Sometimes it's hard to know what a bond measure will mean to a school district. Will better classrooms mean better students? Will more libraries mean more learning? Will an improved football field make for more enjoyable games?

It's easy enough to identify one of the benefits that's come to Mt. Diablo Unified since Measure C was passed in 2010. Its PG&E bill has been slashed, shrinking from $3.82 million in 2009-10 to $687,624 in 2012-13.

That's the most telling report on the district's solar power project, which is spread across its 51 sites -- 47 of them elementary, middle and high school campuses. The more than 28,000 solar panels that were installed across 18 months at a cost of $68 million now supply 92 percent of the district's historic electricity needs.

That is according to Tim Cody, the district's interim special projects manager and a persuasive advocate for the project that Mt. Diablo contracted for with SunPower Corp. of San Jose. It ranks high on the ambition scale -- it's the largest solar operation in any school district in the country. The 12.1-megawatt system generates enough electricity to power about 3,400 homes.

One reason for its success, Cody said, is the extensive planning that preceded it. Board members, community members and construction and design experts all had a voice in the plan. Four solar companies were vetted and invited to bid, and performance guarantees were required before a contract was signed. (SunPower guarantees its panels and at least 80 percent savings for 25 years.)

The solar arrays, as they're called, are located atop parking structures, playground canopies and rooftops -- aimed in different directions, at different angles to maximize sun exposure at each particular site.

"A number of times, the installers would notice something like a flagpole and say when its shadow moves across the array it will cut down production," he said. "They were very careful about trees and shadows."

SunPower maintains the panels and replaces them when necessary, with a few exceptions. ("We handle vandalism losses if someone willfully destroys a panel," Cody said. "Last year there were about 100. This year, it's about 50.") Some panels also were lost early on to careless delivery truck drivers who miscalculated the clearance of overhead arrays. Insurance paid for most of those.

"That doesn't happen any more," he said. "I think the drivers are used to them now."

At its present pace, the project will pay for itself in 23 years, and Cody expects the system to be operational well beyond that. Equally important, today's savings can be redirected toward today's operational expenses.

Word of Mt. Diablo's solar adventure has spread far and wide, eliciting inquiries from other districts. In fact, the Oakland district enlisted SunPower in a similar project.

The solar power project hasn't been the only improvement funded by the $348 million bond issue. Others include air conditioning for schools that were without it, science classrooms for middle schools and upgraded technology infrastructure. Northgate High's aquatics center was also funded by Measure C.

It's fair to assume those additions will be beneficial. No assumptions are necessary about the solar project -- it's paying for itself and then some.

Contact Tom Barnidge at tbarnidge@bayareanewsgroup.com.