It would make sense for the U.S. Postal Service to switch to electric vehicles to deliver the mail, since the average postal truck drives about 18 miles a day. The problem will be how to make it pay off, either with government incentives or through so-called vehicle-to-grid revenues.

That’s the word from a study from the Postal Service’s Office of the Inspector General released last week, that took a look at the operational and financial viability of replacing some or all of USPS’s 146,000 vehicles with electric vehicles.

About 96 percent of the Postal Service’s trucks drive less than 40 miles a day, and all are parked from 5 p.m. to 8 a.m. every day. That makes the USPS a natural testbed for testing electric vehicles with limited range, the study found.

What will be difficult is paying for them, the report found. The USPS is facing a big drop in revenue as fewer people use it to send mail, a long-term trend that the ongoing recession has accelerated. It could lose more than $7 billion in the fiscal year ending Sept. 30, and last week asked as many as 30,000 workers to retire early in an attempt to save about $500 million, Bloomberg reports.

That means that the USPS is looking for quick returns on its capital investments – at least 30 percent over three years, the report found. That’s a lot faster payback than it has previously demanded for energy efficiency projects. More