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Old 09-18-2017, 07:44 AM   #1
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Is an amp an amp?

For those who are trying to figure out your amp usage for battery banks. When using a converter the amp usage is different. By using this calculator.
https://www.batterystuff.com/kb/tool...-inverter.html
we see that 1 amp AC run through a converter uses a tad over 11 amps DC

A typical 120V space heater is 1500 watts on high. Using the formula Amps=Watts/Volts we get 12.5 amps AC run that through an inverter it's 138 amps DC.
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Old 09-18-2017, 07:54 AM   #2
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Quote:
Originally Posted by somewhereinusa View Post
we see that 1 amp @ 120 volts AC run through a converter uses a tad over 11 amps @ 12 volts DC
Just a quick addition there. Amperage calculations are not affected by the current being AC or DC. Voltage is the important part of the calculation.

Someone running a 120 volt DC battery bank to power a 120v AC inverter then 1 amp out of the battery bank would be 1 amp out the inverter, minus DC-to-AC conversion losses.

Likewise, a 1 amp load being pulled from a 120 volt AC inverter would be using 2.5 amps from a 48 volt DC battery bank, minus conversion losses.
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Old 09-18-2017, 08:03 AM   #3
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That's a nice calculator. The one thing that I can't find specifically stated on the site is the inefficiency loss rate. They seem to be using ~10%, though.
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Old 09-18-2017, 10:13 AM   #4
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10% loss is being really optimistic. I usually use 15% as an optimistic value but real world it's closer to 25%.

This is why I run everything I can directly from DC, and avoid using the inverter at all. Wasting my hard earned sunshine on inefficient conversions like this makes baby jesus cry!

Anyway the formulas around volts/amp/watts is:

Watts = Volt * Amps

Watts are an actual unit of work, volts and amps are the inputs required to get that unit of work done. You will do a lot better if you always think of power as watts first, and drill down to volts and amps second.

It's not always easy, especially with things like batteries where they give you a voltage range and an Amp Hour capacity, but if you don't need super precise calculations you can just use the halfway point of your battery's charge range for volts. So for example I have a 200AH AGM battery that operates between around 12.9 and 12.1V, we'll just call it 12.5 and I've got a 2500 Watt Hour battery, which is much more meaningful.
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Old 09-20-2017, 12:00 AM   #5
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Volts, Watts, Amps. What does it all mean anyway? Perhaps a useful analogy is thinking about electricity like water. I mean, both are currents, right?

Voltage can be like water pressure. A higher pressure can "do" more than a lower one.
Amps can be like Gallons Per Hour/Minute. Whether it's at a very low pressure (such as water flowing down a wide stream) or a very high pressure (such as from a water main), it's still measured in gallons per hour/minute.
Watts is like total water used/required. A change in Voltage/Water Pressure is directly proportional to Amps/Gallons Per Minute needed/supplied.
A battery is like a water tank. It can only hold so much juice. The voltage is similar to putting some air-pressure in that tank, and as you use it, it will drop.

Now, carrying on with this example, you'd need a minimum (and in many cases, maximum) pressure to do some tasks. Too much pressure can burst the pipes, blows the seals and innards of your gadgets, just like too much voltage can fry fuses, electronics, bulbs, and even wires. All your electrical gadgets are designed to work with a specific voltage.

Knowing that, however, is only half the battle. We know a starter needs 12 volts to spin, but a 12 V watch battery clearly isn't going to do the job. Now we're talking amps. Back to our water example, even under no pressure, we can still harness water using a water-wheel to power entire mills. We just need a wide trough to get a large volume of water onto that wheel. On your bus you have a nice wide wire from the battery to the starter to serve the same purpose.

Using water, and knowing the weight/density of water, we can actually calculate how much we would need to pass over that water wheel to power a given load, and it's no different with electricity. Volts x Amps = Watts. The math works the same in reverse, both directions. Watts / Amps = Volts. We probably won't need this often since we generally know the voltage we need to be working with, so we use ... Watts / Volts = Amps. We can obtain the wattage of many things we need to power and usually have established the voltage, so we use these two bits of math to calculate wattage or amperage, as needed. A margin of error and safety is essential, so figure adding a 10% (minimum) margin for this at the end of your estimations.

Also take into consideration that changing from one voltage to another incurs a loss, such as inverting (from DC to AC) as well as rectifying (from AC to DC).
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