Hi folks, I am confused by what I'm being told by a couple different salesmen, and I'm hoping someone can help me gain clarity. I'm looking to install Powerwalls as a partial home backup system. That is my main goal, not to capture more solar energy, although I will gladly take that. I have a 11.5kW solar array that I've had for 10 years, and I live in western NY. My main concern is that I have a large, 6 ton geothermal heat pump (Climatemaster Tranquility 27 model 072) that obviously draws quite a bit of power. I would like to provide backup for that plus a deep well water pump plus a couple light circuits. I do not expect backup for the resistance heater on the heat pump. I am only looking for backup for 12-24 hours. I have been told that 2 Powerwalls will be adequate by one salesperson and that I need multiple Powerwalls by another. Do you think that two can meet my objectives? I am virtually clueless when it comes to electrical stuff, so please keep any suggestions simple! Thanks loads!
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Tesla Powerwalls for backup
- Thread starter blperk
- Start date
Your 150LRA HVAC will just barely start with your 2 powerwalls, maybe.
As soon as other loads try to start at the same time, you will not start your well pump and HVAC certainly.
CWhite
2020 Model S LR+
2 Powerwalls can output 10kw, if your heatpump and well draws more than that then you need another Powerwall. That said if everything draws 6kw then you should be good but drawing 6kw constantly would drain the Powerwalls in only 4 hours. Of course devices rarely run 100% of the time but the math is the same
I think it is virtually impossible that 2 powerwalls will meet your stated objective of "12-24 hours of backup power" for something like that. I have (2) 3 ton AC units that draw 4kW each when they are running. thats 8kW if both ACs are on, and my normal house load is 1kW (rounded). That means 1 hour of AC + home is 9-10kW and 1 powerwall has at max 13.5kW.
2 PWs have a capacity of 27kW which means 3 hours of my run rate above would completely drain my batteries (not counting solar replenishment).
See what your house uses over an hour when running the geothermal and pump, multiply that by 12 (the stated backup time) and you will see what you are looking at as usage, then divide that by 13.5 to see number of powerwalls it will take to get that. That doesnt count solar replenishment of course, but for simple numbers works.
You are probably looking at a generator, if the stated goal is 12-24 hours of backup power for something like that. That would be my guess anyway.
If the unit will start...
It will often take a fair amount of power to run it. How long do you want it to run?
For power failure, regrettably, the best answer still lies with diesel/propane/natural gas generators. They can provide days and weeks where batteries barely get days.
There's really two questions that you want to make sure that are answered.
- Do the Powerwalls provide enough power to start the unit
- How long will the Powerwalls run the unit
A soft start kit may help reduce the starting power but it's still very unlikely 2 PW's will last 12-24 hours while running a 6 ton heat pump and other household loads. Several more PW's would likely be necessary.
I'd vote for a backup diesel generator, with a heat exchanger to your heat pump geothermal system. Like @Vines and @jjrandorin, I suspect that when you do the math on how much power your system uses, it will be beyond three powerwalls for 12 hours unless you have a super insulated house. (Which I suspect that the 6ton geothermal argues against...) The quick and dirty would be to take your January power bill, divide by 62 and see what you come up with for kWhs.
All the best,
BG
All the best,
BG
Where does 62 come from? My high bill over the winter was 3600kWh. I figured 3600/30=120 per day. That''s with thermostat set at 70, which wouldn't be the case if we were gone on vacation, which is my primary concern.
Sorry not to have been clear on the math. January has 31 days, and you wanted at least 12 hours of backup (1/2 day), so 31/(1/2)=62...
Given your high bill usage (January?) usage of 120kWh/day, that's 60kWh of usage for 12 hours (+/-, as you probably use a little more during the day and less at night). So rounding the Powerwall capacity down to 12 kWh of usable capacity, you get 60/12= 5 Powerwalls.
That's why I voted for a diesel generator, running on fuel oil, with the option of using waste heat from the generator boosting your geothermal system. A Climatemaster 6ton geothermal is rated at 150 LRA (locked rotor amps) (and 27.2 RLA) in the current specification sheet, and a Powerwall has a LRA of 106A and an RLA max of 18A (per What Can Powerwall Back Up | Tesla Support), so you would have an absolute minimum of two powerwalls to start your 6T, and would probably want at least three to cover other items in the house being on. e.g. Circulation pump...
Does that help?
All the best,
BG
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Thanks for the clarification. I'm really trying to avoid a generator, because I don't want propane or diesel around. I get the feeling from most responses that 3 Powerwalls would work, so I need to figure out how to afford them! Thanks!
To start your system, yes, for your goal of 12 hours of backup power, unless you are counting hours during the day when the sun is providing enough power itself to power your system, no.
EDIT for some additional context (based on what I already posted in this thread about my own house consumption).
I believe that when I was originally looking at powerwalls to add to my solar, the order form said 2 powerwalls would be something like 2-3 days of backup or some ridiculous statement like that. I can drain them in 3 hours if I run both my AC units. I can also make them last a couple of days (even without solar) if I go into major curtailment mode.
I have natural gas heating, I have a whole house fan I can use, 2 outdoor barbeque grills I can cook on (so I dont run my electric oven), etc etc.
In your case, since you are talking about heating, you cant really curtail that. If you used anywhere near 120kWh in a day, keep in mind that (3) powerwalls would be about 40kW of storage, from 100% full to empty. That means, basically, for average numbers, 3 powerwalls gets you approximately 5 hours of backup when the sun isnt shining.
If thats enough for you, then great, move forward with it. Its important to know what it is (and isnt) though.
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They may work (I'm not looking at the starting and running requirements though). But the real question is how long do you want it to run?
I've got 500-gallon propane tank buried near my generator.
As I said, the big question is how long do you want to run? If it's just 12 hours, then batteries are indeed an answer. They are indeed best at short term operation (although not the cheapest).
It's the long-term outages where you will be wanting your A/C to be running. Suffering over a few hours is no big deal. Suffering for days is a huge deal.
Just quickly looking at the Generac site, a 26kW generator is just over $7,000. That's a pretty big generator. That's well less than a single Powerwall. And at 13.5 kWh, the generator can put out the total amount of power that the Powerwall does in just 30 minutes.
That's right, if powering a 10kW load, the Powerwall will ONLY run for 1.35 hours. The generator will run until out of gas, and then the gas can be refilled and the generator restarted.
If you have natural gas in your neighborhood, that's the easiest. You just need to stop the generator to change the oil periodically, otherwise it just keeps running.
In the summer, I would think his 11kW solar can power the house during the day and PWs only needed for overnight. Of course PWs need to be recharged, but that can be done before big AC load is needed
Winter is the issue
Winter is the issue
Just to follow up on @ewoodrick's generator comments, a 26kW generator puts out a maximum of 108A, just enough to start @blperk's 6T geothermal. i.e. 26kW is about as small as you could go, but the exact details depend substantially on the generator construction and design. Compressor and well pump startup surges can be 3-5x the running currents.
@blperk, I think you should check to see how three powerwalls are going to keep the house warm for 12 hours during a storm in January, while you are on vacation, especially if you get snow on your panels.
YMMV...
All the best,
BG
@blperk, I think you should check to see how three powerwalls are going to keep the house warm for 12 hours during a storm in January, while you are on vacation, especially if you get snow on your panels.
YMMV...
All the best,
BG
I don't think so. @blperk is in lake effect upper New York State in winter time, a prime time for storms and power failures. There is only going to be a kWh or so of power per day, and only if @blperk is lucky enough to have snow free panels. @blperk's area gets an average of 55" of snow per year, and mostly cloudy November through March.
I got a quote from local Generac dealer for $17,300 for 24kW. Plus I really don't want propane around.
A big portion of that will also have to be added to the Powerwall cost.
Not sure of your aversion to propane, especially when it is outside away from the house.
But sure, pay twice as much for the Powerwalls, add installation and run for maybe a day or so, that's totally your choice.
Yeah - I always forget about that stuff. Our biggest threat for power outage is PSPS in the summer
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