# How Much Power Do I need

Posted By TrailRunner, Feb 23, 2007 at 6:41 PM

Not open for further replies.
1. #1

### TrailRunner New Member 2. ```NULL ```

Jan 30, 2007
11
0
I have a 33 Elite, and at top speed the fans use 77 watts. I'd like to be able to run them 12 hours, before I had to recharge the battery.

I've read through the posts to figure out how long I can run AC fans off of an inverter from a 12VDC battery. What is the formula to use to figure watts to amphours? And, what efficiency factor should I use for the inverter. If there is a calculator on the web, you can just point me there.

Thanks,

Dan

2. #2

### Corey Minister of Fire 2. ```NULL ```

Nov 19, 2005
2,330
176
Loc:
Midwest
See if I can hammer out the numbers before someone beats me to it!

77 watts x 12 hours = 924 watt hours

924 watt hours / 12 volts = 77 amp hours (good thinkin' using 12 hours - it makes this work out really nice!)

Most 12V inverters I have seen run about 50% efficient ...maybe 60 if you are lucky so conservatively:

77 amp hours / 50% = 154 amp hours

So, you'd need 154 amp hours of capacity in batteries to power your 77 watt fan for 12 hours straight. I think Group 24 batteries typically run about 80 Ah a piece, so two of those in parallel should get you through with a bit of room to spare. You can plug in what ever numbers you like. If you have an inverter in mind, you may measure the volts and amps flowing on both sides to give you a closer number than my 50-60% guess. On a side note, the highest efficiency inverter I measured was a cheap little Taiwan model. It seems like the more refined units give a better sine wave AC power out, but waste more energy to get it. The little Taiwaneese model was putting out a pretty choppy wave-form (plenty good to run fans, if you can put up with a little extra noise) but it was pretty efficient at doing so.

Corey

3. #3

### Corey Minister of Fire 2. ```NULL ```

Nov 19, 2005
2,330
176
Loc:
Midwest
Wow - I was hoping someone would pop in and check my work. Looking at it a second day, it seems OK.

One thing I was also going to mention. If you think you may be without power for long periods and the stove is a 'mission critical' component - like sole source of heat, there are a couple of options to consider. You could replace the fans on the stove with 12VDC units and run them off a moderately efficient switching power supply when household voltage is available, then in a power outage, you could run super efficienly directly from the 12VDC batteries. The other option would be to suppliment the main fan with a 12VDC unit. Run your standard AC fan when times are good and switch directly to 12VDC power when the main power is out. Both of these options are a little more involved than the standard plug-n-play inverter option, but will be more efficient and maybe even a little cheaper in the long run.

Corey

4. #4

### TrailRunner New Member 2. ```NULL ```

Jan 30, 2007
11
0
Corey, Thanks for the ideas. I think that's about right.

I talked to a place that sells inverters and they estimated two Type 27 batteries would do it. They thought that I'd need 200 Amp Hours.

Thanks again,

Dan