# how much power do i need in an inverter?



## Chyrio (Feb 23, 2010)

hey guys im trying to set up a home theater thing in my car. i have a computer that uses 250 watts (but probably will only use like 160 - 200 when watching movies and stuff), an lcd that uses like 60 - 80, and i also want to run some lights off of it so i don't have to use the car lights. probably something simple like some blacklights or something cool. how high powered of an inverter would i need? and could i run it off the car battery and be safe to start my car after like a couple of movies or something (3 -6 hours)? or would i need to hook it up to other batteries? i have two 12 volt motorcycle batteries that i can use. Im not that good with electronics so any help would be great :grin:


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## Done_Fishin (Oct 10, 2006)

First lets take the project stage by stage ...

there will be a surge current as you power on which may go several times the average running current of the inverter. 

I note that you are in Colorado so most likely running a 120V powered PC.

Unfortunate for you that means your 12V car battery will need to supply twice the current that our 240V systems use.

250W @ 120V = ~ 2A

@ 12V without losses you'd require 20A 

allow 80% efficiency and were talking 25A WHILST THE BATTERY is 12V or more. As the current drains though the Voltage will drop so the current will increase.

you need an output stage capable of handling 25A without getting warm and be able to sustain that for several hours. As a safety net I would go for 50A. That would mean an inverter of around 500W at 120V.

3-6 hours would mean having a battery capable of giving 75-150 AH without dropping to a voltage that would force the inverter to stop working and also have a reserve to get your car started. The starter motor will require at least 100A to turn over your engine and get it going .. more if you have a large engine.

Your motorbike batteries will be of no use to you for this.

the overall power requirement would require perhaps 3 or 4 50AH car batteries (without using your car battery) or perhaps even think about using 24V Boat Batteries @ 100Ah or even a petrol generator with some extra filtering, since the generators frequently throw out erratic nasty frequencies and voltages that could damage your equipment.


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## gcavan (Aug 13, 2009)

Look for a single 105 A-Hr deep cycle battery to power your inverter. Alternatively you could use a pair of 80 A-Hr batteries wired parallel. Either way, you will want to isolate your inverter system from your starting system so that you do not drain your starting batttery but still charge all batteries when the vehicle is running. Do not forget circuit breakers.
Also, don't run your lighting off the inverter; use 12 volt CCFL's


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