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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\$
    – user16324
    Aug 5, 2020 at 14:18
  • \$\begingroup\$ The battery has a literal example : 7.4 V * 2.6 Ah = 19.25 Wh. \$\endgroup\$
    – Justme
    Aug 5, 2020 at 14:23
  • \$\begingroup\$ Not at all. They are different physical quantities. \$\endgroup\$
    – winny
    Aug 5, 2020 at 15:00

2 Answers 2

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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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The newest Power Delivery standards have many options for volt/amp combinations, going up to 100 W, depending on the unit. The older ones, which are most common, are always 5V output. The batteries are most typically 3.7v, internally, though it could theoretically be anything.

You also usually get about a 20% loss in capacity for "conversion" (getting charge out of the battery, with whatever voltage step up or other changes it needs to make, plus efficiency loss on power transfer).

So let's say your power bank is 3.7v x 20Ah (20,000 mAh is the same as 20 Ah). Easy multiplication gives you 74Wh (A x V = W). How can we compare that to your 330W UPS? Let's assume the 330W is true, with no conversion loss. To get the Wh, we would need to know the run time. To use an easy example, 20 minutes is 1/3 of an hour, so a 330W x 1/3h UPS is 110Wh, which is about 50% bigger than the power bank example, while a ten minute run time on the UPS would yield 55Wh, which is about 30% smaller than the power bank example.

If your power bank gives the actual voltage, use that in place of the 3.7v I used in the example. If it doesn't 3.7v is a good guess. You can figure out the rest.

In short, to take your mah-rated power bank and get Wh for comparison, drop the thousands and multiply by voltage rating, which is probably 3.7. To get the Wh for your UPS, multiple the wattage by the number of hours of runtime, or more likely, multiply by the number of minutes of runtime then divide by 60. With both now rated in Wh, you can make a direct comparison.

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