What is the most effective way to lower the cost of electricity for solo mining Bitcoin, something you would want to set up and never look back again until several years later, it has to be very energy efficient, the size shouldn't matter that much, I am not looking to solve a block very soon.
It depends. If you are just interested in the principle of mining, and the "lottery" side of solo-mining, you should look at what Sidehack produces as miners: the R606-R909, Gekko Newpac, CompacF, these are small ASIC miners that don't consume a lot of power, and don't need to be monitored if they are well ventilated/cooled. They are very small hashrate miners, it's ideal for home mining, but you won't reach PH/s levels unless you accumulate thousands of them.
Otherwise, recently Canaan had some great promotions on their online site, and the A1166 takes 47J per Th, which is pretty decent.
But in any case it's case by case, it depends on your price paid per kw/h.
I can advise you to look at Willi's Long term Group run: we rent 200TH/s for 6-7 months through a hashrate rental group, and we share the block reward if we have a block. It's a way to get a foot in solo mining without worrying about the hardware and its maintenance, or the electricity ; as it is rental hashrate. We recently hit a block in the Groupbuy#30 but as you know, it is totally random. More informations here if you are interested : https://bitcointalksearch.org/topic/groupbuy-solo-mining-blockparty-long-time-run-2022-5381245
