# What's watts?,,, Say what?



## budgetprepp-n (Apr 7, 2013)

Question,,,,,,, If a 120 volt LED light bulb pulls 18 watts how many watts would it pull if it was 
running using a inverter on 12 volt? I know the inverter uses a little but I mean other than that.

Is 8 watts with 120 volts the same as 8 watts using 12 volt?

I'm going to build a security light with a dusk to dawn photocell and I'm doing some test
to figure out which to go with 12 volt bulb or 120 volt bulb and a small inverter


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## Slippy's-Attorney (Sep 23, 2015)

any time you can stay with DC you will be better off


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## Kauboy (May 12, 2014)

Let me see if I understand this...
Currently, you are running a 120v AC LED light bulb that has a power consumption rating of 18 watts.
You want to know what the wattage will be if you run that same 120v AC LED light bulb from an inverter that is powered by 12v DC, right?

The bulb's consumption will be the same. It will still use 18 watts at 120v AC, or .15 amps.
Now you have to add in the waste wattage of running the inverter, and the loss assumed during DC to AC conversion.
You will end up using more total power doing this than just running the 120v AC bulb on AC power.
If you can, it would be better to just pick up a 12v DC light bulb and skip the conversion altogether.


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## 8301 (Nov 29, 2014)

watts = volts X amps
120 w = 120v X 1 amp
240 w = 120v X 2 amps
so 120v at one amp equals 120 watts 120v at 2 amps equals 240 watts
So a 240 watt lightbulb running on 120v will draw 2 amps current.

watts / amps = volts
120w / 1 amp = 120 volts
240w/ 2 amps = 120 volts

watts are the volts times the amps
60 watts = 120v X 1/2 amp... 240 w = 120v X 2 amps
24 watts at 12v is 2 amps

Got a light that uses 24 watts and runs on 12v. 24w / 12v = 2 amps current draw


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## homegrownrose (Mar 24, 2016)

how much wood would a wood chuck chuck if a wood chuck could chuck wood? (sorry had to)


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## 8301 (Nov 29, 2014)

Slippy's-Attorney said:


> any time you can stay with DC you will be better off


Only if the DC line is fused. A decent inverter is about 92% efficient so not much loss but for long wire runs you will loose a lot more power with lower voltage DC wiring unless you use very thick and more expensive wire. And DC can make one heck of a spark jump with high amperage so the fuse or DC type breaker is needed.


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## Mad Trapper (Feb 12, 2014)

FoolAmI said:


> watts = volts X amps
> 120 w = 120v X 1 amp
> 240 w = 120v X 2 amps
> so 120v at one amp equals 120 watts 120v at 2 amps equals 240 watts
> ...


Power (watts) = Volts X Amps , some fuzzy math above. That is DC current. Switch to AC and the phase will matter too.


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## Maine-Marine (Mar 7, 2014)

FoolAmI said:


> Only if the DC line is fused. A decent inverter is about 92% efficient so not much loss but for long wire runs you will loose a lot more power with lower voltage DC wiring unless you use very thick and more expensive wire. And DC can make one heck of a spark jump with high amperage so the fuse or DC type breaker is needed.


I always think after SHTF.... so I am gearing up for DC for everything... the only conversion I want to do is 12 vdc to 24 vdc if needed...

most things will be 12.. some maybe 24 - refrigerator maybe

we use 10 or 12 gauge wire with mc4 connectors.. most of our installs use din rail mounted connectors and fuses


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## Mad Trapper (Feb 12, 2014)

Maine-Marine said:


> I always think after SHTF.... so I am gearing up for DC for everything... the only conversion I want to do is 12 vdc to 24 vdc if needed...
> 
> most things will be 12.. some maybe 24 - refrigerator maybe
> 
> ...


I'm geared up for manual power, especially for the GF, got to keep going and going.............


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## 8301 (Nov 29, 2014)

Mad Trapper said:


> Power (watts) = Volts X Amps , some fuzzy math above. That is DC current. Switch to AC and the phase will matter too.


As will reactive power from motors and transformers too but trying to keep simple things here.


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## 8301 (Nov 29, 2014)

Slippy's-Attorney said:


> any time you can stay with DC you will be better off


Let's talk wire. If all you ever plan to use is a 5v charger or a few 12v lights 12vDC is the way to go. If you ever hope to use a microwave, well pump, refrigerator, or chest freezer you really should consider 120vAC. Home Power Magazine often has articles from people who are 100% off grid and they almost always wish they had gone with 120v AC instead of 12v for their appliances because of the additional cost of 12v appliances and huge wiring with high amp loads.

example: Microwaves are the most efficient way to heat stuff like a cup of water or soup. 1000 watt microwave, not a huge one, just the average size for this example, 
1000 watt microwave 30 feet from your batteries. 
12v 1000 watts requires 83 amps so at the best 4 ga THHN heavy ( hard to bend wire ) so $66.40 for a pos and neg wire (home depot prices). 
120v 1000 watts requires 8.3 amps so at best 16 ga much easier to bend wire so a $21 extension cord will.

Yes, You need an inverter, but the inverter will cost much less than replacing your appliances with 12v stuff.,, and 12v appliances cost 2 to 3 times as much and and are usually less efficient unless you pay 4 times as much.

We all want some lights if things go bad, but in my opinion having a freezer and no fire smoke way to heat food is just as important.


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## Kauboy (May 12, 2014)

I'm fairly certain Budget is only concerned about running some lights here guys. He never mentioned high amperage devices.
He should stick with DC for this application, if possible for him.


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## chocks141 (Nov 21, 2015)

watts is watts
don't matter if you change voltage.
if you increase voltage, you decrease amps, but watts are still watts.
a 100 watt bulb will use 100 watts regardless of the voltage.


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## paraquack (Mar 1, 2013)

Maine-Marine said:


> I always think after SHTF.... so I am gearing up for DC for everything... the only conversion I want to do is 12 vdc to 24 vdc if needed...
> 
> most things will be 12.. some maybe 24 - refrigerator maybe
> 
> ...


While I'd rather stay with DC applications after SHTF, if you need to have a long wire run, 
12 VDC is going to lose a lot in the getting to the end of the wire unless you have wire 
the size of your thumb. Plus I have 120 VAC things I need to run.


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## 8301 (Nov 29, 2014)

budgetprepp-n said:


> Question,,,,,,, If a 120 volt LED light bulb pulls 18 watts how many watts would it pull if it was
> running using a inverter on 12 volt? I know the inverter uses a little but I mean other than that.
> 
> Is 8 watts with 120 volts the same as 8 watts using 12 volt?
> ...


8 watts is the same weather its at 12v or 120v.
for a small security light 12v works fine.


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## Mad Trapper (Feb 12, 2014)

FoolAmI said:


> As will reactive power from motors and transformers too but trying to keep simple things here.


Yes, but all that math I learned, sure had applications to the sciences.

Lots of today's students think sines, logs and integration are about directions/rules, firewood and race relations, respectively; and be lost without tablets/computers and spell/grammar checkers. I witnessed what calculators did to math students who cannot add, divide or subtract, but the sure do "multiply" before going to work or getting an education.


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