Owners installing in-unit washers/dryers: how to determine water usage bill if master metered?

It's not a given that the people with W/D will use more than people without.

Couple A have W/D but no kids and only take 5min showers each.

Couple B has no W/D but take 15min showers each twice a day.

Which couple uses the most water? The only way to tell is to meter each unit and that probably would require a change of the CC&R's. So would charging people more unless you have a clause in your CC&R's allowing a fee for excess utility usage*.
If you can, then you take how much the monthly water/sewer has increased, divide by the number of washers and you have your number.

Greater concern to me would be if any local permits are required, can the buildings water electric handle the increased load? And what about leaks, our buildings are newer and the laundry area in each unit has a floor drain to handle any leak from the washer.

*Our documents do allow for this as one homeowner found out. Evidently to pass the time she would wash the walkway to her building, she would spend 20-30 minutes wetting down a 15 foot walkway, even after the Board told her to stop. The $50.00 fine got her attention and put a stop to it.

Kris,

Assuming that those who installed their own machines previously used the coin-ops and wash the same amount as before, your water consumption will remain about the same.

The big change is that you have no income from the coin-ops to offset the usage. How much of the income from the machines went towards the water bill versus paying for power and maintenance? One of your problems in figuring this out is that you do not likely have a separate water meter for your coin operated machines and no way of segregating that useage against all other usage.

I really do not know how much water a wash machine uses, but my first guess is that by comparison to toilets, showers, and dishwashers it is not a significant amount. Even though the wash machine may use a lot of water, it only runs a few times a week. Remember, too, that newer wash machines may much more efficient than your coin-operated units.

My first guess is that the cost of water that you no longer get paid for is not going to be significant. This sounds like one of those issues where you could spend thousands trying to refit every unit with its own meter only to discover that the costs were not worth the benefits.

And those that have installed in unit w/ds did so without Board approval. Per our bylaws... you need written approval.