Real-world energy tracking
All,
I am in the process of designing and building a Passive House in NJ, CZ 4a. I just finished the Certified PH Builder course and am currently enrolled in the CPHC class – yes I am a control freak. My ultimate goal is to get off-grid using a combination of PH, PV and Powerwalls. I am trying to estimate what my daily power requirements will be on a no/low PV day (snow/heavy clouds/rain). I have done all my plug loads and found quite a few energy hogs – I now have an excuse to replace that 7-year-old plasma tv that is drawing 470 watts when on. I am trying to estimate what my clothes dryer and hot water heater will require. I currently have gas units and have determined based on my summertime gas bill that I use about 15 Therms/month between the dryer, DHW and gas stove. I have converted those to KWH, but I don’t think that is apples to apples given the current technology today with heat pumps. I have separately estimated usage for an induction stove. What would be really helpful is to know if anyone has the Whirlpool Hybrid Care dryer and/or any of the current heat pump hot water heaters, such as GE, Stiebel or even better, Sanden. I spoke reps from each of these companies at NESEA and they all said I will save a lot over an electric element only unit, but no one could give any hard numbers. Sanden to their credit does show 3KWH usage for their 80 gallon unit per day in their marketing materials and they say that is a big savings over other HP water heaters. I was hoping someone has one of these units and has been able to monitor the electric usage, either using a Kill-a-Watt or a circuit based monitoring system like PowerWise, so I could get a real-world, no-spin estimate. BTW, family of 4, current DHW unit is a 75 gallon AO Smith gas unit circa 2006. Never ran out of hot water.
Thanks,
Jonathan
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Replies
Good day Jonathan!
Using the Kill-a-Watt meter, we've determined that on the cloudiest and coldest days of December through January (with incoming water at 46F and 58F basement but regulated with daily solar), our daily use for DHW is 0.86KW per day using the Air Tap heat pump. I super insulated the tank with 12" of wrapped fiberglass and it sits on 4" of XPS so it rarely falls under 110F. We put the Air Tap on a 50 gallon tank. You can see all the fixings in my GBA blog from November to December. PK
Jonathan,
Your goals are very ambitious for an off-grid house. Once you have finished your calculations, you will be stunned by the cost of the batteries needed to achieve your goal.
A few comments:
1. Deep-cycle lead acid batteries -- the kind that have been around for over 100 years -- are still more cost-effective than lithium-ion batteries for stationary uses like off-grid homes. For more information on this issue, see Tesla Batteries.
2. From mid-November to mid-February, two things happen: your electricity usage goes up while your production of PV-generated electricity goes down. That's why almost no-one who lives in an off-grid house tries to design an all-electric house. (The exception: lucky people who live near a good stream capable of supporting a micro-hydro system.)
Most off-grid homes are heated with a wood stove (with propane backup). Moreover, most off-grid homes have a generator that burns fossil fuel to charge their batteries in November, December, January, and February.
Good luck. If you succeed, I'd be interested in a report after your house has been occupied for one year. And if you succeed, I'd be interested in hearing how much your batteries cost.
Paul - Thanks for the data and I like your boot and mitten warmer. If I read your post correctly, you are using a combination of heat pump and solar thermal for your hot water, yes? If you were building the system today would you skip the solar thermal and just go with more panels? Also, how much does the heat pump cool your basement in the winter.
Martin - my philosophy in life is shoot for the stars if you want to reach the moon. So you are correct, this is ambitious and maybe not achievable, but I think I can get close - for most of the year.
The cost analysis is interesting. While TOU and net-metering give me every incentive to sell to the utility during the day and buy at night, the cost of 3 Powerwalls is comparable to the cost of a my current whole-house generator. However, with the next house being PH, the generator requirements will be much smaller, so in that case the batteries will be more expensive. The only way I can justify it is on a carbon basis. With a national average PEF of 3.16, and pretty good transmission losses on the return of my clean solar to the utility, I can cut out a lot of carbon with this system.
I was touring a PH project on the day that Tesla quietly dropped one of its battery offerings and I was telling the mechanical engineer on that project about my plan. I started to sweat when he said he heard that Tesla was discontinuing its battery project. So I turned to GBA and saw your article and was relieved to see it was the 10KHW back-up which is not what I need.
As for some number, if we take a look at the demand side of the equation first, excluding space conditioning and DHW, I can get my daily energy usage down to 8.5kwh. Plug loads are the biggest component, followed by ERV, then fridge and stove. I work from home so things like dish washing, clothes washing, charging my EV, showering can be done during the day when the PV is producing. Each Powerwall is rated at 6.4kwh, which I believe includes the inverter losses, so 3 would give me a total of 19.2kwh. I am a big wood burner and I currently have a high output EPA certified direct vent unit. Unfortunately, that unit is way too big for a PH. However, I have found units made in Europe that are designed specifically for PH style homes with direct vent, dedicated make-up air and low output that will work. I also plan on cheating just a little bit by including a couple of direct vent gas fireplaces - just in case. So if I can heat with wood/gas on bad PV days and keep my DHW under 1kwh, I could go about 2 days without PV. If my DHW climbs to 3kwh, then I am still good for about 1.5 days off-grid.
As for the supply side, I actually have some decent data to look at. My brother-in-law lives in Northern MA and has a 30-panel system installed on his roof. He gave me access to his monitoring system so I can see what he generates during the bad PV months. His most recent numbers are as follows: Nov - 718kwh, Dec - 437kwh, Jan - 636kwh, Feb - 663kwh, Mar - 993kwh. Like us, they have had a relatively mild winter this year. Last year was just the opposite and his worst month was Feb 2015 with just 389kwh and a 5-day stretch with virtually no generation. Probably a good time to put the house in sleep mode and go skiing. I have the ability to size up to 40 panels and I live in a better solar generating environment, so I am looking at his numbers as my worst case.
So I think I can get pretty close to off-grid, but only time will tell. I won't cut the cord completely, but I also won't be sending a lot of power back to the utility because my car uses about 8,000kwh/year to drive 20k miles. At that really is the end-game for me: To minimize my housing and transportation energy usage.
It's an interesting design challenge to see what it would take to get off grid. However, if your goal is reduction of carbon emissions, I'm not sure that's the best option. If you went all the way and did not have a grid connection at all, there would be times when your batteries were fully charged and your PV producing and you'd do nothing with that electricity. If you are grid connected, you can feed that back to the grid and as a result the utility will burn less coal. I think you are proposing a sort of hybrid model where your goal is to minimize your grid interaction, but you are still connected and could feed back to the grid on those days. But still, there's not much reason to believe that minimizing your grid interaction minimizes carbon emissions. If you simply invested the money you would have used to buy batteries in PV instead, going beyond what you need to get to net zero, you won't get reimbursed for the extra, but you'll know for sure that your investment is going towards reducing emissions. For the same investment and the same expected utility bill, you'll be clearly carbon negative.
In other words, aiming to get off grid can be a fun technical challenge, and I have nothing against playing that game if you want to play that game. But like building robots to play soccer, which is another fun technical challenge, it doesn't directly reduce carbon emissions, and there are other things you can do that will have a bigger impact.
Charlie - you make a good point and you are correct in that I am proposing a hybrid model. I want to keep that line to the utility open, but I hope to only send and not receive for the majority of the time. Minus whatever transmission losses there are sending power back to the grid, if I generate more than I use with PV, then I am essentially carbon free without the batteries. Or another way to look at it is I am carbon free during the day and now my neighbors are using less carbon during the day thanks to me, but we are all back to carbon hogs at night. Batteries put me back to 100% carbon free "most" of the time and places the burden back on my neighbors to do something themselves. I think once they see the finished product, they will at least start to think about reducing plug loads and adding solar. However, at the end of the day solar and other renewables will only get us so far. Without storage, we will continue to have a big carbon load at night. Batteries may not be the best solution in the long run, but it may open some eyes in the short run. In fact the solution may be a combination of things including water heaters? My wife sent me the following article yesterday (she works in the renewable energy space). I have not vetted it out yet, but the basic premise looks interesting.
http://www.popsci.com/need-high-power-home-battery-use-your-water-heater
I think one day we will see distributed energy in communities where panels and batteries (maybe water heater too) are a shared resource, everyone pays for the pro-rata percentage and transmission losses are minimized.
The folks at Sonnenbatterie (the world's most experienced grid-tied home battery installer) is projecting their system prices to fall another 80% by 2020. They have a lot more experience at managing these systems than Tesla/Solar-City, but this will be turning into a fairly competitive market. In terms of cost per cycle (as to the raw price/kwh of storage) they're already price competitive relative to Tesla due to the 10,000 cycle warranty.
http://www.utilitydive.com/news/the-tesla-killer-sonnens-ceo-on-its-us-energy-storage-market-strategy/416866/
The residential scale grid tied battery market in Australia is heating up right now too. It's a hot enough market that Sonnen is investing in local manufacturing facilities to serve that market:
http://reneweconomy.com.au/2016/sonnen-may-manufacture-household-battery-storage-devices-in-australia-36839
LG and others are also diving at the Australian market, where retail electricity rates are high, and remuneration for PV exports to the grid are miserly, making the economics of adding batteries favorable to a large number of existing residential scale PV installations.
Dana - Thanks for the info. The Sonnen package is appealing because it is turnkey and they have much more experience to date. The Tesla Powerwall is just the battery and I need to choose a third party for the inverter. Following the links you provided I was able to find a 2mwh battery system enclosed in a cargo container. That should get me through the roughest week in February!
I read through the water heater battery article - unfortunately nothing new. In fact one of the reasons I found the Sanden unit appealing is that you can program it to run certain times of the day. They envisioned people programming it to run at night when rates are lower, but I would do just the opposite. The other plus about the Sanden is that it won't increase my heating demand in the winter because the evaporator/condenser unit is outdoors.