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Electrical Mathematics Conversation

chappy75's Avatar

Electricity Form:
Watts = Amps * Volts

For DC circuits, voltage times amperage equals wattage.

W = VA
A = W/V

A = 20 / 12

A = 1.67 A

Amps hours is the number of amps drawn times the hours of continuous usage.

1.67(9) = 15 Ah of capacity

A fresh and fully charged 22 AH battery will drive a 20 W 12 V lamp for

22 / 1.67 = 13.17
or a little over 13 hours

Forgot what my question was…lol

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williamj's Avatargold


In a DC circuit current (amps) is also calculated as…

R*V (resistance multiplied by voltage)

So, if you know the resistance (r – measured in ohms) then you can calculate the current needed to operate that circuit. For instance…

An incandescent bulb has a resistance of .5 ohms (hypothetically) and runs on 12VDC. (There’s actually more involved than this for this example. For a circuit like an incandescent bulb you have to take into consideration impedance and some other factors as well)

Which gives us… 12VDC * .5r = 6a (not even anywhere near real life)
Then for wattage it’s… 12VDC * 6a = 72w or
Voltage squared times amps times time equals watt hours

That is of course if that’s what you were looking for… watt hours as apposed to amp hours.

If that’s not what you were looking for then completely ignore this post…. this post will self destruct in five seconds

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chappy75's Avatar

How many volts are in 1 HP?

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chappy75's Avatar

Found this:

Work it backwards, Amps would be Hp/(E x PF x EFF x (1.73/746))
or in simple terms:
HP x 746 / (E x EFF x PF x 1.73)

The 1.73 is because of the 3 phase issue. 746 is how many Watts=1HP in normal, pure power terms

This question is not really answerable because we do not know the Power Factor nor the efficiency of the “motor”. Most power factors are like .97 or something. Efficiency – well nothing is 100%…

If both are 100% (1.0PF), then the answer would be almost 2 Amps

E, I, PF and EFF must be known: HP= E x I x PF x EFF x 1.73 / 746
(See abbreviations explained below)


EFF = EFFICIENCY (decimal)

Have no idea what it means…

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williamj's Avatargold


If you’re looking to find how much current you’ll need to drive a one horse power motor then the electrical equivalent is 746 watts.

If a circuit is 12Vdc then 746w / 12v = 62.2a (approximately)

In reference to your question…

How many volts are in 1 HP?

That’s not how it works, in class to was explained to me as…

An electrical circuit is like water in a pipe. The water itself is the current, the flow rate of the water is the voltage and the resistance is the diameter of the pipe. To make a motor turn requires current driven by the appropriate voltage to over come the motor resistance to turn, the resistance in a motor.

Motor are rated for specific voltage with a specific current capacity. If a motor is rated at 240V and 15A, trying to run the motor at 110V will not push the current sufficiently to turn the motor. The current being pushed by insufficient voltage will nit be able to turn the motor and will dissipate as heat, generally burning up the motor.

If too much voltage is applied the opposite happens… the motor over spins from the current moving too fast through the circuit, over heats and again… burns up.

Basically it’s a balancing act.

hope this makes a little sense)
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biglion's Avatar

Chappy, you could try this site to see if it converts what you need…

William is correct! According to the calculator…

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chappy75's Avatar

Okay, so how then does a 12v, 22Ah rechargeable battery run a trolling motor all day?

If it takes 14.2v to achieve “acceptance phase” to charge a battery and a “float phase” is 13.7v then how many amps are generated by an alternator of a gas powered motor while the electrical motor is under a full load and running at 42 amps?

I mean under the full load of 42 amps, said motor should have 25 minutes of power, right?

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williamj's Avatargold


How long any motor will operate on a 12V 22Ah battery depends entirely on the size and rating of the motor and it’s usage.

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chappy75's Avatar

Usage = Trolling motor

At full on it draws 42 amps
which has 40 lbs of thrust…
If I have a Kayak that I can put a peddle system that I can generate 35 amps with. How many amps do I have to generate to prevent discharge of the battery?

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williamj's Avatargold

In order NOT to discharge the battery at all you would need to generate maximum current draw of you motor (42A @12V or 504W). In other words, if your motor draws 42A then you would have to generate 42A in order to keep from discharging your battery.

A genset generating 35A @12V generates only 420W, generating 35A keeps your battery discharge down around 7A.

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chappy75's Avatar

So, if it is charging down at a rate of 7a/hr then I would have 3.13 hours. hmmm
at 5 miles/hr then that gives me 15 miles…
but if I dont generate then it would be about 40 minutes and dead in the water…

Interesting…. Thank you William.

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gadgetmanken's Avatargold

Thanks for the help in explaining some of that guys. I tried trying to figure out horsepower on something awhile back n couldn’t find anything that would help me figure it out. I think it was because I didn’t know the current. Now I don’t remember. James you technically wouldn’t be dead in the water if you carry a spare battery or have your paddles. lol

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chappy75's Avatar

Is it simple math that if I add a battery that I will get double the performance? Or is there a caveat?

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williamj's Avatargold


Pretty much, BUT….

They must be connected in parallel (positive to positive and negate to negative) and then the normal wiring out to your circuit. Parallel connections maintain the 12V with double the current capacity. You end up with more Ah capacity.

Having said that… You will probably need a battery tender to maintain a charge when not operating the equipment. Trying to charge multiple batteries has it’s own caveats.

A series connection (negative to positive) and the out to your circuit would double the voltage raising it to 24V with the same current capacity as a single battery.

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chappy75's Avatar

Never go up the river without a paddle lest you become a Cliche…


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gadgetmanken's Avatargold

Just, sort of been there done that. Oh man, I’m starting to show my age, now. My step dad almost fifty years ago was skiing on the Ohio river and suddenly the motor jumped off the back of the boat, and he had to dodge it. As a result we were stranded out on the river, with one oar, with a barge coming at us. Luckily someone helped us and towed us out of the way!
I don’t remember if we tried to use the ski to help or not!

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mindmotion's Avatar

All I remember from my electrical classes is that in DC circuits we talk about resistance, and in AC circuits, we refer to it as impedance. I think the best education I got was from a friend, who is an electrician, than my college classes. Being electrocuted a few times also helped me remember the dangers!

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gadgetmanken's Avatargold

I have been called sparky for the same reasons. Nothing like a good mentor!

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