Questions about electricity consumption

[SteveB]

Hi everyone! I have a couple of questions about measuring/planning for 12 volt electricity consumption on my boat. In all cases, assume the engine is off and electricity is supplied only by the house battery (75 amp-hours)

Suppose I have a 12 volt lamp that is rated at 6 watts. Dividing WATTS by VOLTAGE (6 divided by 12), I get a result of 0.5. Is is correct that this lamp will consume .5 (one-half of one) amp-hours when running off of my house battery?

(I think that's correct)

Next, I have a 300 watt inverter connected to my boat's 12v accessory plug.

These are hypothetical questions:

Looking at a standard 110 volt AC-to-DC adapter I have here for a radio, it says "Input 110 volts 60hz : Output 8V --- 2A". How much current will this draw from the house battery when I'm running it off of the inverter? Is it 2 amp-hours or do I have to use another way to compute the current draw?

Last question. I have a regular household AC desk lamp with a 40 watt bulb. Which voltage do I use to compute the draw this will place on my house battery when used through an inverter?

Is it 40 divided by 12 volts (house battery power), for 3.3 amp-hours

-or-

Is it 40 divided by 110 volts (A/C from the inverter) for .36 amp-hours? This seems unlikely.

Or is it something completely different?

Thanks for any help

-Steve

 

[wynn 'n anniedear]

The surest way to know how many DC amps you are drawing is to put a DC amp meter in series with your battery and DC load. That reading over an hours time is amp hours and can be subtracted from your battery rating.

 

[CaptMac]

Looks like my long drawn out answer got zapped! I'll try again

Always work in watts. That's voltage times current and is a measure of power (energy). If the output of the radio cord transformer says 8v and 2 amps, that's 16 watts. If you supply that from your 12 volt system , you'll use 1.33 amps (8 v x 2A = 16 watts. 12v x 1.33 amp = 16 watts).

If you run the radio for an hour, at 12 volts you will use 1o hrs x 1.33 amp = 13.3 amp hours.

There are inefficiencies of transformers/inverters, allow an extra 10 percent.

mac

 

[johnniethek]

Completely unrelated here, but how do I get a 110 volt plug-in on my R-21 so I can use simple appliances like coffee maker and laptop?

Thnx,

 

[captain's cat]

Completely unrelated here, but how do I get a 110 volt plug-in on my R-21 so I can use simple appliances like coffee maker and laptop?

Thnx,

Get an inverter of the correct size (watts). Hook that to 12v, it converts the 12v to 110v. Your 110v appliances then plug into the inverter. For small loads, you can plug the inverter into the cigarette lighter socket, for larger ones, it should be wired into the boats 12V system and fused separately.
Charlie

 

[j&lgray]

The answer to your 110 volt from 12 volt question is get an inverter that makes that conversion from 12 volt to 110 volt. Get the inverter that suites your needs. Too small is a pain and too big is overkill.

 

[wefings]

You need an inverter . The bigger the load you want to power from your 12 V battery system , more inverter and battery bank and charging system capacity size will be required. Its a science RVs and Boats and all 12v D.C. systems converting D.C to A.C. have in common . Much has been written . Figure your load , and get an adequate size inverter , charger and battery bank and go for it .
Marc

 

[dan1000]

Also note that ratings on appliances and on "brick" converters such as the 110v -> 8v one that you have, are based on the *maximum* draw of that device. In actual practice, electronic devices only draw a fraction of the maximum rated wattage, except briefly when they are doing something that requires high power. For example, a radio at full volume will consume more power than a radio at a quiet volume. Similarly, a transceiver will consume its highest amount of current when transmitting at full power. When receiving, or when transmitting at low power, it will consume much much less.

Unfortunately, the only way to know for sure is to measure, or to go by others' experience.

A couple of things to remember:

1) Batteries can survive many charge/discharge cycles if you only use the top half of the charge. They wear out a lot faster whenever you use the bottom half of the charge. If you regularly draw down 90% or more of the charge, they will not last long at all.

2) Whenever power is transmitted or transformed, some of the power is lost to heat. So, to pump out 120V at 1A (120W), an inverter will consume more than 120W. In this case, it will probably lose 15% or more of the energy it converts, so it will likely require 13A at 12V, rather than the expected 10A at 12V.

3) There are many ways to save electricity, such as using LED or fluorescent lights rather than incandescent.

4) Solar power is amazing, and many people use it successfully to avoid needing a genset.

Disclaimer: I'm not yet a boater, but have many years of motorhome experience, where the same sorts of issues come up.

Dan

 

[RDragon]

Dan,
Great post! Thanks for the info. There is a lot od great experience on this board that helps us all out! I was just wondering if folks have some thoughts on solar panels etc. I have been toying with the idea of a couple of the stowable/foldable panels to help with power management. Not sure...just thinking at present. I know Mac has a really neat setup on Island Ranger, but I'm not so sure I want anything that permanent.

I figure when I'm ready, I will make the slog up to Langley and see if my tugbuddy can help put it all together! 😱

Have a great day and look forward to the thoughts.
Drew

 

[baz]

If you have installed inverter and you want to plug into one of the 3-pin sockets located in the boat then why not place a 'Kill A Watt' meter into the socket and then your appliance into the 'Kill A Watt'. The 'Kill A Watt' display should then indicate the applianace's electrical load. I do this with my computers at home to measure their real electrical draw in watts to compare against what my computer's software displays. I find that when computer and screen put into deep sleep the watts drop to 4 watts.

 

[dan1000]

A couple of other bits of information from memory:

Recharging a battery costs about 25% of the energy that you put in. So, if you have a 100AH 12 Volt battery, and you have consumed 50% of it's energy (12 * 100 / 2 = 600 Watts), it will take about 750 Watts of energy pumped back into the battery in order to get that 600 Watts back in (25% of 600 is 150, 600 + 150 = 750).

Where does the rest of the energy go? Heat.

So, if you are considering solar (a good idea, in my humble opinion), you need to do a quite long-winded calculation to figure out how much solar you need, as follows.

1) Figure out how much energy you want to be able to use on a daily basis (this is quite a lot of guesswork, unfortunately). Let's suppose you've worked out that you need 2000 Watts each day.

2) If a lot of this is going to be 110V, the power will have been run through your inverter, which is likely only about 85% efficient. So, divide 2000 by 0.85 to get the amount of power from the batteries that you'll need in order to feed 2000W to the appliances. 2000 / 0.85 = 2353W.

3) Ensure your batteries can hold at least twice this amount, or else their useful lives will be shortened. So, the batteries need to hold 4706W. At 12V, this is 392AH (Amp Hours). This is the size of battery bank that you'll need.

4) Now we need to figure out how to get the energy back into the battery every day. Start by multiplying the amount used by 125%, since recharging costs 25%. 2353W * 125% = 2941W. This is how much we need to pump into the batteries in order to replace the 2353W we use each day. If you're not confused yet, read on!

5) Now we need to add in some more because the charger (probably built into the inverter) isn't 100% efficient either. But if it's a good quality charger, it's probably 90% efficient when charging from solar cells (it will be less efficient when charging from shore power, but who cares). So, to feed 2941W into the batteries, we need to feed 2941W / 0.9 = 3268W into the charger.

6) So, the source of energy, the solar cells, must supply 3268W to the charger in order for us to feed 2941W into the batteries, which is what we need in order to replenish the 2353W we use each day, which is what we use in order to feed 2000W to the appliances. Phew! Talk about inflation. No wonder the boat gets hot inside 🙂

7) The really smart (or bored) ones among us will have worked out that these numbers will likely stay constant for all good quality equipment, and so a shortcut is as follows. Figure out how much you consume. Multiply it by 1.63 to get the amount of solar you need, or divide it by about 5 to get the size of batteries (in Amp hours at 12V) that you need.

8) Depending on geographically where you are, you can now figure out what size solar panels will get you this amount of power. General rule of thumb for sunny summers (but not good for Seattle winter where the sun doesn't shine): Multiply the cells' energy output by 5. This is the total amount of energy you are likely to get out of them over the course of a typical summer day. So, if you buy a single 190 Watt cell, you'll likely get 950 Watt Hours out of it each summer day. In the case of our example, we need 3268 Watt Hours each day from the cells. So, divide this by 5, and we discover that we need solar panels rated at 654 Watts.

9) Solar panels come in many shapes and sizes, but this amount of solar will likely be between 3 and 4 large panels. It will cost several thousand dollars to put this system together, which is a good incentive to figure out how to use less electricity each day 🙂 Also, the panels themselves will produce about 10 watts per square foot, so our system will consume 65 SF of roof space (8 feet by 8 feet = a lot of space!).

10) If you have enough space and money for this kind of system, you can meet 100% of your energy needs without running any sort of genset .You have full "energy independence". But most of us won't have enough space/money, so will need to ask another question. The question is: "With XXX watts of solar, how long can I last before my batteries get down to 50% charged?". For example, if you figure that you can put back all but the last 10% of your battery's capacity each day, then it will take you 5 days to work your way down to 50%, so you'll have 5 days of autonomy. Note: This isn't quite right, because you might dip below 50% on the last day, but it's likely close enough.

By working the numbers out, you'll figure out whether you need more solar, more batteries, or less consumption. Some combination of the three of these will get you what you want, which is independence.

Sorry this is so convoluted, but it really is. Biggest mistake people make is thinking that energy out = energy in. As you can see, it just ain't so.

Dan

 

[RDragon]

Two words.... Wow! & "Confused!"

I'll just keep running the engine when we need power, not green...but easier on my brain. 😳

 

[dan1000]

RDragon: Yes, it is confusing. That's why so many people put in a solar system and then wonder why the battery still drains.

By the way: Since not all days are equal, and since you can't charge your batts to more than 100%, it often pays to have an extra battery or two. That way, you will have some reserve power without dropping the batts below 50%.

Also, winter doesn't equal summer. In winter, you'll likely need twice the solar that you would need in summer in order to do the same job.

Having done this a few times on different motorhomes, I can tell you unequivocally that reducing consumption is absolutely the best way to tackle electrical energy issues. Run the fridge on propane (does this work in a boat?), switch to LED lights, use kettle on stove instead of coffee maker, and so on. Then, add extra batts if possible. After that, add enough solar to give you the number of autonomous days you need.

Finally, there's the question about whether to angle the solar panels, and so on. In my experience, if you have the room, just add another panel and forget about angling them. The costs (up front and maintenance when things break) is similar between having more flat-mounted panels and less moveable/angled panels.

 

[SteveB]

This thread has mutated a bit, but to get back to an earlier question about how to run household appliances aboard an R21-

I have an Inverter aboard to run my notebook computer and charge our cellular phones, but in some other cases, I've purchased some specific 12-volt appliances from this website:

http://www.12volt-travel.com/

My favorite so far is the little 12 volt hand vac, which I use to keep the cabin tidy.

-Steve