Okay SO, this depends on if you can refill them with your water line continually or not. I’m assuming yes as otherwise someone’s answered a swap from a 40 gallon heater.

They do about 12oz in a minute from a quick Google.

Normal tankless water heater is about 3gal/minute. Which is about 400oz.

So, 400/12 gives is 33.33, so since we probably want to make sure we hit at least that, I’d round up to 34. Would need a LOT more electricity for that, as noted by another poster below. Probably more than is actually run to most houses.

Note: unsure how it’ll run without coffee in it, the flow is likely improved and thus you could possibly cut it down by quite a lot, especially with some modification to allow it to flow directly from the heating source in the keurigs you could definitely cut it down.