Jkohuts

What's the problem currently, that you can't keep it warm enough? Or that it is all over the place?

The recommendation I give people is to use two heaters that add up to 3w per gallon. So if your system is a 55g tank, you need 165w of heater divided across two heaters. Two 75w or two 100w heaters should suffice for most days. If one heater sticks on, it won't have the power to overheat your system and harm the livestock, and the odds of both sticking on is unusual.

You should always check your system for things that are unusual, like seeing the heater's orange light on all the time. If it is, that heater may be in need of replacement. If you see it on and later it is off, all is well. Watch your thermometer.

I prefer to keep both heaters in the sump, but if you are in a power outage and have to resort to a generator to keep your reef alive, it may require moving the heater up to the display area. With my system, when the power goes out, I switch over to the generator and run the return pump, skimmer, calcium reactor, Vortech pumps and VHO lighting. Nothing else is critical for the duration, but those will take care of the basics.

i was having trouble keeping up to temp and it would fluctuate a little more thank i would like. i have 2 battery back up units for my tank i just run the return and skimmer off one and power heads off the other, haven't decided about my t5's 108 watts. the battery back up units are different sizes. i think the problem was one of my heaters was bad. i replaced it and we will see what happens.

I would suggest purchasing a controller for your heater.
In my aquarium the temperature was not stable until I purchased the JBJ temp controller. Try to find a controller with a remote temperature probe. The ones built into the heaters are not very accurate.

i just ordered a chiller for the summer that has a dual stage heat cooling control that can handle up to 300 watts. It has a plug for the heaters in the side of it. I think im going to give that a shot.

The battery back ups have nothing to do with your tank's temperature or operational needs, but will come in handy if there is a power outage. I used to have an APC 1500 on both my 29g and my 55g. The 29g used a Mag 5 return pump, and the APC would keep it running for several hours to maintain circulation. Nothing else was plugged in. The 55g's APC kept the Tunze 6100 running, which would last 6 or more hours. Again, circulation in the tank was key.

I wouldn't use a UPS to run a heater or lighting, as both suck up too much power. A UPS is for minimal needs, not everything.

You still haven't explained what swing you are seeing. What's happening specifically?

Controller is the key IMO. I had issues all the time until I bought my ranco controller. I run a reefkeeper lite controller now, and still rely on the ranco to take care of things. They can be had for cheap off fleabay. Best money I ever spent on the reef.