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Old 03-14-2017, 10:42 AM   #1
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Run central air on battery


I would like to put the four ton A/C compressor on a timer or a meter of some kind and have it switch over to a battery for an hour or two per day. Just the A/C, so of there is a power issue it won't bring the whole house down.

I am thinking about using a Tesla Powerwall but maybe regular batteries will do. The stats on the Tesla are
•Inverter: Fully integrated
•Energy: 14 kWh
•Power: 5 kW continuous, 7 kW peak
•Round Trip Efficiency: 90%
•Price $5,500
Initially I will fill the battery at night when the rate is low but eventually I'd like to fill it with solar.

So my question is about the feasibility of a project like this and thoughts on the electrical hook up.

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Old 03-14-2017, 11:04 AM   #2
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Re: Run central air on battery


The electrical hookup isn't that big of a deal. You might have issues with local codes though. I can't see getting enough wattage easily, unless you use a lot of batteries. How many years of marginal utility rate savings will it take to offset the cost of the battery/inverter system? Batteries never seem to work as well in real life compared to on paper.

If this is a serious plan, I wonder if it would be a better idea to rewire the AC blower and compressor fans and compressor to DC? Not sure what voltage the sweet spot would be, if anything. Convert line AC to DC for the AC. Maybe use the heat from that to heat your water? Might help with the solar system.

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Old 03-14-2017, 11:17 AM   #3
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Re: Run central air on battery


Out of curiosity why are you doing this?
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Old 03-14-2017, 03:01 PM   #4
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Re: Run central air on battery


it's not practical at all to run such a large load off a battery. your 4 ton a/c if it's 13 seer will be burning up 4kwh/hr of run time or even more if it's less, so you're looking at displacing maybe 3 hours of run time?

and if the battery is totally discharged every hot day it won't last.

not sure if tesla power wall even puts out 240v, if not you're looking at a very expensive inverter.

it's a waste of money.

if you want to do solar, charging a battery, only relying on the grid for backup you have to get your electrical use down to 5kwh per day or less, otherwise the cost of prohibitive. central air is out of the question.

off the gridders use hardly any electricity; most don't have a/c, a wood or propane or gas stove does the heating and gas is used for hot water and cooking as well.

off the grid houses are fossil fuel dependent for 90%+ of their energy, it's a myth that you can have the north american life-style without fossil fuels. (unless you live on a wood lot and harvest wood for heating, cooking, hot water yourself without any gasoline)

A much more practical approach now is to pre-cool when the rate is low and allowing it to warm up a bit during the day. you can run the fan during the day to keep the air circulating. A decent programming t-stat with programmable fan is all you need.

If you want you can add solar and do net metering, so the power produced by the panels when it's sunny offsets the a/c use. You would need 4 to 5kw of panels, at a very high cost.



solar is stupid when you have the electricity grid, unless it's for emergency backup power to run some leds and small appliances. there's a reason it's not widely used.
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Old 03-14-2017, 04:01 PM   #5
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Re: Run central air on battery


how much can you really save though? What is the payback time on this?
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Old 03-14-2017, 04:29 PM   #6
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Re: Run central air on battery


Here in Texas you can contract to track the wholesale price of power, which can run from 2/kwh at night to as high as the PUC price cap of $9.00/kwh on a hot July afternoon. To make some serious money back you just need to be able to get off the grid for a few hours a day, not all the way. Basically you can get maybe 75% of the savings with only 25% of the investment.

I think the 100+ kwh batteries that come in an electric car will be able to fill the same or better role in a few years, so I'd like to get an early start on that concept while you can still get a 30%(?) tax credit for a 2017 setup.

Finally, I think solar and net metering and maybe a grid crisis will be in my future. This is a way of getting a favorable start on those things.

I hope that answers everyone's questions. Now, about mine....
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Old 03-14-2017, 04:38 PM   #7
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Re: Run central air on battery


your area may have some program with integrated thermostat where they automatically raise the temperature when the rate is high.

do some research and see what's available in your area.

if you're worried about the grid going down, get a generator first, then a battery backup system and wire it to only run essentials.
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Old 03-14-2017, 05:23 PM   #8
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Re: Run central air on battery


someday the cost of solar panels will drop. Then, the only issue will be inventing better batteries
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Old 03-14-2017, 05:35 PM   #9
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Re: Run central air on battery


they already have dropped. they've been working on batteries forever.

the trick is getting the energy use down to next to nothing, then solar becomes very viable. but the masses are never going to be running electric central heat or air off self contained systems.
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Old 03-14-2017, 11:55 PM   #10
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Re: Run central air on battery


Why is it when someone wants to drop an extra 50 grand on a BMW or wants to move into a 4,000 foot house people say "go for it, it's your money," but when someone wants to shell out five grand on a solar or other green deal people post that he's stupid, just pissing his money away?

Okay, any thoughts on the electrical hook up?
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Old 03-18-2017, 11:59 AM   #11
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Re: Run central air on battery


[quote]but when someone wants to shell out five grand on a solar or other green deal people post that he's stupid[.quote]

Because a lot of these things are bad investments. if it's for the novelty or fun, like a crap mcmansion or gas guzzling car, or vacation yah go for it. I actually think those things are a colossal waste of money myself.

If you're trying to save natural resources or money, go with what gives the best return on investment.

LEDs have a fantastic return on investment and should be used.

If you're heating water with electricity, solar thermal is a good option to look at, would provide much more renewable energy at a lower cost than pv. (or just get a bigger electric tank and shift the use to off-peak with a timer)


pv is very expensive and the money is better spent on other conservation/renewable energy projects.

If you could shift your a/c use off the peak periods with a $100 thermostat and pre-cool before hand, why spend thousands of dollars?
(and get further savings doing low tech things like adding exterior window shading?)

Use the right equipment for the right application. PV is really good if it saves you from spending $10000+ to get electricity to your property in a rural application.

It's also good if you want a backup power system to run refrigeration and lighting, but not a central a/c. You only need like 5kwh per day or less to run the basics.

PV is not necessarily better for the environment or saves resources because of the toxic materials in the panels and batteries, but has it's place. The power output is so low that the real environmental savings only come from being forced to dramatically cut your energy use in the first place.
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Old 03-18-2017, 02:20 PM   #12
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Re: Run central air on battery


Quote:
Here in Texas you can contract to track the wholesale price of power, which can run from 2/kwh at night to as high as the PUC price cap of $9.00/kwh on a hot July afternoon.
Those are spot prices, not long term contract prices.

The wholesale cost of power on the spot market is not directly connected to what you pay, even when on a time of use rate structure.

POCOs can pay big $$ to buy the last few mWh to meet peak demand. Averaged out over total generation and other purchases, it only has a slight impact on their average cost of power.The purchases are made to avoid brown outs, blackouts, and/or other power curtailments.

If you are basing your savings calcs on 2-$9 you are using the wrong numbers.

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Old 03-18-2017, 09:13 PM   #13
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Re: Run central air on battery


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Originally Posted by Oso954 View Post
The wholesale cost of power on the spot market is not directly connected to what you pay, even when on a time of use rate structure.

Maybe not in California. Texas is deregulated. My retailer sells me power at wholesale spot + 1.7/kwh + about 4 for the local distribution company. Spot prices are recalculated every 15 minutes by the Independent System Operator.

My bill this winter has run 8.0 - 8.5 I expect it to be higher this summer and much, much higher in the afternoons. Like I said, one kw could get as high as $9.00/hr. That's what I'm trying to save with this plan.

Most people here though do go for long term contracts, which run around 12 or more, day or night, all year long. Probably more like in California. Buying power at night and storing for day use doesn't work for them.
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Old 03-19-2017, 12:12 PM   #14
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Re: Run central air on battery


Quote:
My bill this winter has run 8.0 - 8.5 I expect it to be higher this summer and much, much higher in the afternoons. Like I said, one kw could get as high as $9.00/hr. That's what I'm trying to save with this plan.
program your t-stat so that it doesn't run in the afternoon.

pre-cool when the rate is lower so it doesn't get too hot.

low tech solutions are best.
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Old 03-19-2017, 01:09 PM   #15
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Re: Run central air on battery


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Originally Posted by puttster View Post
Maybe not in California. Texas is deregulated. My retailer sells me power at wholesale spot + 1.7/kwh + about 4 for the local distribution company. Spot prices are recalculated every 15 minutes by the Independent System Operator.

My bill this winter has run 8.0 - 8.5 I expect it to be higher this summer and much, much higher in the afternoons. Like I said, one kw could get as high as $9.00/hr. That's what I'm trying to save with this plan.

Most people here though do go for long term contracts, which run around 12 or more, day or night, all year long. Probably more like in California. Buying power at night and storing for day use doesn't work for them.

I understand what you're trying to accomplish here, but what you want to do is probably cost prohibitive. Even with the Musk Power wall, you would probably need two of them ( at $5000 each) to generate enough energy to power a 240v 12-15 amp central a/c. The batteries, cable and charge controllers would probably run you around $15000, not including labor

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