UPS to run 350 watts for 3 hours

X_xStriderx_X · February 5, 2021

Sorry if this is the wrong place to ask i searched on the forum and couldn't find a similar question. I've been doing some research as to what VA ups i need (im not sure if 2000va - 1200W will work at all for the time needed) to get and im just getting more and more confused to the average power usage is about 160 watts from the wall peak spike being about 320W for 2 seconds. I want something that will run this setup for 3 to 4 hours but i only need it for 2 hours but i want it to be over what i need by about 30% - 50%. My voltage in my country is 230V at 50hz. Im fairly new to this side of electronics so sorry if i said something wrong

SupaKomputa · February 5, 2021

Well UPS is not designed to be used for that duration, usually consumer UPS will only delay the shutdown for 10-20 minutes, just enough time to hit "sleep".

3 hrs you need very big battery capacity, which is cheaper just to buy a gasoline / solar power generator.

PC -> UPS -> Generator.

The UPS still needed for the transition from wall to generator.

mariushm · February 5, 2021

The VA rating is just how much power can the UPS give, peak power. You're interest in the battery capacity, measured in Ah

You can have a UPS rated for 1000-1500 VA which means it can supply 500-800 watts to devices briefly .... but if it has a single 12v 9Ah lead acid battery, it will discharge in less than 5 minutes.

Basically ... lead acid batteries will have a voltage between 11v and 13v - let's just use 12v for lots of margins - and let's say your computer consumes 180 watts.

The UPS will convert DC from batteries to AC with some efficiency, let's say 90% .. so 180 watts will be 90% of what's taken from battery .... this means battery will provide around 200 watts.

At 12v, that's 200 watts / 12v = ~16.5 Ah ... so you'll need approx. 16.5 Ah battery for every hour. For 3 hours, you'll need 3 x 16.5 = ~ 50 Ah

Of course, assuming batteries are full at around 13.5v and they can discharge down to 10.8v, and assuming conversion is done with more than 90% efficiency ... probably 12-15Ah would be enough for one hour, maybe even less.

The point is you need a UPS that has a bunch of lead acid batteries, not just one or two cheap 9-12 Ah batteries, it would not be enough.

I'd look into some refurbished / used datacenter UPS which has a compartment that can take 4-6 batteries and maybe has connector for external batteries, so you can add another set of 4-8 batteries.

Will probably be multiples of 2 or 4 batteries (bigger upses will run internally at 24v or 48v)

here's some examples that may have enough capacity for your needs -- but do your research :

https://www.ebay.com/itm/APC-SURTD5000xli-UPS-New-cells-Fully-working-12m-RTB-warranty-5KVA-UPS/401188057598

And I agree with SupaKomputa ... you should think more at the 20-30 minutes range. if it takes more, you could start a cheap power generator on the balcony or outside in the garden (if you live in a house). A $200 gasoline/oil mix a-few-horse-power generator (as basic as those used on construction sites to power construction tools) can do a few hundred watts for a few hours.

You can have one of these good quality double conversion / on-line UPSes with lower capacity and if it takes more than 15-20 minutes, you can turn on the generator to top up the UPS and supply computer at same time (ups will filter the not-so-great ac power produced by generator)

Archer42 · February 5, 2021

You should not look at VA for runtime. Those are just maximum current UPS itself (inverter mainly) can sustain for short period of time.

If you want 2-4 hour runtime what you need is battery capacity. For example looking at 200w for 4 hours you will need 800Wh battery, +~30% for various inefficiencies (including inverter, the fact that battery cannot be fully discharged and will gradually degrade over time, etc), which is not that unrealistic, just large and relatively expensive. Since many large UPS use multiple batteries in series it will mean something like a pair of 12v 50Ah batteries, for example.

And then there are cooling considerations. Most consumer UPS are built to run on battery for just few minutes, they do not have proper cooling for inverter and such and will overheat/shutdown if used for a long time even if battery is fine. So you have to be looking at something made for servers and such, with fans and capable of running for long periods of time.

If possible, small generator, as mentioned above, + small ups will probably be better for such usage. Big issue with generators is noise though, where i live it would be impossible to run one without a good reason (some kind of serious emergency) as it will be extremely annoying to neighbors. So if what your want is not "just for fun" but for some serious stuff, depending on circumstances, going with large batteries might still be the best solution.

X_xStriderx_X · February 5, 2021

Thanks a lot for the information guys you've cleared up my confusion now I understand a lot better, A generator wont work in my situation so I'll look into getting an inverter rather and have a bigger system with multiple 110Ah deep cycle batteries to run some other stuff around the house because these daily 2-3 hour outages i get are a pain and they are not stopping anytime soon so I will do some more research into what I'll get but since its a bigger project than i was hoping ill make sure i know what I'm getting into first before buying.