When the Exploratorium science museum in San Francisco announced in 2012 that it planned to reach net-zero at its waterfront home, no one expected that seagull droppings would become part of the equation.

But that’s just what happened after the museum installed enough solar panel arrays to provide 80% of the building’s electricity needs—piles of gull guano reduced energy generation by up to 15%. “A seagull dropping in the wrong spot can turn off a whole panel,” says Shani Krevsky, project director for campus facilities at the Exploratorium. For the museum, seagulls were just one of the many variables involved in the complex accounting problem that is net-zero.

Whether it’s a public learning laboratory, nonprofit organization, small busine…