Why is the following true...

If I take a 24V battery bank, hooked to a 24V inverter, which has a 60w incandescent bulb plugged in, the current draw is 2.65 amps (measured between battery and inverter on the (+) wire. So for conversation and argument sake, this is essentially what the battery is 'feeling').

If I take the same 60w light bulb. Use a 12V battery and a 12V inverter. The light bulb draws 6.5 amps. The battery bank 'feels' a 6.5A draw.

If I take my 'Kill-A-Watt' AC meter, plug it into my house wall outlet, then plug the same 60w bulb into the meter, the meter tells me the bulb current draw is 0.44 amps.

Side note: The inverter draw with inverter [ON] but 60w bulb [OFF] were both like 0.20 amps. So not enough to influence my question.

I need a large battery bank just to take into account the additional current draw to convert 12V/24V into 120VAC! Why does a bulb draw different current on different configurations.

I hope I made my question easy to understand and didn't confuse anyone. Thank you in advance to anyone who helps!

I did this test to help me understand if I want to invest in a 12V or 24V solar system. All it did was confuse me more.

The voltage - amps work on a seesaw priciple. The lamp is consistent. (load)

the higher the voltage the lower the amps will be to run it and at the other end the lower the volts the higher the amps.

Depending on how your solar power is going to be used and how you run it then what you need to do is look at whatever the end voltage is that you want to use.

What are you trying to do with it ?

Based on the Question and the answer supplied by cdad, I would think the 24V system would be better. Lower current draw the better, you pay for current not volts.

Eventually I would like to run refrigerator and clothes washer and all lights, LCD TVs, DVD stereo players using solar and leave the 240V poop pump and clothes dryer on grid power. (I would be staying w a pure sine inverter). That's a few years away because I know how expensive they gets!! But I want to start with a 400W 24V solar panel system to supplement the things we use everyday and see how it goes, and learn.

I just couldn't figure out why the batteries were essentially "feeling" a 2.65A or 6.5A draw. Cus that is A LOT for one light bulb. On the bulb (load) side I do know it is always 53.33 watts to run that 60w bulb but couldn't figure out the battery load side.

Ohm's law...
Voltage = current (amps) X resistance (ohms)
Power (watts) = Voltage X current

Add the watts of the equipment you wish to power. For fun lets say the refrigerator (1200 watts) + clothes washer (800 watts) + lighting (1500 watts) = 3,500 watts...
To run the above equipment your solar array needs to provide at least 3,500 watts. In reality a bit more as there are some losses in generation and distribution. Now a watt is a watt... its 1 volt X 1 amp. If you use 2 volts you'll need 0.5 amps....etc. In any case the higher voltage is preferred as it does reduce the transmission losses.

Batteries are rated at the applied voltage in amp-hours. If you want to run a 60 watt light bulb for one hour, you'll need a 60 watt / 120 volt = 0.5 amp-hour (or larger battery). If you want to keep the lights on for a day you'll need a (0.5 amp-hour x 24 hour = 12 amp-hour battery)

Apply the math to your solar project and see if it helps...

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Originally Posted by Catonsville

Lower current draw the better, you pay for current not volts.

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I don't understand this - do you mean solar cell systems cost the same regardless of the voltage they produce? I would think the cost is driven mostly by the system's ability to produce power, in other words the volts times the current they can produce.

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Originally Posted by hfcarson

Batteries are rated at the applied voltage in amp-hours. If you want to run a 60 watt light bulb for one hour, you'll need a 60 watt / 120 volt = 0.5 amp-hour (or larger battery). If you want to keep the lights on for a day you'll need a (0.5 amp-hour x 24 hour = 12 amp-hour battery)

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True, though this assumes that the battery is rated at 120 volts. It may be more practical to think in terms of using 12-volt car batteries to store energy in this system. If you do the calculation for the 60W bulb burning 24 hours being powered by two 12-volt car batteries connected in parallel you would need two 60 A-Hr batteries, which is fairly typical for a car battery. You would probably want to use deep-cycle marine batteries as opposed to standard car batteries, as they are better designed for multiple discharge/recharge cycles.

actually I assumed that the 60 watt light bulb was rated at 120 volts which they commonly are...

In this case the OP was talking about supplying household devices which would be at 120 volts so I thought assuming that in the example appropriate.

Quote:

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Originally Posted by Bravogolf

Eventually I would like to run refrigerator and clothes washer and all lights, LCD TVs, DVD stereo players using solar and leave the 240V poop pump and clothes dryer on grid power. (I would be staying w a pure sine inverter). That's a few years away because I know how expensive they gets!! But I want to start with a 400W 24V solar panel system to supplement the things we use everyday and see how it goes, and learn.

I just couldn't figure out why the batteries were essentially "feeling" a 2.65A or 6.5A draw. Cus that is A LOT for one light bulb. On the bulb (load) side I do know it is always 53.33 watts to run that 60w bulb but couldn't figure out the battery load side.

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How far off the grid are you wanting to travel and do you have gas available where you are ? Also you need to be very careful with running sensetive electrical equiptment in a system like that as the "pulse" from your source has a tendency to wipe out LCD tvs and computers. You may need to buffer them between the source to clean up the signal even further.

My question has gone ascew. I appreciate the replies. But my question still remains. So I thought about a different way to ask it.

If if I have a 24V 35A Caterpillar alternator, hooked up to a 24V battery bank, does that mean I can run a 120V heater that has a 30A-35A current load? If not, how many Amps will that 24V alternator "support" the battery discharge?

Conversely. If I have a 12V 70A Ford alternator, hooked up to a 12V battery bank, does that mean I can run a 120V air conditioner that has a 65-70A current load?

Why?

(I don't want to get into ampere hour battery capacity or watt size of an inverter. I know that stuff. Just the above confuses me). i also understand watt X volts equals amps. That involves load. I am asking charging. I want to know how much current on the AC side of the alternator can a 35A 24V alternator support the battery with? Thank you.

EDIT: I am guessing that I am only able to run 840 watts of 120VAC items? 840 watts / 24V alternator = 35 amps. So with a 24V 35A alternator hooked up to a 160cc Honda lawnmower engine via a V-belt will only power 800 watts of AC power? 800 / 120 = 6.667, or will roughly only run about 6 amp household items? Confirm? And anything stronger current draw than 6 amps will be drawing off of the 24V battery bank and discharging the battery bank?

Example: Attachment 47810

I think I found something that is pretty straightforward so you can do your calculations for load and output.

http://www.vanner.com/brochures/Esti...ments-7-01.pdf

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Originally Posted by cdad

I think I found something that is pretty straightforward so you can do your calculations for load and output.

http://www.vanner.com/brochures/Esti...ments-7-01.pdf

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So far I like this document. Thank you!

"24 volt inverters require approximately one (1) amp of DC input for each 20 watts ofAC output."
"12 volt inverters require approximately one (1) amp of DC input for each 10 watts ofAC output."

My 60 watt bulb took 2.65A. According to that statement, multiply 3 to both sides and you get 3A DC to 60 watt AC output. And makes sense for the 12VDC configuration, multiply both sides by 6 and it's very close to what I experienced. 6.5A for a 60 watt load. I like it! Things are starting to click load vs. output. Thank you!

it appears that in my example of a 24V 35A alternator, it would only be adequate for 600 watts of AC load. Not worth the project. Better off buying a 900W/1000W inverter generator for almost the same price and it's already manufactured.