I've two UPSs. I'm going to connect two devices to them and the combined wattage of the devices is below the rating for each single UPS. Would it be better* to connect the UPS device in serial with both devices connected, or to connect one device to each UPS separately?
Edit: *where better means the devices will remain powered up for longer during a power failure.
Neither.
Just use one UPS unless you have a specific battery runtime requirement, dual power supplies on both devices or want the added complexity of managing two UPS units.
Your other option is to but a battery with the runtime you actually need. It's quite easy to spec a solution that will run for 4 hours or more. You probably don't need that :)
See: Automatic power on after graceful shutdown on UPS
Please don't go down the path of connecting your UPS units in series (or daisy-chaining them). It's not a good solution.
See here:
The most common misconception about connecting UPS units in serial is that you will benefit from additional run time. This is not true. Most UPS systems output a stepped sine wave when on battery power. Almost all UPS equipment interprets a stepped sine wave as bad power. Therefore, as soon as the first UPS goes on battery and outputs a stepped sine wave, the second UPS in the series will read that input as bad power and convert to battery also. With both UPS systems running on battery at the same time, there is no increase in battery capacity.
No, you can't and shouldn't daisy chain (or serialise) a UPS by plugging in another UPS. Run them side by side (in parallel as I think you're calling it) and make them power completely different devices/servers.
Or, better still, buy a much larger capacity UPS.
I quote the APC website which sums up the reasons why not beautifully.
Q: Is it possible to daisy-chain UPS systems together?
A: APC does not recommend that you daisy-chain two or more UPSs together. Each unit should be plugged directly into a properly grounded wall outlet for optimum surge protection. We do not recommend daisy-chaining UPS systems for the following reasons:
Daisy-chained UPSs do not provide any extra surge protection. A UPS is designed to remove any possibility of a surge reaching the output receptacles. In the event of a surge strong enough to damage equipment, the first UPS in the chain would sacrifice itself to protect the load. This would mean that power would be removed from the second UPS in the chain, forcing the unit into battery operation.
Whenever connecting a second UPS to another UPS, the chance of overloading the first one is greatly increased. The number of receptacles in a UPS is restricted by the power limitations of the UPS itself. Even if the number of receptacles was increased, the overall Watt capacity of the first UPS would remain unchanged. The capability of the second UPS would be inversely affected by that of the first unit. Therefore, the overall Watt capacity of the configuration is no greater than that of the first UPS in line.
In most cases, daisy-chaining UPSs does not allow for extra runtime. If you are using a UPS that outputs a step-approximated sine wave when running on battery, as soon as the first UPS goes to battery operation, the second one will also do so because it will see the step-approximated sine wave as distorted or bad power. Both UPSs will discharge together and will not provide any extra runtime to the load.
