Can I power my entire home load with a battery setup for 1-2 hour stints?

Discussion in 'The Projects Forum' started by Offthegrid, Aug 20, 2014.

  1. Offthegrid

    Thread Starter New Member

    Aug 20, 2014
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    Hi All,
    I'm currently looking into the viability of the following;

    Installing a battery pack that will allow me to disconnect from the grid, and will supply my entire home at 240V (peak load 4kW), but only for a 1-2 hours max each day.

    1. Is it an easy calc to work out the required battery size?
    2. Can you fully discharge batteries these days without harming their life?
    3. Ballpark cost of a system like this?

    My understanding is that it is effectively a online UPS setup, but i need to run for longer than a traditional UPS - correct?

    This is not my expertise, and i am researching the pitfalls of such a system, witch I assume are there as this doesn't appear to be a common setup.

    Thanks in advance
    Offthegrid
     
  2. shteii01

    AAC Fanatic!

    Feb 19, 2010
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  3. MrChips

    Moderator

    Oct 2, 2009
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    I think you are going about it incorrectly.

    If you want to go off-grid, begin by unplugging EVERYTHING.

    Then proceed by adding only essentials.
    You also want to find alternatives for the heavy power hogs such as washer, dryer, fridge and stove, furnace blower, etc.

    Is it possible to narrow it down to under 500W total? Now you're really talking off-grid.
     
  4. Offthegrid

    Thread Starter New Member

    Aug 20, 2014
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    This is driven by a desire to benefit from a situation where the distribution company will reward taking load off at peak time, rather than my desire to get off the grid...
     
    Last edited: Aug 21, 2014
  5. Papabravo

    Expert

    Feb 24, 2006
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    I do not see this desire as practical. A simple calculation for a Switched Mode Power Supply will expose the problem.

    Simple Rule #1. Power Out will ALWAYS be less than Power In; sometimes much less.

    Your desired output power is 4 kW or about 5.36 Horsepower. The efficiency of a boost converter to go from 12VDC to 240VDC is maybe 75%. This means you will need 5.33 kW from the battery.

    5.33 kW / 12 ≈ 444.5 Amperes

    That is an enormous amount of current to work with. Your inductor will need to handle peak currents of twice that much. A typical 100 Ah battery will be drained in less than 15 minutes.

    We're still not done. With 240 volts DC you need to make an inverter to get 240VAC. Apply Simple Rule #1 to the inverter using the same 75% factor and you start to see the futility of proceeding.

    The solution to your problem is a generator run by gasoline, natural gas, or water.
     
  6. MrChips

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    Oct 2, 2009
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    If trying to reduce overall electricity consumption, consider the essentials that you would need during a power failure: lights at night and power for the fridge and freezer.

    For lights, get battery powered LED lighting. For the fridge and freezer, switch to energy efficient appliances. Consider getting a 12V or 24V powered fridge or freezer that can run off batteries.

    If your goal is to reduce peak time consumption then that is easily achievable.
    Our baseline consumption is less that 0.5kWh. Energy consumption goes up significantly when using clothes washer and drier, electric stove and air conditioner.

    Avoid using any and all of the above during peak times. Switch to a gas stove or use your outdoor BBQ for cooking during peak time.

    Do the laundry at off-peak hours. Hang the washing outside to dry on a clothes line.
    Turn off or raise the setting on the air conditioner.
     
  7. KMoffett

    AAC Fanatic!

    Dec 19, 2007
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    "Can I power my entire home load with a battery setup for 1-2 hour stints?"

    Sure, the "battery" that starts your 5KW gas generator. :)

    Ken
     
  8. Papabravo

    Expert

    Feb 24, 2006
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    Why didn't I think of that?! Well maybe I did sorta..kinda:D
     
  9. wayneh

    Expert

    Sep 9, 2010
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    For any system to work, it needs to know when "peak time" is so that this information can be converted to action. Your local power company must be supplying that information, in order to offer an incentive. Or perhaps it's just the daily cycle?

    What exactly do you need to do to receive the incentive? Just turning off the largest loads will hugely reduce your consumption. This would be far cheaper than trying to ensure these loads continue running off grid. But again, you need a way to convert the "peak time" information to an action - getting up to flip a switch. At my house, the power company can remotely shut off my air conditioner, and I get cheaper electricity as a result of making that deal with them.
     
  10. MrSmoofy

    Member

    Jul 28, 2014
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    Same here they control the A/C as well as the pool pump think I get $24 off the bill plus the savings of those two items being turned off.
     
  11. KMoffett

    AAC Fanatic!

    Dec 19, 2007
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    My air conditioner is controlled by our power company for a discount on our bill too. They shut it off for brief periods during peak. I assume the shutoffs are rotated among it's customers. I've never noticed the loss.

    Ken
     
  12. wayneh

    Expert

    Sep 9, 2010
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    Me either, except maybe once or twice over the last several years. We use a programmable thermostat, so the AC is off for most of a normal day anyway and doesn't turn on much until rates fall in the evening.

    They're actually doing me a favor by turning it off. My billing is by an hourly rate that depends on the balance of supply and demand. If rates shoot up, to say 25¢/kWh, I don't want to buy any! At rare times I'll see the rate drop to zero, and my wife swears she saw a negative rate one morning. Makes me wish for an electric car with a smart charger.
     
  13. Offthegrid

    Thread Starter New Member

    Aug 20, 2014
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    Thanks for the feedback.

    The network company does send a signal when residential peaks occur, and they already control electric hot water cylinders in this way.

    It is not so much about what is needed to receive the incentive, there is currently no incentive in place - what I'm looking into is aggregrating a number of houses load, and then approaching the network company with the ability to reduce the peak load by 'X kW'.... therefore I need X to be as large as possible for each house, and the fact that i want it to be as seamless as possible (both install and ongoing) for the consumer means that it would make more sense to reduce the entire home's load to 0, even if this is for a relatively short time....

    As i stated in my initial post, i could well be barking up the wrong tree, but at the moment, to assess the viability i really just need to quantify two things;
    1. the cost of a system to deliver this (this is the one I'm stuggling with)
    2. the value the network company (i have a reasonable idea of this)

    Cheers
     
  14. Bernard

    AAC Fanatic!

    Aug 7, 2008
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    What power source is used to recharge the batteries? If line power, there would have to a huge difference in rate from low to peak. If solar, grid connected works & no battery required. Just looked up my average power for July- 873 W. Ran a deficet of 103 kWh for month, grid connected solar. But still have over 2,000 kWh in the bank, which will be paid as a credit at about 3 cents / kWh, in October.
     
  15. tcmtech

    Well-Known Member

    Nov 4, 2013
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    Going back to the OP's original question of course it's doable. The big question is is it cost effective over a practical time frame.

    If it was me and I was going for cheap I would be looking into finding a good used commercial UPS unit rated in the 5+ KVA range which will more than likely use something better than a 12 volt battery. Most likely at that power level it would be a 48 volt or higher input systems with a normal AC line output and moderate overload capacity.

    Battery wise again going the second hand route finding a good set of still functional industrial surplus batteries is not all that hard. Forklift, warehouse equipment and golf cart supply places plus your local salvage yard can more than likely set you up with a functional set of batteries for little more than scrap price.

    Capacity wise I would aim for having a bout 3x - 4x the maximum intended power capacity you need so the batteries are not being heavily cycled although with good commercial units getting moderately drained down for a few hours then recharged later that day really wont bother them much.

    Cost wise I have no clue where you are or what your scrounging skills are but where I am at and with what I do for a living I would say a system like what you want could be put together from second hand materials for under $1000

    all new probably 10X that.

    That's my thoughts.
     
  16. tindel

    Active Member

    Sep 16, 2012
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    I think Mr. Chips had it right in post #3. Instead of figuring out how to keep things running during peak times I think the answer is to keep things off during peak times.

    Follow the Joules! Even if you do make a UPS that keeps you most things running if you consider that your UPS will likely be 80% efficient both in charge and discharge. Sounds like your output energy is 8kWhr, you need a 8kW/80% = 10kWhr of energy in your UPS - at a minimum. But on top of that - to get your battery bank charged up for the following day you'll need to charge your battery. This will take about 10kWhr/80% = 12.5kWhr. So ultimately you will use ~56% more energy (8kWhr/12.5kWhr-1) than if you had just run your stuff through the peak hours! Meaning that your energy savings in dollars needs to be more than 56%! I doubt the discount is that much.

    On top of that - you need one kick ass UPS - like this: http://www.qwiksolar.com/Liberty-Power-Packs
    and at $10k it's not cheep - and it sure isn't small! This means your discount will need to be even larger than 56%.

    Like Mr Chips said - it's a better idea to turn everything off. Can you take the signal your energy company sends to one device and push it to other devices in the house to ensure they turn off, with possibly a manual override if you need them for one day. I've found that the timers that you plug into the wall - like these: http://www.amazon.com/Intermatic-PB314E-240-Volt-Electric-Mechanism/dp/B0039Q7K1E work pretty well for this sort of stuff. Most appliances have no problem being off for a few hours... your beer stays cool, your water stays warm enough, and your house usually says somewhat comfortable - at least where I live.

    Others have pointed out that there may be cheaper ways to produce 10kWhr of energy - solar, gas generator, natural gas, etc... this all true... there's a reason why people use solar to supplement their electrical load. The energy is essentially free and in abundance. The money you do spend you can make your money back in relatively short order.

    Good luck!
     
  17. wayneh

    Expert

    Sep 9, 2010
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    Not to start a battle here, but that's just not true, not without big subsidies anyway. The cost of solar is still well higher than the grid, and an installation does not provide a reliable payback of the capital outlay. Solar is fantastic and getting better where grid power is a poor option, but I pay only about 4¢/kWhr at my house. No way I would save money by installing solar.
     
  18. MrSmoofy

    Member

    Jul 28, 2014
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    I agree with this, 10k+ USD to install a decent solar system that would take 10+ years to recover the costs. While solar companies are pushing hard to sell it it's only the tree huggers with deap pockets that are buying it.

    It's just not a solution for the general public because if it was everyone would have it.
     
  19. wayneh

    Expert

    Sep 9, 2010
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    Take the mile-high view. You have an aggregated demand. Without a change in consumption, who is going to be more able to supply that demand, you or the power company? My money is on the power company. You're setting yourself up to be an alternative power company, with your power source coming from the same place as the regular power company. The BEST you can do is time-shift the consumption a bit at the expense of all the efficiency losses, which actually will increase overall consumption by at least 20%, maybe as much as 50%.

    Unless the power company is irrationally desperate for what you intend to offer, I can't imagine they'll pay you enough to make it worthwhile.
     
  20. tindel

    Active Member

    Sep 16, 2012
    568
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    I think you guys missed my point - or my point wasn't very clear.

    If you want to remain powered during peak times and get the benefit of lower rates the only solution I see is a solar system. Once installed, the sun is abundant (in CO anyway) and it's free energy. The startup costs are large, but you might be able to make your money back in short order if you get the discount of not using grid energy during peak times - maybe you can even sell energy back to the energy company during peak times. This could be a viable solution for the OP to get solar and make his money back quickly with hopefully some government subsidies to help him along - although I believe they have run their course in the US.

    Of course how much you pay for energy is a huge factor. I personally pay 12.3cents per kWhr... and I use about 9000 KWhr per year (~$1100/year). Solar is viable for me. I suspect that I would make my money back in 5 to 7 years if I shop around for a deal, get a government subsidy or two, I do some of the work myself, and sell some of the energy back to my electric company (A friend at work came up with similar numbers after installing solar so I know I'm on the right track). Unfortunately I don't have a southern exposure. If I were able to remain off the grid for an hour or two each day with my system and get an additional cut on my electric bill that just decreases my time to getting in the black.

    Is it an instant savings - no... but it you think long term... You can make your money back in relatively short order, just as I said in my previous post.

    One thing to note is that it is against the law in the US for HOA's to not allow you to install solar or artificial grass.

    Maybe I'm a "tree hugger or have deep pockets". HAHAHAHAHAHAHA! I think I just know how to do math.
     
    Last edited: Aug 22, 2014
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