Understanding AH

If I have a 110v motor that pulls 156 watts and it runs for 24 hours then it uses 3744 watts (Wx24) right?  So that means 34.03 amps (W/V) Right?  So without accounting for 50% discharge rules then I would need a 34 AH battery?  This is all hypothetical but a way for me to try and rap my head around it.  Any thoughts? 

Plans are only good intentions unless they immediately degenerate into hard work -Peter Drucker

The motor probably maxes out around 40% efficiency (there should be specs for it) so your input power will be at least 2.5 times your output power.  If the 1 HP rating is maximum power, efficiency at 1 HP is probably closer to 25%.  So at 1 HP you’re consuming 4 HP of electricity or about 3KW. 

If your battery is a 12V battery then you'll need to deliver 250 A continuously.  Add a safety factor (20% is a good idea) to get 300A. So you need a battery that can deliver 300A for as long as you want to run the motor. 

I’d use a 96V system to cut the current requirements 8 times, that's still almost 40A/HP delivered. 

If your motor is much more efficient that can help a lot, redo the math with your total system efficiency to scale the current." 

110v I assume is AC not DC...

156 Watts is .156 KW and is that at maximum efficiency or at a measured amount?  If it's at maximum you'll have to do the math given up there.  If you measured it, then take your number.  It sounds like your motor is quite small...

At start a motor can load to about double it's running pull, so do take that into account when figuring your battery. 

1.42 Amps (figuring your 156 watts is at any moment and an actual measure, divided by the volts).  Your battery needs the capacity to deliver the amps as long as you need it to.  Again it sounds like a very small motor.  And would load to about 3 Amps when starting.  So the battery may need to deliver 3 Amps at any one time. 

3744 watts would be the total consumption over 24 hours (your words).  You don't need to produce 34 amps, you need to produce the working load for as long as the motor is to run.  Are you trying to run it on a solar cell with battery backup? 

There relevant units are:
Watt (Power): Is the rate at which energy is used, like miles/hour or the amount of wax a candle burns in a certain period of time.  This unit describes the energy per time, not the energy itself. 
Volt (Voltage | Spannung in german): Is the difference in electric potential.  Similar to a difference in height between two volumes of water, or gas pressure. " The higher the better", but too high gets dangerous and more difficult to handle. 
Ampere (Current): Is the flow rate of electrons.  Like gallons/hour. 

If you multiply 1 Volt and 1 Ampere, you get 1 Watt.  Both parts are needed to produce something useful.  The water has to flow and there has to be pressure. 

The next ingredient is time.  1 Watt * 1 hour = 1 Wh (Watt-hour).  This is a measure of energy. ( The amount of wax burned.) 

To the actual numbers: 156W = 1.42A * 110V
To get the time the motor will run, multiply 1.42A with the hours it has to run.  for 24h * 1.42A = 34Ah. 
Now battery manufactures tend to use theoretical numbers, that when actually used, will kill the battery.  So the number above has to multiplied with something between 2 and 4. 

Motors are usually rated at their maximum capability, so the actual Power drawn by the motor depends on the load and is likely to be below 156W.

As Deb Rebel already pointed out, one major factor is the battery voltage.  At 110V the current is 1.42A. At 12V it is 13A. 13A is covered by the normal cables uses for household power. 

Are you using an inverter to get the 110V?  If so, it will have losses (below 5% for the best ones, otherwise about 20%). 

Joseph Johnson

Posts: 128

Location: Sierra Blanca, TX

3

posted 1 year ago

Sorry it took so long to get back to you guys.  I am still driving OTR so it is hard sometimes.  The motor is 110v AC and when it is running the kill-a-watt says 156 watts.  I am using the motor on my evap cooler as an example here so I can build a few stand alone systems for various tasks around my property where running a mile of wire is very impractical. 

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