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Old 12-12-2009
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Question Running engines for electric power

I hope this is the right area to ask this...

How can one determine how long an engine has to run (to recharge batteries) in order to provide enough power to run an electric appliance for a certain length of time?

For example, how many hours a day would a typical, modern, 30HP engine have to run (per day) to keep a set of batteries topped off if you were running a chart plotter and radar unit 24 hrs, nav lights 10 hrs and recharging a laptop once (per day).

And how would you figure out how many/what type of solar panels could reduce that by X hours (assuming decent sunlight).

Is there an "easy" way to figure it out? I know there's math involved, but hopefully nothing too complex.

To top it off, I'm not all that clear on the difference between watts, volts and amps.

-Greg
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Quote:
Originally Posted by GregX999 View Post
I hope this is the right area to ask this...

How can one determine how long an engine has to run (to recharge batteries) in order to provide enough power to run an electric appliance for a certain length of time?

For example, how many hours a day would a typical, modern, 30HP engine have to run (per day) to keep a set of batteries topped off if you were running a chart plotter and radar unit 24 hrs, nav lights 10 hrs and recharging a laptop once (per day).

And how would you figure out how many/what type of solar panels could reduce that by X hours (assuming decent sunlight).

Is there an "easy" way to figure it out? I know there's math involved, but hopefully nothing too complex.

To top it off, I'm not all that clear on the difference between watts, volts and amps.

-Greg
Hello Greg,

Watts = volts x amps.

The boat's electrical system is usually a mixture of A/C and DC. The DC is most often 12 volts, or 12vdc (12 volts dc). If you have an inverter then you can convert that 12vdc into 120vac (120 volts a/c) which is what is in your house. Watts = volts x amps, so if you have something using 12vdc boat power at 1 amp then it is using 12 watts. If you have something that needs 240 watts and it requires 12 volts then it uses 20 amps. If you have a solar panel that is 100 watt and it is producing that amount then it is making approximately 100 divided by 12 or about 8 amps.

So if you aren't worried about losses, etc, then just add up what everything is using - let's say a fan uses 20 watts, a television 30 watts, etc, etc, and you end up with lets say 300 watts, then you can figure out how much power you need per day in amp hours by dividing 300 by 12 equals 25 amps for 24 hours or 600 amp hours. One of those big deep cycle batteries is often 100 amp hours at 12 volts, so you can start to see about what sized batteries you would need as a "buffer".

Why a "buffer" ? Because you don't want to run the engine all the time, you want to charge up some batteries, let the load drain them to some safe level where they aren't overly discharged, then charge them back up. The bigger the buffer the less often you have to run the motor. Solar panels are great because they are like running the motor at a low charge level all the time that the sun is up, so they charge during the day and then don't do much of anything at night. Still, no matter how big the battery bank you still have to generate as much power as you use, so if your load is 300 watts constant then you need to be generating and storing enough power to replenish that 300 watts per hour into your battery bank.

There are tons of "but, if's ..." involved, there are losses that mean that you don't get all the power that you generate, there are concerns about how deeply you discharge the batteries before you charge them back up, there are concerns about wearing out the diesel engine by running it for short periods of time which some say doesn't allow it to warm up and be properly lubricated, etc, but the basics are pretty basic and it doesn't take much learning to get the basic idea. One concern you would have is that no matter how big your generator is you can't charge the battery bank any faster than what it will accept, let's say 10% of its capacity for easy math, so if you have 10 deep cycle batteries which is 1000 damp hours then you can only charge it at 100 amps (10% of 1000 amp capacity equals 100 amps), so 1200 watts (12 volts times 100 amps) is the most power the bank could even accept, hooking the bank up to a 10000 watt generator would just waste fuel.

There's more to it ...
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Thanks wind_magic! I think I understand most of what you're saying. Except I am confused about...

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Originally Posted by wind_magic View Post
So if you aren't worried about losses, etc, then just add up what everything is using - let's say a fan uses 20 watts, a television 30 watts, etc, etc, and you end up with lets say 300 watts, then you can figure out how much power you need per day in amp hours by dividing 300 by 12 equals 25 amps for 24 hours or 600 amp hours.
You say "if a TV uses 30 watts"... how do you know how many watts it uses? And is that "per hour"? Or is that "while it's on". I guess I'm confused about if watts is a measure of "flow" or a measure of "amount". (Like "gallons per hour", or just "gallons")

So how much "power" does the TV suck out of the batteries if on for 3 hours? And what unit of measure do you use to express the absolute amount of power still in the battery, volts? or watts?

And about the engines... how many watts does a modern, 30HP marine engine produce? (I assume it's actually an alternator making the electricity, correct? Or do marine engines use something different?)

I'm learning... little by little...
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Quote:
Originally Posted by GregX999 View Post
Thanks wind_magic! I think I understand most of what you're saying. Except I am confused about...



You say "if a TV uses 30 watts"... how do you know how many watts it uses? And is that "per hour"? Or is that "while it's on". I guess I'm confused about if watts is a measure of "flow" or a measure of "amount". (Like "gallons per hour", or just "gallons")

So how much "power" does the TV suck out of the batteries if on for 3 hours? And what unit of measure do you use to express the absolute amount of power still in the battery, volts? or watts?

And about the engines... how many watts does a modern, 30HP marine engine produce? (I assume it's actually an alternator making the electricity, correct? Or do marine engines use something different?)

I'm learning... little by little...
Watt is a measure of work, you can think of it like flow. In terms of the water analogy volts is the speed the water moves at and amps is the size of the river, and multiplying flow rate times size gives you volume [edit, really it is more like flow, or total inertia, not volume], which is sort of like what watts are.

How many watts does the motor produce, that depends on the alternator which is the electrical generator on the motor. Your alternator will be rated to produce a certain number of amps, you can look up the model of your alternator and see what it will produce. You can also change the alternator to get a bigger one, usually, that will produce even more power. The alternator typically uses very little of the power generated by your motor so a lot of power gets wasted unless you are also using the motor to actually move the boat somewhere, in which case the electrical power is kind of "free" since you're using the motor anyway.

How much power do things use ? Turn your television around and look at the back and it will have a little plate saying how many watts it uses. Or you can look in the manual that came with it. Or if it is something small you can look at the little wall wart (the power adapter) that came with it and see how many amps that generates. Or you can just get a table of common values and guess. Or you can get something like a "watts up" meter and actually measure how much power the device is using.

Power usage is while it is on, typically, so if it uses 30 watts for 1/2 hour then that is the same as using 15 watts for a full hour, or 60 watts for 15 minutes. Does that make sense ? 30 watts at 12 volts is about 2.5 amps, so if you want to run it for an hour you need 2.5 amp hours of battery capacity, so a 100 amp hour battery (in theory) would run a 2.5 amp load for 40 hours before being completely discharged.

Power usage isn't always constant - there are usually two kinds of power usage, one when you are using it, and one when it sits idle. So for example your old VCR player might use 20 watts while you are using it but still use 0.2 watts when it is turned off (to keep the clock set, etc), so even if you aren't using something it may be using small amounts of power. Usually this is called "standby" power and you can find it in the documentation that came with whatever device it is.

Keep asking questions and I or someone else will answer them ...
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Last edited by wind_magic; 12-12-2009 at 07:06 PM. Reason: correction
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Let me try using your water analogy again, I wasn't very clear ...

Pretend you have "X" amount of water going past a bridge in a given period of time, that is kind of like watts, it is the rate times something like the area of the cross-section of the river. Now, if you reduce the size of that river then the speed of the river has to increase to get the same amount of water to flow past the bridge in a given period of time, that is kind of like increasing the voltage which reduces the amount of amps to get the same number of watts. Watts is kind of like the ability of the river to do work, its rate times its size, and amp hours at a given voltage is kind of like the volume of the water. So if you have something that uses 12 volts at 10 amps that's 120 watts. But you could also have something that uses 24 volts at 5 amps and that is still 120 watts, they both use the same amount of "power", but the 24 volt electricity is "moving faster" (24 volts), but the river is half as wide (5 amps instead of 10 amps). The lake holding the water is kind of like a battery and that is amp hours at a given voltage, so if you have a 12 volt battery bank and it holds 100 amp hours then it can supply 12 watts (12 volts at 1 amp) for 100 hours which equals 100 amp hours, or it could supply 6 watts (12 volts at 1/2 amp) for 200 hours, etc (in theory).
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Last edited by wind_magic; 12-12-2009 at 07:04 PM. Reason: edit
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Old 12-19-2009
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Buy Nigel Calder's books on boat maintenance. Tons of excellent info all spelled out for you on alternator power, battery banks, battery meters, inverters, solar & wind, load calculations, etc, etc. Big book full of info that could not be translated in a few posts here.
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Old 12-20-2009
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Thanks Xort, I'll look it up.

Edit: It now on my Amazon wishlist.

Last edited by GregX999; 12-20-2009 at 07:37 AM.
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