical U.S. home can reduce their energy bills the most by installing a solar energy system. Somewhat surprisingly, the authors find that simply taking energy-efficiency measures — like installing low-energy lighting or upgrading windows — won’t cut homeowner’s electric bills as dramatically when compared to a solar home electric system.

The study spanned 10 U.S. cities and included 30 homes that were evaluated across three age groups: old, typical and new. Executive Director of CALSEIA Sue Kateley explains why solar came out to be the number one cost-reducing measure:

“In almost all of the U.S. housing stock built since the mid-1980s the ‘low hanging fruit’ of basic energy saving measures have already been harvested through energy efficiency regulations and rebate programs for energy efficiency measures. Consequently, for a typical home in the U.S., rooftop solar energy systems (electric and thermal), will generate six times more energy than can be saved with lighting, weatherization and insulation retrofits combined.”

The paper’s results are likely to encourage homeowners to consider installing solar panels — and prompt others who previously passed on the opportunity to give it a second look. With solar rebates and other incentives fast-disappearing, the time to consider a solar home energy solution is now.