# 12v light amperage

Say i have a 12v light bulb that is 40 watts that can hook directly to 12v battery. I want to know if hooking it directly to the battery uses more amperage than if i were to hook my power inverter to the battery and go through it.i know its 40w÷120v=0.3333 and 40w÷12v=3.333 so does that mean my battery will last longer using the inverters ac 120v? Im very new to all this and i dont have anyone to ask, i dont have electric so im just trying to have light at night time and i need the least amperages i can for my battery to last as long as possible. The 120v bulb through the inverter...or 12v straight to the battery. All my lights i use are LED. even when i had power thats what i used. More efficient. Which is gonna kill my battery quicker?

• I don't understand your question. You want to go DC -> AC -> DC? Why the inverter? Is this an incandescent lamp, LED lamp, florescent lamp? All that aside, that 12v battery will not do much of anything if the lamp requires 120v. – Gunner Dec 20 '19 at 10:46
• All are led light bulbs. One is 12v straight to the battery. One is 120v through the inverter. To be technical i use the 9-12w LED bulbs. I prefer the 14w because its just dark here more than anywhere it seems like. The 14w daylight ones are the best. Plus i do a lot of work to my place and am a night owl. So going straight to the battery or through the inverter? – Amanda Dec 20 '19 at 12:33
• Im just using the 40w as an example so i didnt have to throw all that other in there – Amanda Dec 20 '19 at 12:33
• Some spacing, paragraphs, blank lines would make that much easier to read. – FreeMan Dec 20 '19 at 14:48
• Excuse me....spacing, paragraphs, and blank lines? What are you talking about? – Amanda Dec 20 '19 at 18:44

In theory yes, you're correct, a 40W bulb will draw 10X more current at 12V than 120V.

But nearly all bulbs I'm familiar with are either 12V or 120V but NOT BOTH.

If you connect a 12V/40W bulb to a 120V power source you will almost certainly burn the bulb out in moments.

Your inverter, however, will not make the bulb light longer since, again in theory, 40W is 40W regardless of what voltage it's supplied at. The battery will be called on to provide that 40W and the amount of time you supply that power determines how much ENERGY (i.e. Energy = Power * Time). So 40W for an hour will draw 40 Wh (Watt-hours) from the battery.

In practice, however, the inverter does not have 100% efficiency. So when powering things through the inverter you also must factor in its conversion losses. Let's assume that the inverter is 90% efficient. Now your 40W load becomes 40/0.9 or 44.4W and you'll draw 44.4 Wh from the battery in an hour of use.

The best thing to do to maximize your battery life is to limit the power demands on it and use more efficient things. For example a good rule of thumb is that an LED bulb will draw 1/10 the power for the equivalent light output. So rather than a 40W incandescent bulb, use a 4W LED bulb.

• All are led light bulbs. One is 12v straight to the battery. One is 120v through the inverter. To be technical i use the 9-12w LED bulbs. I prefer the 14w because its just dark here more than anywhere it seems like. The 14w daylight ones are the best. Plus i do a lot of work to my place and am a night owl. So going straight to the battery or through the inverter?Im just using the 40w as an example so i didnt have to throw all that other in there – Amanda Dec 20 '19 at 12:35
• As noted above in the answer, NO going through the inverter buys you nothing and, in fact, costs you the efficiency losses in the inverter itself. Don't confuse current draw with battery capacity. 40W is 40W regardless of the voltage. – jwh20 Dec 20 '19 at 13:35
• Ok but the battery goes by amps. 40 watts is 40 watts no shit, i just didnt know if the 40 watts would draw less amps through an inverter or not. – Amanda Dec 20 '19 at 18:46
• No, batteries are NOT rated by Amps except the MAXIMUM current that it's safe to draw. Battery capacity is rated in CHARGE which is almost always in Amp-Hours or milliamp-Hours (for smaller batteries). So a 1AH battery will provide 1 A for 1 hour or 0.5A for 2 hours, etc. Without the time factor, you might assume that a battery will provide a certain current forever which it most certainly will not. – jwh20 Dec 20 '19 at 22:56

First off, remember you can't apply 120VAC power from an inverter to a 12V bulb. You'd have to get a regular 120V light.

You actually waste some energy inverting the DC to AC, so that is not going to help you.

The most important thing to do is get a really efficient light that's no brighter than you need, or at least has a dimmer know or a high / low setting. LED lights are generally far more efficient that incandescent or fluorescent - more light for the energy it uses.

So if you have a 12V battery source, you want a 12V LED light.

If you shop around you should be able to find a lantern that works for you, I'd look for

• rechargeable battery
• comes with a regular wall charger
• comes with a car charger
• long run time
• dimmer so you don't have to run it too bright

Something like this:

Along with that a small jump starter with a charger port or two or three and you should be able to get by for a whole weekend

• Ok. Im talkin with the 12v bulb straight to the battery. Or the 120v bulb through an inverter. Which is gonna kill my battery quicker? Thats all – Amanda Dec 20 '19 at 12:22
• @amanda - answered above ... – batsplatsterson Dec 20 '19 at 12:38
• Whats that thing do? The portable power pack? – Amanda Dec 20 '19 at 18:58
• @Amanda - It's a jump starter, you can jump start a car with a dead battery, but it makes a good portable power pack. It has a compact powerful battery in it, and you can plug car chargers into it to charge your phone, charge that lantern, etc. etc. – batsplatsterson Dec 21 '19 at 6:27

## You're thinking about the wrong units

And/or you don't know how the units work.

Volts are like pressure/force. It's like how fast you pedal the bike.

Amps are like flow/distance. It's like how hard you push when pedaling the bike (the part that's hard on your knees).

As you know, you can get up a hill on a bicycle in different gears. One is cranking fast but lightly, another is pushing hard but slowly. It's the same amount of total effort, though, because it climbs the same hill at the same speed.

We measure total effort in watts. Watts is volts x amps.

Around here we use systems called constant-voltage, which means the system has one nameplate voltage it does its level best to operate at, and the appliacnes decide how much current to use. This is a bit like your water pipes, which are always at 35 psi or whatever, but you decide how much flow to take.

You know volts, since it's constant. So given watts, you can compute amps. (But you don't really need to; watts is enough to get this done).

## Direct

You know 12 volts and 40 watts. You can plug those in and get 2.66 amps at the battery.

## Inverter

Inverters are magical things, but they're not free. It costs power simply to have them "spun up". Let's say it takes 55 watts for the inverter to be idling with no load.

Inverters also have an inefficiency, let's say that is 10%. You are converting 40 watts, so that'll take 4 more watts in conversion losses. So we have

• 55 watts for the inverter simply to be on
• 40 watts for the load
• 4 watts of conversison losses

Total 99 watts to run the light off the inverter.

Amps of current on battery: 8.25 amps.

## Verdict

Running things directly off the battery is much better. Shut that inverter off!

• Ok thank you for that simple and direct answer. I dont know how much the inverter uses actually i never looked. Its only a 400w but the only reason i had got it was because i didnt know you could run a light directly from the battery until recently. Like i said im very new, like few months new, at all this battery and inverter crap. I just dont have any electric right now so i was kinda forced into all this and have to try to learn everything myself. Its really hard sometimes. – Amanda Dec 20 '19 at 18:57