The trouble with goats....
... is that having polished off the weeds they'll turn their attention to the fence, the cables and the solar panels...
Japanese solar power plant operator SB Energy Corp will use goats and sheep to boost the output of its 43MW plant in Yonago City, Tottori Prefecture, Japan. Nikkei Technology reports that the plant will employ four sheep and two goats to eat weeds. The site is apparently threatened by vines that grow on the plant's fences and …
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"that problem goes away ... and so does the CO2 emissions that come from using a powered weed-whacker."
No CO2 emissions from the weed whacker sure, but you've now got the CO2 emissions from 4 sheep and 2 goats, not to mention the MUCH worse N2O (nitrous oxide) and CH4 (methane) emissions and the emissions from the farming (including the crap they pump in to make the stuff grow and spray to make the bug die) and transport of their supplementary feed.
In this case "green" is not so much green as more of a dark purplish red, which is appropriate as it's the colour the animals will be after they chew through the electric cables.
Green would be swapping the gardeners weed whacker for a manual set of shears.
"Pretty sure human powered pruning sheers are even less energy efficient. Humans tend to need a lot of high energy foods too. Not to mention all the other stuff a humanoid weed extermination device likes to have."
The human is already part of the equation in that they already have a groundskeeper to cut back weeds, with the sheep and goats there will also be a human to corral them and provide their supplementary feed, therefor simply providing said human with manual shears IS the most green option. BTW the CO2, N2O and CH4 emissions from 4 sheep and 2 goats is much greater than from 1 human.