The savings were because of new capabilities, not new efficiencies. Adding power (which they happened to use solar power) gave them new capabilities (compaction, signalling), which saved them almost a million bucks.

Solar was still the enabling technology. Marginal energy cost comparison with grid electricity is irrelevant, since the cost structure is dominated by fixed costs. Can you imagine installing, reading, and maintaining an electric meter and utility interconnect for every single trash can?