battery sizing for fridge backup

[strider3700]

Basically what I'm looking to do is create a battery backup for my fridge and/or freezer in the event of extended power outage.  What I'm having trouble figuring out is how to size the battery bank.   I know from measurement that my fridge uses 1260 watts in 26 hours on a hot day in the middle of summer   so lets say 1200 watts/day.   It ran for 8 hours of that time.   and this is all measured at 120V  so thats 10 amps

Now when I look at batteries they are measured in voltage and amp hours.  so Lets say I have a 12 V battery bank.  How do I determine the Amphours needed to run it for 1 day?      Thanks

[alcul8r]

Your units are confusing.  Watts is an instantaneous measure of power, but you are using it like a cumulative measurement of energy.

If it uses 1260 watts, then over 8 hours it uses 1260 x 8, or 10,080 watthours, or 10+ kilowatthours.

If it used 1260 watthours in 8 hours time then it is using it at a rate of 1260/8, or 158 watts.

Sorry I can't be more helpful.

Rex

[strider3700]

sorry yes it uses 1.26 kwh in a day.  the fact that it ran for 8 hours just tells me that it uses 157 watts while running.  as you pointed out it would be 50 watts when spread out over the day.

50 watts at 12 volts woult be  just over 4 amps  

does this mean that if everything was perfect without losses and I was willing to completely drain the battery I'd need a 4.1 amphour battery?

The fact that it draws 157 watts should also be useful I think.  

157 watts at 12 volts would be 13 amps  so I need a 12 volt battery bank and wiring capable of delivering 13 amps correct?  

These numbers are all way too low of course.  I'd want to not discharge the battery more then 80% during regular use and I'd have to size for losses and so on.  

so I'd assume 200 amphours at 12 Volts should give almost a weeks worth of power to the fridge.

Or am I completely wrong?

[wdyasq]

Strider,

I think you are mixing apples and oranges.  Or maybe in this case, Kiwi fruit and horse turds, getting names of similar stuff mixed.

First of all we will work basicly in WATTS, a unit of power.  Your watts hours are 1200/day or 1.2kW.  Put your inverter losses and that is how much power a day you need. A 6V, 200AH battery has ~1200WH if used over 20 hours and you don't care about the number of charges.  

Your inverter also has to supply the STARTING SURGE.  On my freezer it is 1100W.  I don't like to run my batteries down below 50%.  So for a day of off the grid using my freezer and a 100% efficient inverter (boy, could I sell a boat load of those) I would need 2 "108" size batteries. I would also need an inverter able to handle the surge of startup.

The best thing to do is make sure you are talking in the proper energy units and have several folks figure it and come up with similar numbers.  For an efficient refrigerator 1kW/day is close to right.

Ron

[strider3700]

ok the watthour measurement is the number of watts used in an hour.  

so when you say "A 6V, 200AH battery has ~1200WH if used over 20 hours and you don't care about the number of charges"   how do you know that?  is there a conversion between watthours and amphours?  

[wdyasq]

Strider,

I have to get into discussions trying to teach BASIC electricity.  Do a bit of reading and research and things will start to fall in place. But, Volts X Amps = Watts.

Watts can be thought of as a quanity - like a gallon of water, and is usually thought of as watt hours, If we continue the watter senario,  Volts may be thought of as the pressure of the electricity and Amps as the size of the water pipe.

HTH,

Ron

[ghurd]

Maybe amps could be thought of as how fast the gallon is draining?

G-

[nothing to lose]

If your fridge uses 1200watts in 24hours, 1200watts/12V=100amps. So to run your fridge 24hours you need 100amps plus extra for losses in the wire and inverter and such. That would be from full charge to totally dead, and when near totaly dead the batteries probably would not supply enough power for the startup surge even if they had enough to run it, pluss they won't last long abused like that. So you probably want 4X that.

400amp battery bank -100amps power=300amps left, your drawing it down to 75% capacity or lower that way, it will last much longer than draining even lower or dead, plus it has the power to supply for a startup surge I beleave and covers the losses also.

Actaully 500amps or more would be better for longest battery life, but if this is mostly just a backup and your not going to run it off the batteries allot then you could get by with less.

As for the other numbers, 1200watts/8 hours actaull runtime would just mean it uses about 150watts per hour when running. Rather that's usefull to know depends, if you want actaull runtime of 10 hours it would be good to know, 10x150watts=1500watts/12V=125amps

Or 150watts/12V=12.5amps etc..

Also if you know your amps at 120V then multiply your amps by 10 to figure 12V. It takes 10Xs as many 12V amps as it does 120V amps, then add extra for losses in inverter and wires etc...