Battery power

I am attempting to calculate my amp need for my garage in order to set up a 12v battery system. Using the formula; Watts / voltage = amps drawn, I have hit a question. If using a 12v battery system and converting to 110v, should I be using 12v or 110v in the formula?

Example; 100 watt light bulb / 110 volts = .91 amps. Nice!
But if I use the 12v; 100watt light / 12v = 8.3 amps.

I'm believing I would be drawing 8.3 amps from the battery to power this light.
Plus the converter draw...

Please tell me which is correct?

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guyfrom7up9 years ago
correct, you'd be drawing 8.3 amps from the battery, plus the converter
olddawg (author)  guyfrom7up9 years ago
Thank you! This is helping me! Let me ask another; Let's put a 15amp outlet circuit in my garage. How do I calculate the draw in amps for the battery? For calculation, figuring 15 amps for 1 hr. The 110v light took 8.3amps from the battery... Will the circuit take 120amps? (8 times the draw)
look at it this way, the watts into the converter is the same as the watts out.

for example:
1 amp at 12 volts in would result in ~0.1 amps out at 110 volts.

for 15 amps at 110v:
15 times 110 = 1650 watts

1650 watts / 12 volts (battery) = 137.5 amps

if you were to draw 15 amps at 110 ac, you would be drawing 137.5 amps from the battery.
olddawg (author)  guyfrom7up9 years ago
Once again... Thank You! This makes sense. Figure the watts, then watts to amps by the battery. I'm coming up with almost 1000 amps needed for my needs... 3000 amps needed for my wants... Guess I have to work on those wants... May I push this another step? I'm looking at batteries and seeing that a standard 12v 'Marine' deep cell battery has 80AH (as I figure it 1900+amps). I've seen the specialty batteries for storage @ 2250, but 6v. Both being equal in price. Needing 2 of the 6v to make 12v, even with the specialty lasting twice as long, it seems to be a wash... I'm I missing something here? Assuming amp use & amp replacement is equal across any voltage pattern (ie: 6v, 12v or 48v) .Does it matter if the voltage of the generator is lower than the battery rating? I do assume if higher there are consequences... 6v generator to 12v battery.... With that, is a amp truly an amp? Generator is 6v, produces a amp, would it be 1amp to a 12v battery or .5amp? I've come a long way today!
first off: There's no way that your going to be able to drain 1000 amps from almost any battery, and definitely not 3000 amps. Even iff you could draw 1000 amps from a battery it'd last for like a minute and be really really really really really hot. lets say you have a 100 AH battery drawing 1000 amps would make it last for 6 minutes
olddawg (author)  guyfrom7up9 years ago
The 1000 amps is a weeks usage... probably should have said that. Lights on & off daily, vacuum on weekends, radio while hanging out there... I also plan on batteries, not just a battery. Battery of choice right now 225AH and hope to start with two. This should be 4500 amps (wired to increase voltage to 12v) and only using 1000 in a week. Is this impractical? If it is, I'm really missing this....
Patrik olddawg9 years ago
You may want to try keep your units consistent - you're really mixing apples and oranges at the moment.

Saying that "1000 Amps is a weeks usage" doesn't really make any sense. That'd be like saying that your gas tank holds 40 miles per gallon...

If you want to calculate how much *energy* you require for a week's usage, you would typically write it in kilo-Watt hours, aka kWh - that's the units you see on your energy bill as well. (If you're a physicist, you might want to use Joules, where 1 Joule = 1 Watt second).

For example, if you need to have one 100 Watt bulb burning constantly for a week, that would be 100 Watts x 24 hours/day x 7 days/week x 1 week = 16.8 kWh.

Now, if you were to use a 110V, 100W lightbulb, it would draw 0.91 A. If you were to use a 100W bulb intended for 12 V, it would draw 8.33 A. But both bulbs would use the same amount of power. And if both bulbs had the same efficiency, they would both give off the same amount of light. It's just that 1 Amp carries a lot more power at 110 Volts than at 12 Volts.

If you're using AC, you could use a transformer to increase the voltage but decrease the current, or vice versa. You're essentially just "translating" one set of voltage and current to another, without too many losses. Same principle holds for AC to DC conversion or vice versa.

In a scientist's ideal world, all batteries would be labeled with how much energy they can store in Joules. In practice, batteries put out a relatively fixed voltage, so their capacity is listed in Ah or mAh, rather than Wh or kWh. However, you can't compare the Ah capacity of two batteries, unless they have the same voltage (again, one Amp at a higher voltage drop carries more power than one Amp at a lower voltage). Also, the Ah capacity of a battery has nothing to do with how *fast* you can discharge the battery - only with how much energy it can hold.
olddawg (author)  Patrik9 years ago
Patrik, thank you for the response! Let me ask you something. I now have 2 6v solar lights in the garage that are doing they're particular job (stopping me from walking into things) very well. This is what's leading me to turning the garage 'Green'. I thought of going to a 12v system and upping the power figuring I would be increasing what my 6v lights are doing right now and purchasing new 12v lighting. I am not certain this is what I really should do for the long term goal, which is to 'grid connect'. If indeed I ever reach my goal, then it seems more practical to set up with efficient 110v lighting. So with this in mind and trying to determine what I might use in a week use of amperage out in the garage, I came up with 1000 amps would be used. Nothing would be used constantly, just that much would be used in a weeks time. By knowing this, I hoped I could determine what kind for battery storage I would need. I came up with the 2- 6v 225AH batteries. Costs are within my budget and I thought they would do the job. 225AH , figuring I can not use more than 50% of those available amps per hour, I need to adjust either my battery bank or my usage. Question 1; Am I still shooting too high? Is 50% too great a number? This lead me to the next step, in my mind. How many amps can I realistically replace each week to make up my usage and recharge the batteries? I am looking at both wind & solar and calculating gains vs. costs right now. I have to say I'm leaning towards wind... I'm gathering parts now to build a test turbine and see if I'm figuring everything right. With no guidelines (I could find) to go by, I thought this would be the best way to start. Figure a weeks worth of usage. Find a source to draw from that would work. Find a way to replace the energy used. I now realize there's more to this, yet I think I'm on the right path. Any comments & please, suggestions are not only welcome, but wanted.
Patrik olddawg9 years ago
Again, saying that you would use "1000 Amps in a week" does not make any sense. It's just as nonsensical as saying "I travel a total of 1000 mph in a week" - the units just don't match up. Amps tells you how much current is flowing right now, whereas what you want to estimate would also need to include the amount of time time and the voltage.

Do you mean Amps x hours, at 12Volt, for example?

Could you post how you arrived at that one-week estimate - maybe we can unravel where you're going wrong...
olddawg (author)  Patrik9 years ago
Okay, Now I'm ready!

12v battery bank, converted to 110v current.

Lights; 2- 20 watt motion detector spots running 1 hr constant twice a day
3.33amps morning & evening = 7amps daily x 7 days = 49 amps
( I currently have 2- 6v solar powered spots in there now and they work
great! They may stay..)

1- 15 amp appliance figured at 45minute actual use cycle = 103 amps
Say I'm ambitious and do this for 4 hrs = 412 amps
Do this Sat & Sunday 412 x 2 = 824 amps

Go 1 step further and say it's a gray day and I need lighting
4- 20 amp lights = 6.7 amps per hour + my appliance 103 amps
= 110 amps per hour

I take 50 amps (lights) + 825 amps (appliance) + 60 amps (switched lights)
I get 935 amps used in 1 week

I tried to add 10% converter loss and rounded all numbers up for math's sake. I also rounded to 1000 amps to make up for unknown usage or losses.

Trying to stay under 50% of the battery capacity per hour, I assume is correct usage... I see my highest usage is 110 amps in any 1 hour.
Using a 225AH battery @ 12volts, I figure I just make it.

Now that I know (think I do) what my usage would be, then I should be able to determine how much amperage I must generate in a week to make up for my usage.

Does that clear it up?
How close am I?

I do realize that another 12v 225AH battery would be better, but will this work with one?
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