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Thread: Batteries For Home Standby Power

  1. #1
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    Batteries For Home Standby Power

    I have a customer that asked for a quote for a standby generator for his townhouse condo. I gave him the quote for a 16kw unit. It was going to power his entire load with load shedding relays on the electric oven, dryer and AC. Now he says he does not think the condo board will approve a standby because of the noise during exercising or running during an outage. He is suggesting a battery and an inverter.

    My response without any real experience with batteries powering a home is that it would be expensive. He wants me to look into it and I am willing. If nothing comes of it I should still learn something. He has plenty of room in his unfinished basement for batteries.

    He is willing to forget about powering all that a 16kw standby could handle. He listed all the critical things that we can all imagine....gas burner for heat system, refrigerator, tv, WiFi, microwave, a few
    Lights, etc.. I explained that some of these loads are connected to circuits that supply other than the critical stuff and the system would have to be large enough to power whatever is connected or plugged in.

    I’m looking for some feedback on how practical this concept is. I would assume the system would need about 5kw peak load and have an average output of 2kw. These numbers are just shooting from the hip. If these numbers are anywhere near close, how big of a battery would I need to survive a power outage for four days?

    My gut tells me the battery bank would be enormous, the inverter battery charger would cost a fortune and I’m not sure what the transfer equipment would be. Anybody have opinions on this?

  2. #2
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    Inverter/battery boils down to Amp-hours on the batteries, based on battery voltage, inverter efficiency and load. But four days? Expensive is an understatement. The larger systems I'm familiar with in the power industry are UPS systems for critical system backup. The building load is on a generator. They make generators that can be insulated and muffled to the point of near silence, so that would be my suggestion. Transfer switch costs would be the same either way. My whole house generator (propane) can't even be heard until I get within a few feet and it's nothing special....just a Honeywell sold by Home depot. It exercises once a week for 15 minutes. The neighbors would be drooling with envy during an outage.
    Last edited by meternerd; 06-12-18 at 10:38 PM.

  3. #3
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    Those who power their entire home off-grid usually have thousands of watts of panels and scores of high-capacity batteries.

    Myself, I have a measly four 105-Ah of batteries and I'm lucky to get a day out of running just my furnace.

  4. #4
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    180612-2201 EDT

    I agree with your 2 and 5 kW guesses. Three days at 5 kW is 5*24*3 = 360 kWH, and at 2 kW is 144 kWH.

    A typical car battery might be 12*80/1000 = 0.96 kWH. Car batteries are about $100 each. 360*100 = $36,000.

    The extremely rare power outage does not justify this kind of capital cost, maintenance, and replacement. And you don't want that many batteries in your basement.

    Get a small quiet car engine and alternator on a trailer, or power take off on a car.

    In my life time, a fairly long time, I have had less than two weeks of total outage. The longest single outage was about 5 days for the great eastern blackout. I use a Honda 5kW portable for backup, not quiet.

    I also believe everyone should have a backup generator. In my neighborhood probably less than 3% have backup, and every neighbor could afford a generator. Few would really need more than 5 kW capability. However, most would not know how to get along with 5 kW.

    Our area has relatively low cost natural gas.

    .

  5. #5
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    Quote Originally Posted by gar View Post
    180612-2201 EDT

    I agree with your 2 and 5 kW guesses. Three days at 5 kW is 5*24*3 = 360 kWH, and at 2 kW is 144 kWH.

    A typical car battery might be 12*80/1000 = 0.96 kWH. Car batteries are about $100 each. 360*100 = $36,000.

    The extremely rare power outage does not justify this kind of capital cost, maintenance, and replacement. And you don't want that many batteries in your basement.

    Get a small quiet car engine and alternator on a trailer, or power take off on a car.

    In my life time, a fairly long time, I have had less than two weeks of total outage. The longest single outage was about 5 days for the great eastern blackout. I use a Honda 5kW portable for backup, not quiet.

    I also believe everyone should have a backup generator. In my neighborhood probably less than 3% have backup, and every neighbor could afford a generator. Few would really need more than 5 kW capability. However, most would not know how to get along with 5 kW.

    Our area has relatively low cost natural gas.

    .
    Based on my bill, my consumption, even during the summer, is max about 30 kW-hr/day, so 3 days, with AC would be 120 kW-hr. The Tesla Powerwall 2, less inverter, is $6,000 a pop for 13.5 kW-hr at a rate of up to 5 kW. You would need 9 units for a total of $54,000 in batteries alone

  6. #6
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    For less than the cost of all those batteries you can buy a totally self-contained RV trailer which you could easily live in for a week before having to restock gas for genny, water and propane, and dump waste. Just find a local RV storage lot or a willing homeowner with some unused backyard and park it until needed.

    Bonus: It's really useful for those weekend getaways!

  7. #7
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    Quote Originally Posted by gadfly56 View Post
    Based on my bill, my consumption, even during the summer, is max about 30 kW-hr/day, so 3 days, with AC would be 120 kW-hr. The Tesla Powerwall 2, less inverter, is $6,000 a pop for 13.5 kW-hr at a rate of up to 5 kW. You would need 9 units for a total of $54,000 in batteries alone
    Your calculation is not taking account for the energy the PV system would be contributing. The Tesla batteries are designed to work with a PV systems that is generating most of the energy. It will help make up for peak loads that are greater than the PV is producing and energy at night when there is no PV energy. In most cases you would not connect the AC to the critical load panel supplied by the Tesla batteries. If you really want everything backed up and to function like you have utility power then you will need to pay the price for more batteries.

    BTW
    The Tesla batteries have a builtin inverter. They simply connected to a 2P30 back feed breaker in the critical/backup loads panel.
    Curt Swartz
    Electrical Contractor

  8. #8
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    tesla makes a pretty big battery that is lithium ion and is made for this kind of thing. Not cheap but do you really want a bunch of lead acid batteries in someone's townhouse?

    another option is to suggest the customer buy one or more of these units.

    https://www.amazon.com/Goal-Zero-Por...rds=yeti+power

    They are not cheap but one can plug just about anything into it. You would need to come up with some kind of xfer switch arrangement for their gas fired furnace so they can plug into the power station. They can be recharged by solar if you have enough solar PV. But if they are judicious in their use of it they can probably last a day or two with a couple of these units.
    Bob

  9. #9
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    Quote Originally Posted by curt swartz View Post
    Your calculation is not taking account for the energy the PV system would be contributing. The Tesla batteries are designed to work with a PV systems that is generating most of the energy. It will help make up for peak loads that are greater than the PV is producing and energy at night when there is no PV energy. In most cases you would not connect the AC to the critical load panel supplied by the Tesla batteries. If you really want everything backed up and to function like you have utility power then you will need to pay the price for more batteries.

    BTW
    The Tesla batteries have a builtin inverter. They simply connected to a 2P30 back feed breaker in the critical/backup loads panel.
    The OP's customer lives in a condo. "No solar panels for you."

    Even with a built in inverter you'll need some infrastructure to set up 9 batteries to feed the condo.

  10. #10
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    Quote Originally Posted by petersonra View Post
    tesla makes a pretty big battery that is lithium ion and is made for this kind of thing. Not cheap but do you really want a bunch of lead acid batteries in someone's townhouse?

    another option is to suggest the customer buy one or more of these units.

    https://www.amazon.com/Goal-Zero-Por...rds=yeti+power

    They are not cheap but one can plug just about anything into it. You would need to come up with some kind of xfer switch arrangement for their gas fired furnace so they can plug into the power station. They can be recharged by solar if you have enough solar PV. But if they are judicious in their use of it they can probably last a day or two with a couple of these units.
    Uhhhh, see post #5?

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