AC Watts vs. DC Watts.....

CliffA

CliffA

Well-Known Member
Dec 29, 2009
4,712
Lake Norman, NC
Boat Info
2001 Sea Ray 340DA
Name: 'Happy Place'
4.5kW West. Generator
Purchased Nov. 2014
Fresh Water Use
Engines
Twin Merc. 6.2L (MPI)
640 hp (Total)
Raw Water Cooled
V-Drive Transmissions
i tried to look up a conversion table on the net for this but i got confused in the process....electrical issues are not my strong suite...

i am going to purchase a tower fan to use in the cockpit during hot days/nights....from what i can tell these fans draw somewhere around 100 Watts of 110VAC power when they are running on 'high' speed....

what would be the equivalent DC power draw for this fan running on high speed?....i have a 400W power inverter....would that be large enough to run the fan on high for a few hours assuming a deep cycle group 27 marine battery is used as the power source?....

while on shore power i am not concerned about the power usage.....but if we are anchored out and watching TV in the cockpit i would like to be able to run this fan on inverted DC power instead of running the generator for AC power due to the noise from the generator....

thanks...

cliff
 
Watts=Volts X Amps. Your fan will run with the inverter and draw close to 10 amps DC per hour of use. Added to the other DC requirements on the boat (lights, radio, fridge, etc) you will only be able to run the fan for a few hours on a single group 27 battery
 
BlueYonder said:
Watts=Volts X Amps. Your fan will run with the inverter and draw close to 10 amps DC per hour of use. Added to the other DC requirements on the boat (lights, radio, fridge, etc) you will only be able to run the fan for a few hours on a single group 27 battery
Click to expand...

thanks....actually my 'House' battery is two Group 27 batteries in parellel so it sounds like i will be OK for a single evening on the hook just to watch a movie....if i run the fan on low or medium speed that will lessen the power draw and lengthen the available run time...

cliff
 
Last edited:
I would have to say I doubt all that will run all night on two group 27 batteries. Remember that inverters are not the most efficient at converting power. More power you need, the more inefficient they become. And you don't want to run your group 27 marine batts all the way down, may ruin the batteries.
 
Yep, watts is watts... AC or DC

Given P = I x E (Power = Current x Voltage), I = P / E. So 100W / 12V = 8.3A. Most inverters are only about 80% efficient (as trit21 indicated), so to get 100 watts out, you will burn 125 watts on the battery side. So about 10.4 amps for the fan on the battery (like BlueYonder indicated). So add up all the other DC stuff (TV, lights, etc.) and you're probably double, but lets assume 18 amps total for the simple math...

A single group 27 battery will deliver 90 amps for 1 hour or 9 amps for 10 hours... or 18 amps for 5 hours. Multiply that times 2 for 2 batteries... and you're back to 18 amps for 10 hours. So, in theory, you could run your fan and associated stuff for 10 hours. :smt101

Running your fan at a lower speed will reduce the wattage requirements and therefore run longer.

Disclaimer: Your actual mileage may vary! :smt024
 
thanks for the replies....i expect that if/when this situation comes up it will be of relatively short duration, probably 3 - 4 hours max in the evening....i would run the generator during the day as needed to cool the cabin down and run all the 'stuff' including this fan as well as keeping the batteries charged....then in the evening after dinner we often like to watch a movie....now that we have a larger boat with a TV in the cockpit we would likely use this TV for the movie....so i would shut down the genny to watch the movie in quiet....after the movie we will turn in for the night....

now i need to do the LED bub conversion throughout the boat to lower battery power consumption....

cliff
 
Last edited:
CliffA said:
now i need to do the LED bub conversion throughout the boat to lower battery power consumption....

cliff
Click to expand...

Cliff - On my 340, my halogen cabin lights alone were using 10 amps. LEDs lowered that to 1 amp. The numbers are X2 on my 410...with twice as many lights...
 
Cliff, just for kicks, check out a fan called Endless Breeze. It's a 12V fan to begin with, not converted from AC. I once ordered almost a dozen 12V fans from one of my suppliers to check them out (perk of the job - I can send them right back without ever paying for them). The goal was to find one that was quiet, moved a lot of air, yet still pulled minimal amps. This one is REALLY nice - it met all those requirements... to be honest, I'm surprised at just how good it really is in all those categories.
 
carterchapman said:
Cliff - On my 340, my halogen cabin lights alone were using 10 amps. LEDs lowered that to 1 amp. The numbers are X2 on my 410...with twice as many lights...
Click to expand...

thanks Carter...i knew the difference in power draw was significant but not that much...i'll have to move the LED conversion up on my project priority list....

cliff
 
Lazy Daze said:
Cliff, just for kicks, check out a fan called Endless Breeze. It's a 12V fan to begin with, not converted from AC. I once ordered almost a dozen 12V fans from one of my suppliers to check them out (perk of the job - I can send them right back without ever paying for them). The goal was to find one that was quiet, moved a lot of air, yet still pulled minimal amps. This one is REALLY nice - it met all those requirements... to be honest, I'm surprised at just how good it really is in all those categories.
Click to expand...

Dennis, i actually have that fan....i bought it a couple years ago when i saw a post where you recommended it....you are correct that is does move a good amount of air with mimimal power draw and is 'reasonably' quiet.....there are a couple of small problems with using this fan in this situation...i don't have a DC power plug near where the fan would need to sit to blow on us while watching the TV....so i would either need to install a power plug or use a long extension power cord to the power outlet at the helm....since the Endless Breeze fan is a box fan i would need to sit it on a table or chair to direct the wind towards us....both of these are small problems so i may reconsider using this fan at night in lieu of the larger tower fan....

the good thing about the tower fan is it is VERY quiet and moves a lot of air even on the low settings....and i have a 110VAC plug in the right place in the cockpit....and it is tall enough to blow directly on us while seated in the cockpit...

if anyone is interested in the fan i bought here is a link.... http://www.amazon.com/Ozeri-Ultra-i...id=1431610329&sr=8-1&keywords=ozeri+tower+fan


cliff
 
Last edited:
Do Over said:
Cliff... I'm surprised that your genny effects watching TV in the cabin.
Click to expand...

the genny noise will intefere while watching the TV in the cockpit, not the cabin....

cliff
 
here is a pic of the cockpit TV i installed a coulple weeks ago....the tower fan would sit below the TV at the edge of the cockpit galley....

cliff

View attachment 40361
 
Oh, I didn't realize you bought the tower fan already. Based on your first post I thought you were still contemplating the purchase. Never mind! :grin:
 
Lazy Daze said:
Cliff, just for kicks, check out a fan called Endless Breeze. It's a 12V fan to begin with, not converted from AC. I once ordered almost a dozen 12V fans from one of my suppliers to check them out (perk of the job - I can send them right back without ever paying for them). The goal was to find one that was quiet, moved a lot of air, yet still pulled minimal amps. This one is REALLY nice - it met all those requirements... to be honest, I'm surprised at just how good it really is in all those categories.
Click to expand...

Thanks Dennis for posting this just ordered it from Amazon 66.75 plus a 12 volt 12' ext cord for 7.75 love Prime it will be here tomorrow.
 
Bob & Sue said:
Thanks Dennis for posting this just ordered it from Amazon 66.75 plus a 12 volt 12' ext cord for 7.75 love Prime it will be here tomorrow.
Click to expand...

Just ordered it too with the 12' extension...gotta love Amazon. It was $59.95+$7.75+shipping...
 
mwph said:
Just asking....
Wont these 12' extension cords significantly affect the draw on the battery/inverter?
Click to expand...

Dependent on the gauge of the cord. I would think anything 16g. and bigger will have a minimal effect if any.
 
You must log in or register to reply here.

Forum statistics

Threads
113,224
Messages
1,428,896
Members
61,116
Latest member
Gardnersf
Back
Top