Hydrogen fuel cell buses are being tested in several cities in California as a clean, environmentally-friendly alternative to diesel buses. In Berkeley, a hydrogen bus system consists of grid electricity, electrolyzer, hydrogen compressor, hydrogen tanks, and a bus propelled by electrical motors driven by a 60 kW hydrogen fuel cell stack and a 600V auxiliary battery. We show that the overall energy efficiency of this system is 2-3 times lower than that of a diesel bus. The prototype hydrogen bus generates elsewhere 2-3 times the CO₂ emissions, and several times the NO_x and SO₂ emissions of the clean diesel bus equipped with a catalyzed soot filter, and it costs 18 times more.