This is a long one, for intrepid geeks and those who appreciate energy intrigue - I couldn't help myself, I'm sure you understand... There is so much fun to be had in the energy world these days, the limits to your imagination are bounded only by your budget.
Allow me to introduce myself first, then on to the meat of hot water, off-grid, and energy monitoring. Lots of links and name dropping here for those who want some nitty gritty how to.
I've been a hands-on energy geek for over 20 years. I like to think I've done it all – from basements to board rooms. I've done thousands of energy audits and ratings, investigated hot smelly attics and cold, smelly basements. I pointed fingers at insulation contractors and told them to “fix that and don't charge the customer". I’ve broken water pipes in customer’s basements, found silver bullets of savings, metered hundreds of appliances, and invented thousands of spreadsheets to support predicted savings. I’ve provided program design and analysis to utility managers, installed solar panels and wind generators, converted gasoline cars to electric, made thousands of gallons of biodiesel from waste vegetable oil, and (unfortunately) built my house before I knew what efficiency was. And then I wrote a book about it called The Home Energy Diet. Its for readers like you. So I like to think I've done it all, but new things keep happening and I won't ever have learned enough.
I live off grid with my family using solar and wind for electricity and wood for heat. We have a backup (bio)diesel generator for those rare occasions when the sun doesn't shine and the wind doesn't blow - usually it's one or the other. Recently we upgraded our solar electric system so that nearly 100% of our electrical needs are met with renewables throughout cloudy northern New England winters. We use 6 to 8 kWh a day, with all electricity monitored at the circuit level through my Powerhouse Dynamics E-monitor. Our 4 kW PV system keeps 50 kWh of battery storage charged quite well when the sun shines, with wind as an added bonus when it's cloudy. Solar power generation is monitored through the Outback Power MX80 charge controller, and wind data is collected with an anemometer feeding and NRG systems data logger. Happily, when we compare annual power production graphs of both solar and wind, they are nearly opposite each other, with wind providing more power in the winter and sun taking over in the summer.
A couple of months ago, shortly after feeling quite smug about having too much electricity, I received a propane bill for over $1000. Most of that propane is used to heat water, some is used for cooking, and some as a source of backup heat if we go away and can't load the wood stove. This presented a challenge that I could not resist: how to get off the propane “grid”. I knew that part of the answer was in efficiency and conservation, part in harnessing excess summertime PV production, but perhaps another piece lay in biogas production.
You can read about my efforts to generate biogas on my blog. It's quite an exciting process, and much simpler than I had thought. It will be a good hobby someday when I'm not so busy. To conserve hot water I took out my 10-year-old, 40 gallon, Bradford White sealed-combustion water heater and replaced it with a Navien NR180 on-demand, condensing water heater. While I was at it, I had my plumber install a GFX drain water heat recovery unit from Waterfilm Energy. I've known about DWHR for years and was thrilled to feel firsthand the 20° temperature rise in the water circulating through the coil. Now the cold water entering the water heater is 75°F instead of 55°F.
But I didn't stop there. I did something I would not generally recommend anyone else do, simply because it is not terribly cost-effective. However, in my case (off-grid with too much power production) the numbers worked a little bit differently. Admittedly, I will do things that get me off the fossil fuel mainline even if they aren’t cost effective - I can’t help myself, and I won’t try to talk anybody into trying this at home.
I needed a dump load for excess electricity generation. I considered a mini split air source heat pump but decided against it because most of my excess power is in the summer and it doesn't really get too hot here in central Vermont. I next considered a heat pump water heater but my off-grid power system limits me to 120 V and I could not see how to retrofit an HPWH to 240 V without losing efficiency through a transformer. So I bought the most efficient electric hot water heater I could find, a 40 gallon Marathon, and swapped out the heating elements for something that was suitable for the low voltage DC power that would feed them. If you want to try this approach, you will need some custom machining done. Call first, I can save you some time.
Finally I had to put together a control system that uses a signal from the charge controller to activate a solid-state relay that connects the battery output to the heating elements once the batteries are fully charged. Did I say finally? Since I know how much propane gas I've used for the past 15 years, and I will of course want to see the effects of the changes I've made, the only choice (really, it couldn’t be helped) was to install a data monitoring system. This is not such a difficult or expensive thing anymore with high-speed Internet, practically free online data storage, and some neat innovative tech products.
An Itron whole house gas meter with a pulse output allows me to see when and how much gas is being used, delivering one pulse for every half cubic foot (0.0278 gallons) of propane. Cooking energy is almost negligible so most of the gas used in summertime is for water heating. From this I can establish a baseline to which winter gas consumption for the propane heater can be added to determine its consumption. To monitor how much hot water the family is using, I installed an Omega in-line water meter with a pulse output (75.7 pulses per gallon). Half a dozen temperature sensors complete the water monitor sensor array. All these sensors are plugged into a WEL data logger that uses a web interface allowing the user to see real-time and cumulative data. So far it looks like gas used is less than half of what it was last month.
As of this writing, this is all pretty new and there are a few bugs to work out – such is the life of an early adopter. In terms of satisfaction, though, the payback was immediate. On a sunny day, the batteries are charged before noon and the water overheats before dinner. Nice to see the electrons going to good use. I may soon be looking for a dump load excess hot water. Perhaps a hot tub is in the future and I'll become a profligate energy waster of the sort I've been complaining about for the past 20 years - but at least it's all renewable sourced, so it's “green”, right?