Hello again. Long time, no speak. I have a buddy that needs to put a battery bank into an old house boat he is fixing up. And I've had a heck of a time finding a calculation that'll allow a person to figure out AC wattage, that'll drain DC deep cycle batteries, with X amp hours. I've found online tons of calculators where you can plug in your appliances etc (usually from solar companies etc). But they don't really help in this situation. He doesn't have appliances yet, and that's what we want to figure out. Plus, we'll need to figure out how big the battery bank will have to be so that the generator only has to kick in to recharge in as little amount of times as possible etc. I know I can figure out the AC amps of an appliance, but I highly doubt they are related to DC amps.

So lets say that on average his boat uses 500 watts AC, with a single 100 amp hour deep cycle 12v battery (an inverter to match etc, and take into consideration efficiency loss etc). This is redundant from above, but just to clarify, we want to (1) figure out how long this will last on this battery and (2) figure out how it changes with changes in AC wattage etc. Its easy to calculate how large the battery bank needs to be after we figure out the above issue. We can just add DC batteries, and double, triple, etc, etc. Or conversely allow for more AC wattage by adding batteries etc. Anyway, I hope this makes sense! Thanks again guys!

I am a DC Volts person, For Boats, remote operations, Solar or what ever, get 12 volt everything you can, The losses are great with inverters.
500 Watts @120 volts is 4.1 amps.
500 Watts @12 Volts is 41 Amps.
Volts X Amps equals watts. I would hook up amp meter to DC side to see actual load on batteries with each item to be connected.

We're going to try to find as much as possible in 12volt, or propane. He's going with an RV propane fridge (actually 3 way), propane heat and cooking, propane hot water etc. 12v pumps. We're trying to source out 12v LED lights that we can use for cabin lighting. LED's use so little power and last so long. But inevitably, he's going to need some 120. For stuff that you just can't get, or are really hard to get (expensive) in 12v. I know it is easy to calculate amps like you mentioned, but is AC amps the same? Meaning that if I calculate the AC amps of an appliance, can I subtract that amount from the amp hours of a DC battery. For some reason, I don't think it is that simple? Maybe I'm wrong. Thanks again for your help!

Edit: I just noticed the part where you said to hook up a DC volt meter. That's a great idea and perfect. But the issue is, he doesn't have the gear yet. I found a calculator on line that shows how much AC watts will drain a DC battery at x amp hours. I'm sure its approximate, but it'll give us an idea. Thanks again!

PS: If you have any other recommendations for 12v or propane, especially in the air conditioning department, I'd be greatly appreciated. Thanks!

Watts (ac) does equal Watts (DC) and so do the other ohms law formulas.
12(I1) = 120 (I2); I1@12V needed = 1/efficiency*I1 or
inverter power needed = Power@120V * 1/efficiency

HOWEVER,

The inverter will only operate to a given minimum voltage, so lets say it can only operate to 8V, so you really need to look at the battery discharge curve to 8 volts.

You have an essentially constant power drain on the battery and not a constant current drain on the battery, so I will keep on increasing as the battery discharges.

So, in one case, you will have constant 120 VAC till your battery becomes lower than the cut out voltage of the inverter, but you will not have constant DC.

You could use a DC-DC converter to keep some of the 12V loads constant.

There are also I*R losses in the DC cables that should be accounted for. The higher the DC voltage, the less loss.

Here is an example of a DC-DC converter with a 48 V input which will give 12V down to 24 VDC. These could become useful. 48 volt to 12 volt DC/DC converter, 24 to 48 volt inpupt, 400 watt.. I did notice that the efficiency wasn't given.

Does this offer any insight?

Check out Xantrex Technology Inc. as well. They should have generator controllers.

Check this out as a resource too: Home Power Magazine: Solar | Wind | Water | Design | Build. They have a CD set, about $100, that includes almost of their issues.

The inverter guys ought to be able to provide run-times for different battery banks. APC, who makes UPS's has run-times based on loads.

So, you might be better with a higher DC system voltage, a generator charge controller, DC-DC converters for the 12V stuff and DC-AC converters for the AC stuff. The DC-DC converters probably have an efficiency >85%. You might be better distributing power at the 48V level and then use local DC-DC converters, so you don't waste energy at idle.

Probably a 12V light buss makes sense. This would be an always-on 12V source.

This might help to caclulate run-times:

http://www.xantrex.com/web/id/1911/DocServe.aspx

Wow, thanks Kiss, that's a lot of info. So you suggest he run 24 or even 48 volt DC, and use a DC-DC converter at the 12v source item. And this is to decrease loss over DC cabling? I just want to make sure I'm getting this right. Is the loss on a 12v DC system big enough to warrant spending the money on DC to DC converters? And from the generator's 12v DC, will the charge controller recharge the system at a higher voltage, 24, 48 etc, or would he need another DC-DC converter there? And I do know you can get 24 volt inverters. Can you get 48v? Or again, would it make more sense to use another DC to DC converter and use a 12v inverter?

Sorry for throwing so many more questions at you, I just want to make sure which is the best option for efficiency but for the more efficient price too. Thanks again!

EDIT: I think I missed a point. The DC-DC converters essentially keep the 12v at a constant rate? Without them the voltage will drop off? Correct?

Check out xantrex for their charge controllers that involve generators. I didn't get a chance to do that.

For your edit: DC-DC constant, the answer is yes. You may be able to obtain 12 V down to 50% capacity, otherwise as the batteries discharge, some 12V things might complain. e.g. lights will get dimmer. 12V TV might just die.

You might have to do some looking around. I gave you places to start.

As an example, you might go with a 120 VAC generator (they are common) to a 48 V charge controller like this one. IOTA Dls54-15,15A,48V 2Stg Charge Controller

There should be ones out there that can automatically start the generator or devices that can start or stop the generator based on battery voltage.

I'm not recommending products at this point. I'm just suggesting places to start.

You really need to do a load analysis and go from there. Include distances to power source and whether they are affected by low battery voltage.

kingsX,

In your OP 500w@120vac=4.16a

500w@12vdc=41.6a

so a 100 amp/hr. battery would last a little over two hours or 2.40 hours to be exact for a
constant 500w load.

Thanks again Kiss for all our help and info. I spoke with him yesterday and he agrees. We're going to have to see what all items he needs and go from there.

I do have two more questions. First, what gauge of wire would someone use to run DC voltage through the boat? Can you use 14g AC household wire, and just drop the ground? Or is it more specialized?

And lastly, will he need a pure sine wave inverter for his electronic stuff, AC Tv for instance? He has an uncle with an off the grid cabin, and uses a normal inverter and says that he hasn't seen any noise or static in the device he's using.

Thanks again everyone for all your help!

1. There is a specific grey UV resitant 2 conductor stranded cable that's available at most boat places. I would try to avoid solid wires because of the vibration. It's going to have a lower current capacity because the % voltage drop will be higher. There are voltage drop calculations which have been described on this site which will work for DC as well and that's dependent on length and wire gage. You should try to keep the drop less than about 3%.

There is no such thing as a normal inverter. There are 3 major types: 1) Square, 2) modified sine and 3) sine wave. I have seen a few devices that won't work and generally it's because of the power factor correction circuitry and the switched mode power supplies. For example: if you want to operate a Laptop, I'd consider a supply from iGo.com - Power Adapters and Chargers for Mobile Electronic Devices. It's 120/12V input to various outputs. The "tips" select the voltage. They even have devices that will charge cell phones and PDA's. That will probably be a wait and see.

One thing to watch out for is that SOME inverters are not AC generator friendly.

Marine supplies have 2 or more conductor cable for cable that is tinned copper, works great. They also make crimp connectors that are heat shrink with adhesive, and heat shrink tubing with adhesive, they work great. #14 and #12 will work most things. Any large drawing items will need larger conductors, low volts(12) drop fast, run big wires to those, and or mount battery and inverter close to that item.

Got a catalog in the mail today from Burden Sales Surplus Center - Hydraulics, Engines, Electrical and More They have 2 DC-DC converters of interest: 12 V output, ~85% efficient

6-989 18-50 VDC in, 12 V DC @ 5 A out ~$65
6-990 20-60 VDC in, 12 V DC at 25A out ~$200

Former looks good for a 24V system. Later for a 48V system.

Thanks a million again guys, as per usual you were all very helpful. Now I have to convince my buddy to install an air conditioner. He thinks it should have surround sound inside for his TV, but not air conditioning? This dude needs get his priorities straight :)

Anyway, cheers again!

Here is a propane refrigerator

Search Results: propane refrigerator | Apple Appliances

Another interesting site:

How to Analyze Your Site for an Alternative Energy System | Alternative & Renewable Energy - ABS Alaskan, Inc.

Definitely go with stranded wire over solid wire for a boat. The previous post is correct about vibrations causing breakages on solid wire and you get a higher amperage rating from thinner stranded wires. Usually the insulation is better on the stranded wire too.

This is sort of going off topic, but there doesn't seem to be propane air conditioners. Which seems strange? Maybe I just can't find them, but you would think that the principle would be the same as a propane fridge? Could a person convert an old RV fridge into an air conditioner? Or is that just plane insane, and totally inefficient? Dangerous?

Took me a while, but I found some information.

Propane Cooling/Refrigeration

And

Home Page

Seems like the search term is "gas cooling".

Aha... gas cooling. It makes sense now. But at first glance, it appears to be a bit more in the industrial department. I googled marine gas cooling and didn't really find anything, although I didn't do an exhaustive search. I think he should just get a AC or DC marine/RV air conditioner and just be done with it. The cabin isn't that big, and as long as it doesn't get too hot on the lake, he won't really need to use it too much anyway. The boat has big windows that open up and could create a really great cross wind into the cabin off the lake. I think it won't be too bad. And if he begins to cook, he just needs to run his generator longer. Thanks again guys!