Whole house ELECTRIC backup - PowerWall? Alternatives?

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ExtraCool
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Joined: Sat Jun 29, 2024 7:54 pm

Re: Whole house ELECTRIC backup - PowerWall? Alternatives?

Post by ExtraCool »

A 2-stage 4-ton unit may consume a running load of 2500-4500W (eg., 20A x 220V at high). I am ignoring the compressor high startup load for now. As examples
- the AC cycles like 10-min off and 10-min on, ie., 30min ON in each hour. So, in an hour, 4500W x (30/60min) = 2.25kWh.
- AC runs continuously at high from 11-ish AM till 8-ish PM (hot 95-100deg Houston day!). So, 4.5kWhr every hour, ie., 4500W x 9hrs = 40kWh…just during these 9hrs! Additional usage with on/off cycles during the rest of the time.

Depending on how much the AC cycles (or not), the energy usage can be quite high. Having solar panel system helps to cover some of the usage during peak solar time window of 11am-3pm), but, can use up the battery capacity quite quickly.

With one AC using 4500W, and, additional home loads like microwave, hair dryer, etc, one may need 6-10kW power from battery system array of at least 2 batteries. Hence, avoid considering use of AC during backup event.

(Aside: I monitor my usage for each of the circuits in my SPAN panel)
cmr79
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Re: Whole house ELECTRIC backup - PowerWall? Alternatives?

Post by cmr79 »

We have a small 10 kWh Enphase home backup battery that we had installed along with our solar system. Its purpose is mostly to deal with short nuisance power outages that we get frequently with storms here in the mid-Atlantic. It is not in a sub-panel, but we certainly can't run our entire house with it and typically would turn off AC, wouldn't cook/bake and wouldn't charge our EV. We have too many critical circuits spread about for a sub-panel to have been practical. We have a backup propane furnace (primary heating is an ASHP) if power were to go out in Winter.

We determined that for ~12 hrs, we can continue to use lights normally, keep the house from getting dangerously cold, keep stuff in the fridge/freezer from getting dangerously warm, run our well pump and HPWH, open garage doors and have internet/TV access. That was enough for us.

For OP, the devil is in the details with the desire to run AC for ~15 hrs...sizing a battery for peak/continuous power output and enough energy to do that could end up being quite expensive, though with smaller variable speed units with soft start and with a very energy efficient house, it might not be. OP, perhaps what you should try is to simulate what you would run ba what you would shut down during a power outage. Turn the thermostat up as high as you'd be willing to tolerate. Adjust your activities accordingly. Then at the end of the test, review your energy usage like you posted here earlier. That won't give you a clear picture of the instantaneous peak use you might need if both AC units are booting up simultaneously, of course, but it should give a good indication of the continuous power draw and the total kWh use that you might expect, which will get you in the ballpark.

All of this would be a moot point if the batteries can't be installed in your home in a manner that your HOA would permit, of course.
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just frank
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Re: Whole house ELECTRIC backup - PowerWall? Alternatives?

Post by just frank »

I assumed backup AC was a small window unit and everyone sleeping in one room.

Lot cheaper than a powerwall, or two.
alfaspider
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Re: Whole house ELECTRIC backup - PowerWall? Alternatives?

Post by alfaspider »

need403bhelp wrote: Sat Jul 06, 2024 1:48 pm
Sandtrap wrote: Sat Jul 06, 2024 1:42 pm
need403bhelp wrote: Sat Jul 06, 2024 12:59 pm We have a 2200 sq ft townhouse on the Gulf Coast.

We have a small L shaped backyard. I basically dragged a permanent generator installation company out here against their will few years ago and they said no room for permanent generator.

We also have an HOA.

Looking for whole house power backup (2 ACs etc) solution that could work, seems Tesla PowerWall might be good option at least for <14-15 hour outages that could be installed completely indoors.

One quirk is that our circuit breaker is in the backyard not in the garage. So not sure if they would want to install outdoors anyway requiring HOA approval etc.

Any thoughts re Tesla Powerwall for this situation? Any good alternatives? Yes I am aware could buy gas generator but I think in the storm situation it would be prone to theft if left unattended outdoors.

Thank you!
How many hours would a Tesla power wall system provide whole house power?
how large would that system be for you and how much would it cost?
3 powerwall units for 24 hours for "whole house" vs only essential circuits?

j :D
So Tesla website claimed up to 13 hours with 2 powerwalls for around $23k including install.

This is before anyone comes to look so who knows what ends up happening at that point.

I agree not very cost-effective for sure but DW doesn't like idea of power going out and DS is 3 years old.

We had one really bad outage 3.5 years ago when DW was pregnant, and scattered recent ones lasting a few hours which were significantly disruptive to DW.

I was hoping for some kind of solution. We can certainly afford $23k for peace of mind.

If we evacuate, DW also wants fridge running, not sure if 2 powerwalls might be able to do that if everything else is off for a few days.
I have a Tesla power wall as part of a solar system (you ideally want to do it with solar because that makes it tax credit eligible). The post-tax marginal cost of the power wall was only $6k with the solar.

The estimates they give are really sans HVAC. You aren't going to get more than a few hours out of a power wall running a central AC. With my dual units, one power wall would only last ~2 hours running A/C in summer. My system is setup to bypass the A/C circuits for this reason. However, I can run a small window unit in the afternoon during an outage (uses substantially less power than the central a/c).
Luke Duke
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Re: Whole house ELECTRIC backup - PowerWall? Alternatives?

Post by Luke Duke »

snackdog wrote: Tue Jul 09, 2024 12:47 am The instantaneous amperage on compressors can be large. I have a few 4 ton heat pumps which run at 15 amps but can be as much as 83 amps on startup (locked rotor or LR amps). If just two should happen to start simultaneously, I'd be looking at 166 amps on HVAC alone.
My 4 ton Goodman A/C (not heat pump) was at 120A on startup and 11A running. A Micro-Air soft start got it down to 40A on startup.

A Ford Powerboost F150 may do what the OP wants or if he goes the generator route this is probably the most economical route for a quiet inverter generator.
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