It looks like I can get a new, vastly more efficient freezer of similar size for around $625 delivered. If I assume 8 years of $100 energy savings and nothing past that I get an IRR of 6% and change. Would you do it?
First of all, are you sure you need that extra freezer
at all? If not, recycling it and not buying a replacement would clearly be the most profitable option.
Second, are you sure you need a freezer of the same size and configuration? I hear that chest freezers are usually more efficient than upright ones, for example.
Third, where is this freezer located, and do you use it all the time? A freezer located in a basement that stays cool all the time will have a lower return on investment than one located in a garage in Florida, and one kept outside that's used only for storing the game from fall hunting season until spring and then shut off will have a lower return than one used for keeping beer cold during the summer.
Fourth, if you do actually need the freezer and if you do use it in the right circumstances to actually save $100 a year, then sure, 6% return with low risk (mainly, that electricity gets cheaper or that it breaks out of warranty) sounds reasonable unless you can get a better risk-adjusted return elsewhere. (In other words, assuming you have no high-interest debt, have already filled up all your tax-advantaged accounts, etc.)