calculating battery back up

Dear Forum,

I just joined and I am very new. Thank you for a great place to ask and learn!

Quick question on battery back up calculations. I see batteries listed with 'x' number of amps and usually at 6 or 12 volts. If I want to draw power at the standard rate for normally wired homes, can these batteries be used singly? If 'yes' how would you make that calculation? If 'no' what do you do? Buy a bunch of batteries and wire them in series?

cheers!

Comments

  • BB.
    BB. Super Moderators, Administrators Posts: 33,431 admin
    Re: calculating battery back up

    Ah... The $64,000 question.

    First, you need to define what a "normal" home uses... For me, a bit of 200 kWhrs per month (4 people, no AC, near San Francisco California--the average home use is around 600-1,000 kWhrs per month in our area)... For other, 2,000 kWhrs per month.

    Lets assume you are running a $100 US/CAN power bill per month at pay $0.10 per kWhr... That would be $100/$0.10perKWhr=1,000 kWhrs per month.

    1,000 kWhrs/30 days per month=33.3 kWhrs per day average.

    33.3 kWhrs per day / 24 hours per day = 1.39 kW or 1,390 watts 24 hours per day... Equal to an electric heater plugged into your home outlet running on high heat 24 hours per day.

    Just to give you an idea--a good place to aim for off-grid (battery backed) power is around 100 kWhrs per month or less (about 1/10 what an "average" home may use)... The more power you use, the larger your off-grid system needs to be.

    So--here we are, using in our imaginary home 33.3 kWhrs per day (33,300 watt hours per day). Just to give you an idea--the average car battery holds about 1.0kWhr (1,00 watt*hours) of useful energy (if it was the correct kind of deep cycle battery)... So, using our 6x rule of thumb, a 33.3 kWhr per day home would need:

    33.3 kWhrs * 3 days of storage * 1/50% maximum discharge * 1/1.0kWH of battery storage=600 "average car sized" storage batteries.

    As you can see, this is not a practical battery arrangement. One way to do better is to use much larger batteries. In this case, lets look at ForkLift Truck batteries.
    Sale price: $4,953.00
    935 amp-hours at 20 hour rate, 24 volt.
    Dimensions: 30.63" x 12.69" x 31" high.
    Weight: 1,440 pounds
    Now, note that batteries are usually rated in Amp*Hours... A 1 amp light bulb running for 10 hours would be 10 Amp*Hours.

    But, normally, we talk about Watt*Hours... A 12 watt light running for 1 hour would be 12 Watt*hours...

    What you need is the math/formula to convert between Watts and Amps:

    P=V*I... or I=P/V or V=P/I

    So, if we convert that battery into Watt*Hours:

    935AH*24Volts=22,440 Watt*Hours or 22.4 kWhrs of energy storage.

    Take the original 33.3kWhrs per day (1,000 kWHrs per month):

    33.3kWhrs per day * 3 days * 1/50% discharge * 1/22.4 kWhrs per battery=9 of these $5,000 1,400lb batteries...

    If you could cut your power use by 9/10 (down to 100 kWhrs per month), then you would only need one of these batteries instead (or more, smaller batteries).

    Note that when you add batteries in series, the voltage adds, and the AH stay the same. When you add batteries in parallel, the AH ratings add, and the voltage stays the same... There is no magic here--you are still storing the same amount of energy, but in one case the energy is available at "12 volts", in another case it is available at "24 volts" (or whatever you choose).

    There are some more details here that we have yet to discuss--but it sort of gives you a 50,000 foot view of sizing/pricing batteries for an Off-Grid system--and why we always tell everyone that conservation is the first step before going into solar power...

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • niel
    niel Solar Expert Posts: 10,300 ✭✭✭✭
    Re: calculating battery back up

    firstly, batteries are dc and utility power is ac(varies at either 50 or 60 hertz). to use it from dc to ac one must invert it, which is simply an upconversion to 120vac and is done with an inverter and some can up it to 220vac or 240vac. secondly, that amps in conjunction with the volts makes watts and that is carried over from one to the other plus losses in the conversion. volts x amps = watts. knowing this if you have a 240w load at 120vac this is 240w/120vac=2amps ac. now that 240w/12vdc=20amps dc. adding to that the losses in the conversion(% varies per inverter) and roughly making it 10% would be 10%x20a=2a so 20a+2a=22a total at 12vdc to run that load at 120vac. the input watts needed will be 22ax12v=264w.
  • System2
    System2 Posts: 6,290 admin
    Re: calculating battery back up

    Hi Bill and Niel!

    Wow! Thank you so much for the great explanation! What a terrific welcome to the forum.

    As I am digesting this, I just looked over my own power power consumption in my townhouse. I seem to average 300 kilowatt hours in a month.

    I was having some problem understanding your calculation. 1/50 is point 02 I think? I keep getting 1.1998? I know I must be doing something wrong here. Can you tell me where I made my mistake?

    Two questions, one specifically relating to Bills example of the fork lift battery.

    Firstly, using your conversion method, how long could I run my own little townhouse off of that forklift battery, if I did not rewire my house to 24 volts? Can I leave my house wired the way it is now?

    My second question is just general. Does it matter if you draw at 6, 12, 24 or 110 or 220 or what ever? If 'yes' how does that affect things? Do you just divide into the amps to see how many amps you get at the rate?

    Oh-a third question. If I wanted to ever use power tools that drew more power than my computer or fridge, is there some way the system (the inverter?) can be switched to do that? I'm thinking here of the occasional 220 saw to make a book shelf or drill to do some car work.

    Thanks guys for all of your help and time. I am just kind of flabbergasted that anyone even wrote me never mind took such time to help! Many many thanks!

    Frank
  • System2
    System2 Posts: 6,290 admin
    Re: calculating battery back up

    Hi Niel,

    Thank you so much for you help and information. As I wrote to Bill, what a terrific welcome to the forum. I never expected such help, not to mention such expert and detailed help. Thank you again!

    I am just trying to work though my own sample calculation using what you wrote and what Bill wrote and using my own townhouse power consumption.

    I use 300 kwh per month. Using your calculation then, I get 300 kwh, times 1000 to get 300,000 watts. I divide 300,000 watts by 120 to get 2500 amps. To calculate how long that forklift battery of 935 amps will last in my situation, I divide the 2,500 by 935? Which gets me 2.67 days. More or less.

    You refine the calculation, with some power losses on conversions, of about 10 per cent. So I would get 2.67 days times point 9 or 2.40 days?

    I am sure I made errors here, and would be grateful for any pointers.

    Thank you again Niel!

    Frank

    niel wrote: »
    firstly, batteries are dc and utility power is ac(varies at either 50 or 60 hertz). to use it from dc to ac one must invert it, which is simply an upconversion to 120vac and is done with an inverter and some can up it to 220vac or 240vac. secondly, that amps in conjunction with the volts makes watts and that is carried over from one to the other plus losses in the conversion. volts x amps = watts. knowing this if you have a 240w load at 120vac this is 240w/120vac=2amps ac. now that 240w/12vdc=20amps dc. adding to that the losses in the conversion(% varies per inverter) and roughly making it 10% would be 10%x20a=2a so 20a+2a=22a total at 12vdc to run that load at 120vac. the input watts needed will be 22ax12v=264w.
  • niel
    niel Solar Expert Posts: 10,300 ✭✭✭✭
    Re: calculating battery back up

    you should bring that down to the daily average as that battery won't have to go 30 days. 300,000w/30days=10,000w/day. 10,000w/12v=833.33amps dc/day. if losses are 10% then 1.1x833.33=916.67amps dc/day. input power for that 10kwh ac load=916.67x12vdc=11,000wdc. i could've just multiplied the w by 1.1 (110%) in the first place for the input watts needed, but i wanted to show you how the current is much higher for the 12v batteries to produce the same power needed for 120vac at a smaller current.
    for the sake of calculating for pvs/batteries needed is why we break it down and would take me into areas that get lengthy and you can read them elsewhere in the forum.
    oh, and to answer your question the 12v fork lift battery will be nearly dead. 935ah fork lift battery and almost 917ah load current. 917ah/935ah=98.1% dod (depth of discharge). for the record we don't recommend going below 50% to account for better battery life so 2 of those would be needed.
  • BB.
    BB. Super Moderators, Administrators Posts: 33,431 admin
    Re: calculating battery back up

    Hi Frank,

    That was 1/50% or 1/0.5=2 ... Basically, as Niel said, try not to cycle a lead acid battery below 50% capacity for long(er) life... So, you need 2x as much battery for any given load.

    Regarding all of the calculations... Your power bill is in kiloWatt*Hours... Kilo=1,000 (1kWH=1,000 Watt*Hours).

    Watt is like speed... tells you the rate of electricity you are using (like a 100 watt light bulb). But you also need time to figure out how far you went (distance) or how much energy you have used. In the case of "home power", we use Watt*Hours--so a 100 watt light bulb burning for 10 hours = 1,000 Watt*Hours or 1.0 kWattHours.

    You can do the calculations in Watts, kWatts, or Amps... The difference is that with Watts--you know everything. With Amps, you need to know the voltage you are running...

    P=V*I (Power = Volts * Amps).

    300kWHours of power*time / 30 days = 10 kWHrs per day

    10 kWhrs / 12 volts = 10,000 Watt*hours / 12 volts = 833 Amp*Hours @ 12 volts per day (if you are trying to size a 12 volt battery bank)

    If you wanted a 48 volt battery bank:

    10 kWhrs / 48 volts = 10,000 Watt*hours / 48 volts = 208 Amp*Hours @ 48 volts per day

    Both the 12 volt and 48 volt daily use is the same amount of "energy"... Just one is at 12 volts and the other is at 48 volts (1/4 the current).

    When choosing a battery bank--generally, if you are running a "small" inverter, you can run a 1,000 watt inverter on a 12 volt bank... If you want a large inverter, 4,000 watts and above should be operated on a 48 volt bank (to keep the current and copper wiring a reasonable size).

    P=V*I -- if you make V 4x as large, then current is 1/4 as much (for the same amount of power).

    In reality, battery bank solar is very expensive power... You are probably talking about $1.00+ per kWhr (based on US pricing, sunny region, no property taxes, no rebates included, etc.) whereas your own home power is $0.10 per kWhr. This is because off grid solar is less efficient, and the batteries are expensive and need to be replaced every ~5-10 years (depending on what batteries you purchase). Also, since solar power depends on the sun, you may need a backup generator for stormy weather or winter season use (if you cannot cut back on your electricity usage).

    So--the question is--why do you want off-grid power? Emergency backup like a UPS for a few hours or day or two over the course of a year? To save money? To "go green"? You need 100% reliable power for a data center operation or Iron Lung?... etc.

    If your power is reliable--on you have only rare / short outages... Get a generator that runs on your fuel of choice (propane, natural gas, etc.). And if you want to go green, put the money in a solar thermal system (domestic hot water, heating) and look into Grid Tied solar (if your local utility allows). Grid Tied solar is the most cost effective form of solar electric generation/usage. In some high cost states (New York, California), GT Solar electric can sometimes cost less than utility power (especially if rebates and/or use A/C cooling for hot weather).

    If your power is unreliable and you need weeks of off-grid power--then running a small part of your home load on off-grid solar (fridge, furnace, few CFL lights, well pump, simple TV/radio)... You can build and run a pretty successful small system for emergency power plus a small generator for backup. Great for Ice Storms and such (assuming you have sunny weather after the storm clears).

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • Brock
    Brock Solar Expert Posts: 639 ✭✭✭✭
    Re: calculating battery back up

    I know it isn't really the correct way, but I think of everything in total watts consumed per hour. What should technically be called a "watt hour".

    Now say you have a 100 amp hour 12v battery. You can also call that a 1200 watt battery. Again you don't want to take 1200 watts out of it or it would last very many cycles. So using that 50% rate, you can call it a 600 watt battery. Then take you house loads per day and divide. If you start to think of batteries in watts it makes the conversion much easier.
    3kw solar PV, 4 LiFePO4 100a, xw 6048, Honda eu2000i, iota DLS-54-13, Tesla 3, Leaf, Volt, 4 ton horizontal geothermal, grid tied - Green Bay, WI
  • BB.
    BB. Super Moderators, Administrators Posts: 33,431 admin
    Re: calculating battery back up

    Brock,

    Not sure if Typo or just trying to "simplify" to help understanding... But really a 1,200 watt*HOUR battery...

    I agree with you about doing the calcluations in Watts and Watt*Hours vs Amps and Amp*Hours... Using A/AH people forget what the voltage is and get wrapped up thinking 1 amp at 120 volts and 1 amp at 12 volts is the same thing.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset