Calculating UPS Run Time?

[riggz]

I'm a bit scatter brained today and I am trying to figure out how to calculate the run time of a battery back up for a 4 camera system including the NVR and cameras only. The cameras are listed as 5w max (qty. 4) and the NVR is listed as 12w (without HDD). Assuming the HDD uses 6w this should bring the total to 38w.

If I used a 600VA (360w) UPS backup how long would it last assuming everything was using the max wattage?

 

[pozzello]

VA (and watts) is the amount of power the unit can support (and you're fine there,)
but it tells us nothing about the capacity of the batteries in the UPS,
which will determine how long it can support that much power output...

You'd need the AmpHr rating to determine that...

 

[riggz]

If it lists the Volt-Amp-Hour rating at 84 I assume it is a 7Ah 12v battery.

 

[pozzello]

84 / 38 ~ 2.2 hours, but there are losses in conversion from 12v to 115v (inverter) AND you will not
see that much output as they typically rate the batteries at a 20hr rate. (if you're drawing 10 times
more than the 20hr rate, you'll get substantially less time than at rated output...)

So I'd guess about 15-20 minutes @ 38W, maybe less...

 

[riggz]

Wow. I was hoping for a bit more than that. I guess I will need to do some real world testing. Thanks for your help!

 

[bp2008]

Yup, batteries are frustrating. For any significant capacity they have to be huge and heavy, and they never last as long as you want. I'm sure half my UPS batteries are bad by now. I really should put a date sticker on every UPS I buy so I know how old the batteries are.

 

[johngalt]

Yeah, Efficiency is not calculated into any of those things. I know APC and MinuteMan had pretty good calculator tools. I'm guessing CyberPower does too. Sometimes you can find some common electronics already in their drop downs / selections.

 

[riggz]

I guess my confusion is based on watts listed by the devices and voltage. They list at 5w 12v. So how does this convert to what the draw is on the 120v the UPS is supplying?

I used APC's calculator and it said a 550va 330w UPS with a 7ah 12v battery would last 95 minutes when I input 38w in to the calculator.

So is 5w at 12v really 50w at 120v? It's been so long since I've calculated any of this...

 

[Ssayer]

Those calculators are only as good as the battery in your UPS. On day one the numbers may be pretty good. A year in, and not so good. Three years in (you get the point...). We put new batteries into our UPSs here every three years and are just about ready for another changeover...

 

[riggz]

Those calculators are only as good as the battery in your UPS. On day one the numbers may be pretty good. A year in, and not so good. Three years in (you get the point...). We put new batteries into our UPSs here every three years and are just about ready for another changeover...

Yeah. I have a few rack mount UPSs that run servers and equipment at my house. I'm about due for a changeout with those. $$$

 

[riggz]

So I have an install tomorrow for a customer that includes 8 3MP dome cameras (5w max each) and an 8 channel NVR w/ 8 POE ports.

I hooked everything up to a UPS rated at 1500VA / 865 watts. (Looks like it has 2 9ah batteries.) Running with the IR leds on and recording motion it never peaked above 40watts. The battery backup lasted 84 minutes. It originally estimated the run time to be 116 minutes. Battery may not have been fully charged although it said full.

I will try to test a 4 camera setup on a smaller UPS later this week hopefully and also try to do some more scientific testing.

 

[pozzello]

nothing like real-world testing to make us armchair engineers look bad.

seriously though, good numbers! thanx for the info.

 

[alastairstevenson]

That's a huge UPS for the size of the load.
There is a good chance that when it's so lightly loaded the efficiency will be very low, and that may well account for such a poor result, just 25% of the battery capacity supplied to the load (assuming 12v batteries and a steady 40w load).
And assuming if it says the batteries are fully charged, that they are.

 

[riggz]

Well, this was a much larger battery backup than i had originally asked about. I have a similar battery backup to test with a 4 camera system later this week.

 

[Mike Mckay]

One of the most annoying things about batteries or a UPS is that they only tell you the maximum capacity of the batteries

They will reach a cut off voltage where the conversion circuit ceases to try and achieve the output voltage but the batteries still have X amount of "capacity" left in them so you never get anything like the total capacity out of them unless you are running something that works down to quite low voltages

With alarm panels they tend to shut down or sound the alarm when their batteries reach around 9/10 volts but if you put a nail across the output there is still plenty of power left in the battery

If you are desperate to squeeze the maximum amount of time out of a battery back up with a bespoke system you "can" use a buck circuit to step up your battery voltage to the required amount but this also has its own set of problems and limitations too and is only really useful for very low powered applications like micro controller based projects using things like an ESP or Arduino

For important UPS backed systems I tend to supply or recommend a UPS with 3 times the "calculated" requirement to allow for the voltage cut off inside the UPS and power "trapped" inside the batteries when that is reached as well as to give some leeway as the batteries get older, hold less and lose their output voltage more quickly, with the output I tend to look for 1.5 times the maximum I will need so the circuitry isn't being driven at its maximum but that's not really a concern with things like a DVR/NVR as they tend to be quite low power anyway