So i'm not an expert but I have played with UPS systems before and I would say 1000w is big enough.  You should also figure that your grandson will most likely never use all 850 watts of power the PSU outputs even with 3 graphics cards etc.  A UPS system for home is designed only to give you enough time to shut the computer down.  If you are lucky you can finish what your doing (under 3 min) and shut down, but in no way is it designed to continue gaming or doing whatever until the power comes back on.  It does work effectively against spikes and surges.  Someone may chime in with a more detailed explanation as well.

Hi Maxd,

Dell would not recommend you to go for a 1000 watts lower UPS as the system if used with high usage application then the wattage will shoot up to 850 watts.

I would suggest you to purchase the 1.2KV UPS.

The Area 51 with 3 way SLI has 1500W power supply.  I always spec an UPS based on the Circuit its used on.  Typical house outlets are 10 to 15 AMPS max.  

watts = 1 × 15A × 120V = 1800W

80 percent efficient power supply means

watts = 0.8 × 15A × 120V = 1440W

That means if I had an Area 51 and 3 monitors I would get a 30 Amp Circuit and Spec a 2500W UPS.

In your case a APC Smart-UPS 1500VA would be sufficient IMHO.

https://www.amazon.com/APWSMT1500-APC-Smart-UPS-1500VA-Tower/dp/B009YBP1MI

Thanks to all for your thoughts.

Speedstep, Firstly, he cannot afford an APC Smart-UPS; your link would run him just over $600 CAD. I'm looking at suggesting an APC Back-ups Pro that will handle only his tower/PSU (850 Watts).

Would this unit , -which has basically the same wattage as the PSU (865/850)-, with the proviso that his wattage draw doesn't exceed 80 % when an outage hits, be safe and effective ?:

www.amazon.ca/.../B003Y24DEU

Because I don't understand how these things work I am concerned that I don't hand him a time bomb that may take out his tower or worse by being under-powered.

btw, The PSU people, EVGA, said not to go higher than 1500 VA.

Comment ?

That really cheap unit is not pure sine wave and likely a fire bomb waiting to happen. It would neither be safe nor effective.  You cant say wattage draw would be 80 percent.  The power draw will be 20 percent more than maximum due to power loss.  You put 1000W in you get 800W out.  Nobody makes power supplies that are 99.99 percent efficient.  12.5 amps ac is 1500VA  more than that requires a nema 5-20 power cord and outlet aka 20A circuit.

Based on price this is the MINIMUM I would recommend.

It will give you 5 minutes run time.

https://www.amazon.com/Tripp-Lite-Display-Protection-SMART1500LCDT/dp/B009TZTGWK/

Also comes with Insurance against damage.

Your local licensed bonded insured electrician would put one in

where the outlet looks like this.  Which would be the plug for a 2000VA UPS.   2500VA and up would need a L14 30R plug at 208V 30A.

The Tripplite uses PMW which apparently is the equivalent of step technology. Correct ? If so I don't see what's gained over the APC unit. The Cyberpower does have a pure sine wave and I appreciate that benefit. Why the change ?

btw, We use 15 amp, 120 volt for most electrical here.

Thankyou for sharing  your expertise on a very difficult and poorly understood subject. I feel more confident in passing this along and selecting a viable unit.