If you put a tank in and run the water until the incoming hot water from the on-demand HW heater displaces it, it would waste both water and fuel. (If your water is pumped electrically from the well, this could be an electrical power hit as well.)
You could skip the tank, use a recirculation pump operated by motion detector (probably not your cuppa tea) or pushbutton controls to call the hot water (& turn off the circulator once the hot water has arrived) on more of a demand basis. (But DO insulate the recirc lines to R4 or better, as aardvarcus recommended.) There are automated controls for pushbutton-demand recirculation, but off grid you probably don't want the electrical standby hit of automated controls- a simple switch would do it.
You could also do a once/day pushbutton recirculator to fill a tank too. By heating up the tank with push-button demand you get reasonably long burns out of the hot water heater (important for IT'S average effficiency), and you can stop when the mixed flow feels "warm enough".
Plan-B would be to put a small (2.5-6 gallon) electric HW tank under the sink with the element disconnected, letting the thermostat on the tank run the recirculation pump. (The contacts that switch the element on/off are usually quite capable of handling recirculation-pump currents- even 12VDC, unless you're using a real monster of a pump.) Standby losses on electric tanks are already pretty low, and can be made arbitrarily low depending on how much insulation you want to wrap it in. The duty cycle on the pump would be quite low, and you'd basically never have to think about it other than to turn it off if you planned to be gone for more than 12 hours. Electric HW thermostats range from box o' rock dumb bimetal technology (the kind YOU'RE) looking for to digital-bla-bla with mode indicator LEDs and temperature readout (exactly what you DON'T want.) This would be high convenience-factor (you'd never wait for hot water), and it would be reasonable on fuel (short handwashing burst of an on-demand KILL it's inherent efficiency- a buffer tank restores it substantially, albeit at a small standby loss cost.) But it would occasionally chug some electricity you didn't really need to (not much, but some.)
20 gallons by the sink is overkill- 2-3 gallons of local buffer is enough. If you went with demand-only recirculation, if you got a slug of cold water first thing in the AM, flipping the recirculation demand switch to get the HW there faster (without wasting water), the tank would probably warm the tank enough stay well above mere tepid all day even without further recirculation or thermostatic control- at least enough to mix to reasonably with the incoming slug of cool wate without that blast of cold. The volume of 60' of plumbing just isn't all that much. (Points of reference: 60 feet of 1" pipe is ~2 gallons, 60' of 1/2" pipe is about a half-gallon.) Anywhere from 1-3x the feedpipe volume in the local buffer is probably more than enough to keep from freezing your hands off. Whether you maintain the tank temp thermostatically or with demand-only recirculation, storing 10x that amount would have higher standby loss for marginal gain in comfort/function. |