Sizing a UPS

[HCHTech]

Ok, I kind of fell down a rabbit hole on this little project, because I found myself with an uncharacteristic excess of free time today coupled with my own curiosity.

So this is a common-enough issue, "what size UPS should I get for a particular job?" In today's version of this question, I had a generic discussion with the client about size vs. cost vs. runtime, and landed on the statement "It really depends on how much runtime you want." Some time passed, and they came back later with the statement, "We'd like a guaranteed runtime of 30 minutes." This is for a network equipment stack, not for computers.

Ok so I thought I'd dig in and really make an attempt at understanding the load and requirements here, so I could intelligently answer the question, showing my work for proof. Here's how I got there - please poke holes where necessary:

1. UPS "capacity" is invariably shown in Volt-Amps. e.g. 500VA or 1000VA.
2. I found this website that stated for runtime, a rule of thumb is "The formula commonly used is 10 times the capacity of the battery (in ampere hours) divided by the load on the device (in watts)." giving you estimated minutes of runtime for a specific load.
3. So, step one, convert Volt-Amps (VA) into Ampere hours (Ah). For 2-phase power, the formula for this is VA / (Line Voltage x 2). So at 110v line voltage (I'm in the states), a 500VA UPS provides 2.253 Ah (500 / (110 x 2)).
4. Next, measure the total UPS load in watts. Here comes the "Watts = Volts x Amps" formula. Getting the input numbers for this is trickier than I thought since I'm not onsite, but Google is my friend. I have the following equipment in the network stack:

a. HP OfficeConnect 1950 48-port switch. I found a spec page that showed 110v input at .5A, or 55W. I later found another site showing 54W maximum power consumption, so that tracks.
b. Sonicwall TZ400 - This has a power brick which outputs 12 V @ 3A, so an easy one at 36W. Note this is FIOS internet, and we're taking ethernet directly from the ONT, so no modem in the mix.
c. Comtrend POE switch for IP phones (this is tricky since load depends on what phones and how many) I found specs for the switch excluding POE load which showed 110v input at .35A = 38.5W. Let's round this to 39 to get rid of the decimal.
d. They have 7 POE phones connected to the Comtrend switch, which are POE at 24V & .2A = 4.8W ea. x 7 = 34W.
e. Unifi US-8-150 POE switch for the 3 access points. Specs on this note "20W maximum power consumption excluding POE, 150W including POE". So 20W for the switch itself
f. 3 Unifi AC Pro access points. These spec at 9W ea. They are 48V POE, so that means .1875A ea. for extra credit. 9W x 3 = 27W total

So.... in total, I've got a maximum load of 55 + 36 + 39 + 34 + 20 + 27 = 211W load.

5. If I get a 750VA UPS, that converts to 3.41 Ampere Hours, and the rule of thumb from step 2 gives us (3.41 x 10) / 211 = .162 hours or 9.7 minutes.

That's nowhere near the 30-minute goal. It looks like a 2200VA UPS would be necessary. That's (10Ah x 10) / 211 = .474 hours or 28 minutes.

Note that all of these calculations assume 100% efficiency, which is never the case, so I imagine the more practical answer would be "Plus X%".

I have a feeling the client is going to relax their 30 minute requirement once I tell them how much a 2200VA UPS is going to cost.

Other than that, though - did I get the math right? Is there an easier way or rule-of-thumb that I'm missing?

 

[YeOldeStonecat]

UPS Selector

Select the right UPS for your home, home office, small business, Server Room and Network Closet, or Data Center Facility.

www.apc.com

 

[HCHTech]

Well, that takes all the fun out of it!

I want to know HOW they got their answer - I just can't help myself. Plus, they are showing different numbers for different units that have the same VA rating, so there is more going on, at least with their calculations.

Funny or sad, here is an example: The last time I bought a car, I calculated the monthly payment - by hand - in front of the salesman to prove they had made a mistake. He took my math back to the finance guy and they redid their work - victory is mine!

 

[Sky-Knight]

You know how many watts? Then... assume that for an hour, now you have a watt hour. Which is easily converted into a kwh. Once you know that, you can do some math on each battery in the storage unit to know how many KWh the unit has.

You're only going to get 80% of that duration... multiply.

I despise VA ratings... they mean almost nothing. Has more to do with what the inverter can output than the batteries hold. Both matter I suppose...

Also, if you need high uptimes? It's cheaper to get a "camping generator" that's got LiFePo cells in it than a UPS these days... by a HUGE margin.

 

[YeOldeStonecat]

Yeah, I'm old, I'm not into reverse-Einsteining anything anymore...

I'm guessing each manufacturer knows their products in more details, and with APC generally being higher quality, they expect more out of their batteries...
Reading your list...I'd have plopped an 1100 Pro there....when I read you figured a 2200 I thought that was a bit much, I've used those and 3000 models in stacks of 2 or 3 in big full server cabinets.

Access points typically use a 1/3 to 1/2 of their peak.,..figure Unifi APs at 4-5 maybe 6 watts.
Phones...most of the time probably just sitting on the desk doing not much.

 

[mikeroq]

Side note for the Sonicwall you calculated the DC output wattage not the AC input wattage.

For me I spec based on MAXIMUM power draw regardless of what the current load is. To account for inrush and future capacity.

For example a server with 750W power supply, I load calc for 750W not the 200W it normally pulls.
For POE switch I load calc for maximum POE consumption (or maximum the switch can pull) to account for future POE devices, etc.

Doesn't hurt to pad it a little so there is some buffer, and to size the UPS a little bigger so you're not 100% loading it down in case you hit the maximum. Bonus you can say "This will beat your 30 minutes of runtime based on your current load and provide an hour...I sized it for some growth in the future which will still deliver at least 30 minutes".

 

[VISA MC]

I have seen more than one customer use the big 1,000 watt battery packs from Harbor Freight for emergency use. It’s not automatic but for most of our customers needs, the price is right.

550 watt hour model keeps networks up for 1.5-2 hours depending. Costs less than $600 and can be used for other things if needed as well. They go up in size but that’s usually enough for our customers.

 

[Sky-Knight]

@VISA MC Yes! There are slightly more expensive systems that will work just like a UPS does, meaning they charge and provide power at the same time. They're quite common in the camping space, and vastly less expensive than equivalent capacities in a UPS.

The downside is remote control options, alerting etc. But there are other solutions to monitoring the status of the mains. The only real challenge these solutions have that I can't solve is how to test the batteries. But with LiFePO packs in the mix, that's a ten year from now me problem... Assuming we don't just replace the things outright every half decade or so and send the older units off to other sections of the building or home with someone to use for their actual intended purpose.