# Are AC amps not the same value as DC amps?

Posts: 1Registered Users
Hi, the first part of my question is whether AC and DC amps have different values or rating.
Here's way I'm asking. I have a chest freezer to be used in my cabin. It's rated at 0.5amps /110v.
Should I expect it to only consume 6amps from my battery for the 12hours I plan to run it?
Or, should I multiply 6amp value by 10 in order to get ( 60amps) as the DC amps required.
Or, do you get to the same point by converting to watts. eg. 110v * 0.5amps =55w therefore 12hours * 55wts = 660w

• Posts: 2,852Registered Users ✭✭✭✭✭
With 12VDC, amps will be 10x amps at 120VAC for a given wattage. 55 watts is 55 watts. What the freezer uses over time depends on lots of things, but say it's already cold with frozen food in it, and runs on a 1/3 duty cycle (ie actually running 20mins/hour). Estimated consumption would be 55w x 12hr ÷ .85 inverter losses ÷ 3 duty cycle = 259 watt-hours. 259wh/12v=~22 amp-hours from battery.

If it's in a hot place, filled with warm food, etc, expected duty cycle and consumption would obviously be higher. It may be worth actually measuring consumption with a kill-a-watt type meter to get a sense of what it actually takes to run in the location and way you expect to use it if planning a system to run it.
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• Posts: 7,923Solar Expert ✭✭✭✭
AC power has another component that DC seldom has,  Reactance.  Motor windings cause Amps and Volts to get out of line, and the inverter has to be able to support higher loading than simple resistance would be.   The net drain from batteries is the same.
This is measured at Power Factor.  Motors, Generators and Inverters often have a PF rating.

BUT - with an inverter you have
a) Conversion Losses  (most sinewave inverters can be figured 80-90% efficient. Specs will sometimes state more, but only at a cherry picked loading.)
b)  Idle losses  The larger the inverter, the more power required to keep it powered up.

And, AC motors have starting surge, often 10x running load.  So for a fridge, you may require a 800W -1200w inverter to be able to start the motor, even though it runs on 55w

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• Posts: 78Registered Users ✭✭
Something worth trying. I run a chest fridge at my camp and only run it during the day. I have large amounts of fluids to "store cold" through the night. Consumption in the day is double that calculated above. Adding extra panels is far more cost effective as a bigger battery. I use a 2,000W inverter only for the fridge which switches on by the thermostat to eliminate idle currents. All fans are removed as they are not needed with oversize inverter.  120A at startup @ 12V.
• Posts: 284Registered Users ✭✭✭
Maybe this helps,  maybe it does not, but  I found these calculators to be a "quick and dirty" for AC and DC amps

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