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I'm trying to figure out how to power a few devices during power outages. I've looked into UPS devices and also portable phone power banks. My biggest problem is how to compare the devices, apples-to-apples. The UPS we're looking at claims to provide 600VA/330w, and it has only 120V (AC) outlets. The power bank claims it provides 20,000 mAh. It has both USB ports and a 120V (AC) output. How do I evaluate how long each device will last? It would be helpful to know how to convert VA or W from the UPS and mAh from the power bank in like terms.

We have two devices to power. First, we'd like to be able to charge a Moto e4plus phone. The battery itself is rated to 5,000 mAh. It charges through USB, and as far as I can tell it draws 1.9A while it charges at 5v. A full charge takes 2-3 hours.
The second device is our cable modem. The modem is a Technicolor TC8305C. The modem plugs in to the AC mains and the label says it draws 1.2A. The modem itself does have the capability to use its own battery for reserve power. We don't have one, but I've attached a picture of one. Perhaps that might give a clearer picture of what DC power it actually consumes. battery pack

Thank you!

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  • \$\begingroup\$ W to mAh : divide by voltage, multiply by time. mAh to W : the reverse. \$\endgroup\$ – Brian Drummond Aug 5 at 14:18
  • \$\begingroup\$ The battery has a literal example : 7.4 V * 2.6 Ah = 19.25 Wh. \$\endgroup\$ – Justme Aug 5 at 14:23
  • \$\begingroup\$ Not at all. They are different physical quantities. \$\endgroup\$ – winny Aug 5 at 15:00
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To make an "apples to apples" comparison, you'd need the capacity rating for both devices in watt hours.

What you have is VA for one device and mAh for the other.

VA (volt amperes) and watts are a power rating.

mAh (milliampere hours) is (sort of) an energy rating.

The unit "watt" doesn't have time factored into it. Energy always has time factored into it.

You can't compare the two ratings without more information than you've given.

A UPS will often have a power rating and a time rating (600 VA for 10 minutes, for example.)

The powerbank rating is usually the capacity of the battery rather than a rating that directly tells you how much energy you can get out of it for how long.

Energy is the product of voltage, current, and time.

Your power bank gives you the product of current and time for the battery discharge, but doesn't give you the voltage of the battery.

The UPS gives you the product of voltage and current for the output, but no time.

You cannot calculate the available energy from either one, and are missing different bits of information you would need.

You can't really make a valid comparison of the two from the information you are given. It's like comparing an apple core and an orange skin. Does not compute.


I'd find a UPS that gives you the VA rating and a runtime. From that you can estimate how long your devices will run.

Multiply VA by the time in minutes. Call that E.

Multiply the volts and current for each device to get a VA rating for it. Add up the VA ratings for all of your devices. Call that P.

E divided by P will give you an estimated runtime in minutes that you can expect from the UPS. It will be somewhat pessimistic because the VA ratings for the devices will be the maximum rather than average values.

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