Heat pump electrical usage for Powerwall design
My skills are pretty weak for determining electrical requirements for a dwelling beyond adding up typical lighting and outlet loads, etc.
I’m designing a high-end dwelling of about 3,600 sqft on Martha’s Vineyard that will be lived in year round. This will be what I get people here are now calling a PGH. The owners definitely want to install solar panels and are looking into Tesla Powerwall battery backup for energy storage primarily as a backup (in lieu of a propane powered generator) for the hours, and very occasionally days, when utility power goes down due to storms.
We’d like to heat and cool the house using minisplits and/or multi-splits probably with some use of ducts. The Owner has expressed a desire to also install a propane backup system, possibly radiant hydronic, possibly using common ductwork with the minisplits. His reason for this is he feels that the power draw of minisplits will be much to large for the back-up batteries to maintain for the hours or days needed. I would rather have him invest is money in additional battery capacity than a duplicate heating system. To due that, I need, at least initially, some sense of what kind of the power needs of the minisplits, perhaps a rule of thumb of the KW/hr draw on the system per ton. Should I assume that the draw would be continuous or will it modulate except at severe low temperatures?
I guess I’m looking for enough info to see if its reasonable to go with just batteries. Once we make that decision, we can get into more detail with Tesla or whoever is providing the batteries. My understanding is that Tesla’s batteries have a capacity of 13.5kw with a maximum draw of 7kw, and that houses of this size would have two or three batteries.
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Battery backup for days with heat pumps is not really practical at this point.
We have a similar sized house (though not at 'pretty good' specs). We heat half of it with heat pumps (with natural gas boiler driven hydro coils as backup), and the other half has a natural gas forced air furnace. The heat pump heated side also has a steam humidifier that is a pretty significant load.
The Mitsubishi heat pump we have (a MXZ-8C48NAHZ) is spec'd to peak at 5,250 watts.
A typical winter day for us is around 120-130kWh.
The Powerwall 2 is rated at 13.5kWh ( https://www.tesla.com/sites/default/files/pdfs/powerwall/Powerwall%202_AC_Datasheet_en_northamerica.pdf ) which means we'd need 10 of them to run for 24 hours without power.
I'd look at a propane generator that would be appropriately sized for the load for this.
The worst month (after 2 heating seasons) I've had is 690 kwh, or 23 kwh/day, so I'd need 2 of those powerwall 2's for 1 day. This is a 2200 sqft, sun tempered house in CO, using an american standard heat pump. I'm not sure it makes sense in Martha's vineyard, but you really need to look at the details of the house, location, and occupants.
what climate zone? what are typical winter temperature?
6B by county, but closer to 5b by deg days. Typical low 40's day, teens at night, below zero occasionally, sunny
>The warm season lasts for 3.3 months, from June 9 to September 20, with an average daily high temperature above 71°F. ... The cold season lasts for 3.3 months, from December 10 to March 20, with an average daily high temperature below 46°F.
I don't know whether the fact that it's Martha's Vineyard means we're not price-sensitive, but...
This would be a great application for a dirt-cheap water thermal storage solution, for most of the year in this heating-dominated climate. Spend almost all your money on large quantities of PV panels. Get or build a big insulated tank, fill it with water-glycol. Put a cheap resistance heater in the tank (heat pumps are expensive). Put a heat exchanger in the tank for hydronic, or for water-heater preheating. The water-glycol doesn't get pumped anywhere.
This guy makes it look dead simple, but still a custom setup:
https://www.youtube.com/watch?v=ryJmtItfaXQ
And the build:
https://www.youtube.com/playlist?list=PLVe7nJBrjHReCvpEjC_M_rBGiT74JpTxV
He's getting about 30kWH of thermal storage for $300 in materials spent on a custom 250 gallon storage tank, which is 93% cheaper in his analysis than battery storage. Bury a 5000 gallon cistern tank with sufficient insulation under a rough patio with an access hatch and you're looking at enough to weather weeks or months. Just take care to ensure it's rated for temperature.
The following table published by the NEEP shows the power draw of a Mitsubishi MUZ-FH12NAH ductless mini-split under various load conditions and operating temperatures.
As for backup heat, would your client consider one or more propane fireplaces as an alternative? (The integrated blower fan consumes a relatively small amount of power.)
Yes...I've seen this table, but don't know how to figure the actual power kWh power draw under real world conditions. For instance, the draw at 47 degrees is 2.3 Kwh for 21,000 btu/hr. If the actual load at 47 degrees was only a third of 21,ooo but/hr, would that mean with modulation and/or cycling that the power draw would be about 1/3 of 2.3 kWh?
I'm guessing it would be somewhere around 600-watts. Yesterday, at an average outdoor ambient air temperature of 2°C, one of the two mid-efficiency ductless mini-splits (9.3 HSPF) that serves our 51-year old, 2,900 sq. foot Cape Cod consumed 8.0 kWh. Its draw held fairly steady at 300-watts, with a slight rise between 04h00 and 06h00 in response to the operation of our heat pump water heater. Note, however, the momentary spikes as it recovers from a defrost cycle.
+1 on using propane for occasional use and water storage if the use is frequent (eg, daily).
Spending around $50k on batteries that'll store around 8 bucks worth of electricity is beyond nuts, as is having a whole duplicate heating system for backup.
Since money is apparently no big deal (3600 square feet on MV will cost at least $1.5 million, plus land) spend $10k on a whole house backup generator and the rest on improving the building envelope in this already planet unfriendly project.
I agree about the apparent illogic of spending a lot of money on batteries, although probably 25K, not 50K.
THe building envelope will be thought out carefully and despite your snarky comment about "this already planet unfriendly project" we will do what we can do to make it as friendly as possible given the size of the house.
Eversource has a pretty reliable electric grid, use a generator(Kohler) and keep a caretaker on rotation. Typical Vineyard set up.
Hugh.
Four years ago I was quoted $14,000 for a single Powerwall in Maine. MV costs will be higher. I lived on MV for several years decades ago. A house that size, with ( more snark coming) the usual bells and whistles common to MV second homes, won't get through a day with only 25 kwh.
I'm certainly not blaming you for giving the client what he wants. I just hate what money has done to what used to be a pretty nice place.
It is all nuts, Stephen,
But it's not like we can go back to the days of Billy Joels downeaster alexa, and Rupert Homes pina colada.
If not your home anymore, keep it in your heart.
I agree using batteries for long time backup isn’t a good option. In the telecom facilities I design, the battery systems are only intended to run critical loads in the relatively short (usually under 10 to 30 seconds or so) time between a utility failure and a generator starting and picking up the load. Long-term power backup is best handled by generators, not battery systems.
You could potentially try to use battery power to ride through the night so that you’d be running on solar power essentially all day, but then you’d basically be building an off-grid type of system.
I would get a small generator running on natural gas if you have that available, otherwise propane with diesel as an option if you need a larger generator which probably isn’t the case. I would personally use one of the small Kohler units which are much more reliable than the more commonly brands found in the box stores. You’ll need to size the propane tank large enough to flow enough gas to operate the generator at the coldest time of the year, which will likely mean a 250+ gallon tank. Note that grid tied solar systems don’t usually like to operate with generator power since it’s not as frequency stable which makes it hard for the solar inverters to stay synchronized.
Regarding battery capacity sizing, if you have a 16kWh battery that should run 1kw worth of load for 16 hours. There are sometimes other limitations like non-linear discharge curves (twice the load can only run 1/3 as long, for example), but the specifics of that depend on the battery system you’re using. There will be other losses to allow for as well. I would expect two 16kWh battery systems to be able to run a typical home of that size for less than 24 hours, probably quite a bit less. Multi-day runtimes aren't really practical with battery systems in normal applications, as can be seen from the challenges the off-gridders have to deal with.
Bill
It’s a shame steam heat has been phased out- while not the most efficient in terms of combustion efficiency, it requires essentially zero electricity to run other than a 24v ecobee, and could run on a battery powered basic thermostat if needed (no caretaker needed in outage). With new construction demanding AC I understand the need but it is a great heat source.
For backup of my 2400sf home, I have a 5000w inverter generator with interlock backfeed that runs in 25% quiet mode as my whole house can be under the 1250w demand most of the time (without micro, dishwasher, washer/dryer, minisplits). LED lighting and energy star appliances help keep the loads low.
Does a Tesla Powerwall have a generator input? If not, I would look into a battery that does.
Sonnen is one example.
It's true that these new batteries do not make sense economically, but that does not mean people don't want to buy them.
As a solar installer and zero energy rebuilder, I can say you don't want to try to run that powerwall that much. If you can keep the house to less than Passiv house .6ach the mini splits will be effecient, just not enough to run all day on the batteries ( if you want them to last more than a few years). Blue Ion batteries are far more environmentally friendly and can be pushed more but also cost $17k for a 12kw system.
I also would recommend a propane generator. It is much cheaper (and with better usable life) to oversize the propane tank you are already planning on to support a small genset.
My Vermont house was off grid until a few years ago. I always had a generator that I needed in the middle of winter when the sun didn't shine for several days, but the plan was to run the generator for just a few hours to charge up the batteries. I have kept the same plan now that I am on grid. If the outage is less than a day I can run off my batteries and heat with the wood stove instead of the mini-splits. If the outage lasts longer than a day and there has not been enough sun to recharge the batteries then I run the generator long enough to bring the battery charge to a reasonable level.
I am glad not to have to run the noisy generator non-stop during an outage when my power needs are modest. A mixed system like this serves my needs well.