calculating amp hours?

[mseifert]

I'm thoroughly confused on trying to calculate my electical requirements. If I know information such as the following can I calculate how many amps per hour I need?

For example my chartplotter says 25 W at 10VDC and my XM weather receiver says 18 watts max... Can I calculate amp/hrs from this? Can someone please explain?

Also, is there any place that lists generic power requirements for things such as nav lights, anchor lights, cabin lights, etc.?

Thanks for all your help!

--mike

 

[Don S/V ILLusion]

Below is a simple conversion table link.

To simplify, V times A = watts
Your chartplotter 10V x A = 25 or A = 25/10 = 2.5 amps. If you run it for one hour, you have pulled 2.5 amp hrs from your battery.

There is no generic cabin light, anchor light, etc but each should have a wattage rating stenciled on it and you know the voltage so it's easy to calculate.

http://www.jobsite-generators.com/power_calculators.html

 

[sailingdog]

I'd point out that many of these ratings are maximums rather than average use...and the equipment will not draw that much in typical use. For instance, the VHF won't use the full power rating if it is only receiving or transmitting in low-power mode. This means that your budget will often be higher than expected.

If you flip through a good catalog, it will often have the specs for various pieces of equipment.

 

[Stu Jackson]

Energy Budget form is also here: (download the pdf from the first post):

http://c34.org/bbs/index.php/topic,3976.0.html

West Marine Advisors are also helpful.

 

[Eric M]

Unless you have already upgraded your lighting to LED lights, chances are that your lights are the largest consumer of power on your boat. Just one cabin light on my boat draws more than the chart plotter, speed, depth, and wind instruments combined. Next after lights are electric motors, things like autopilot, pressure water pumps, electric head pumps. These draw even more power than a light, but are usually not on for extended periods of time.
Good Luck,

--Eric

 

[Bill Roosa]

Let a spread sheet do the math

How much each item consumes is on the first part of coming up with an energy budget. You have to also determine how long each item is on in a given time period. The given time period is usually between charges but not always. The you also have to determine how fast you can (or can't) charge the batteries. If you day sail on weekends the charger has all week to bring the batteries up to full charge if you are dong extended cruising then you are most likely going to be charging every day..... depends on how you use the boat. For most of us it is a mixture of day sailing and limited cruising. You would have to figure out which is the worst case. Then there is the matter of battery bank sizing, which will also depend on how you use the boat and what electrical items you use.
I have built an excel spreadsheet that does the load-charge-storage math for several different boat usages. You will still have to do the watts to AH or amps consumed but you only need to do that once. Then you can play around with load times, bank size and charging schemes to get a low cost solution.

Some things I found while playing with it and insights from the group:
Moving heat around with electricity is expensive (reefer)
Super large alternators are generally a waste of money
The cheapest scheme is usually reduce the consumption
AH into a battery is more than AH stored in a battery
AH stored in a battery is more than AH out of a battery
The best scheme is to use it as you produce it (just enough, just in time)

Let me know if you want a copy of the spread sheet william-roosa@us.army.mil

 

[larryw]

Eric M was pretty close, but if you add in refrigeration your power needs will rise. On my boat, I use about 120 AH/day. That includes all the above, plus a 13 in. CR TV with a built-in VCR. A switch to digital cut power use somewhat. I could put back about 80% of the capacity. That's where you'll live, between 50 and 80%. Don't go below 50, and don't bother trying to charge up past 80. Most people, when cruising, stay in one place for four or five days, then motor enough to the next place to fully charge the batteries, or they go into a marina to get fresh water, hose down, and plug in. There are purists who do otherwise, but what are you going to do is what counts.