It may be $20/mo for each unit, but you are paying for it, right?
I've never replaced a water heater like this, just furnaces and air conditioning units, but I also worked for an ESCO that did projects like this for large building owners, and here is how I would look at it.
Call three or four companies that replace this kind of unit. See what they would propose to replace what you have now, and ask them how they derived their sizing information.
They should be able to show you some calculations in order to determine the size of the unit.
It may be possible that new low flow shower heads would cut your hot water consumption, which would cut your overall energy spend as well as your up front capital costs.
That said, using the income valuation approach an improvement in net operating income increases the value of the property. So if you can cut your energy spend by 50% ($2,000/yr) and your cap rate is, say 8% then you can take 2000/.08 and find that the project increases the value of the building by $25,000.