## 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?

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?

active| newest | oldestfor 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.

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.

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...

12v battery bank, converted to 110v current.

Weekdays;

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..)

Weekend

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 ampsGo 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?