We can take this a bit further. Lots of EVs provide V2L these days (Vehicle to Load), which means you can power mains appliances from an EV (e.g. if you're camping). A Mac Plus takes 60W. A typical modern EV has at least 50kWh of battery, so you could power your Mac Plus from a charged EV for: 833 hours, which is approximately 110 days or 30% of a year. We only have a 22kWh Zoe, so we could only provide 48 days worth (and it doesn't have V2L, so that's a bit moot).
A Mac Plus's CRT is probably taking most of the power, so it's a pity you can't mod a Mac Plus and turn off the CRT completely. No spare VIA pins I think. Is that true? But if you could, then you could write a simple screen saver, let's call it CrtSleeper and reduce the average power consumption even further.