I really like my JBJ Arctica 1/4hp chiller. It is almost silent (some chillers are very noisy) and has a very good reputation for reliability. I got it used on Craigs List for $250 ($300?) and it has been running perfectly for me for about four years. New price is about $800, so I'm a huge fan of used chillers. I later got 1/5th hp JBJ Arctica on Craigs List for $150, because it had a bad thermostat. The guy threw in the high quality external thermostat he was running it with, so it works fine, and was cheap. If you live in a heavily populated area, and watch Craigs List for a few months, you'll see a couple of deals go by. Just watch a couple times per day and be the first guy there with cash in hand. Know which brands are good and which stink, test it first, and if it works, buy it!
I run a cold water system (Bimac) that I keep at 62 degrees, in a room that is kept at about 72 degrees. I used to keep my tank at 54 degrees, and over the years I've learned a lot about what it takes to keep a tank cool. It's great that you get cheap electricity, so it sounds like you are concerned with the purchase price of a chiller, and want to know what size chiller will work for you. Think of a chiller as a device that removes heat from tank water, and puts it into the air that runs through the chiller. The size of the chiller you need depends entirely on how much heat (per hour) is getting into your water. You need to estimate the amount of heat (in watts) that is getting into your water from each source, add that up, and get a chiller that can (easily) pull out that many watts of heat per hour. The sources of heat are:
1) Heat from Warm room air conducting through the surfaces of your tank, sump, and plumbing.
2) Heat generated by the warm parts of pumps that come in contact with your water.
3) Two types of heat from lights: a)Heat radiating into your tank and warming the rocks or whatever it hits, and b) heat conducting from the warm air around your lights, and the warm parts of the light housing touching the tank.
There is a lot you can do to keep heat from these sources from getting into your water, and needing to be removed by a (larger) chiller.
1) Room Air - cover every surface you can with insulation to slow the heat coming in from room air. I use 3M Styrofoam (brand) insulation panels (From Home Depot?). It is easily strong enough to not crush when put under your tank, and you can cover 5 sides of your sump with it. Also insulate all pipes (Home Depot). I cover the back and sides of my tank with this insulation, after covering the insulation board with black vinyl (for looks and water resistance). Styrofoam has an R-Value (resistance to heat transfer) of 5 per inch, and you can look up the R-Value of glass and acrylic. You can look up the formula to calculate heat transfer rate given R-Value, and temperature difference between the water and the air. Then for each sq. in' of tank, sump, pipe surface, add up the rate of heat gain per hour (in watts).
2) pumps - Submersible pumps dump about 90% of the watts they use into the water as heat, so just read the back of the pump to get that number. If you can use external (air cooled) pumps, like Iwaki, it will put most of that heat into the air, and probably only 10% of the watts they use go into heat in your water, so consider switching to external air cooled pumps. I use a single Iwaki pump on my system. As a return pump, to push water through my chiller, and to push water through my Aqua-C skimmer. Korallia type pumps for flow inside the tank are okay because they move a lot of water for just a few watts. A closed loop system run by an external air cooled pump would also be a good way to get flow without adding much heat. For each pump you have, multiply the watts used (from the label) by 0.9 (submersible pump) or 0.1 for external air cooled pumps. For pumps with plastic housings that can go either way, multiply by 0.5 (WAG (Wild A$$ guess)). Add up the results, and add them to the number (watts) from 1) above.
3) Lights - a) radiant heat. This is harder to calculate, and differs for different types of lights. If it feels warm just shining on you (like sun light) then probably about 25% of the watts used by the light are going into the water as radiant heat (WAG), so multiply by 0.25. If the light feels cool (fluorescent or LED?) then maybe a lot less (0 - 5% ?). b) Conducted heat from lights - Don't let a warm light fixture lay on top of your tank. Laying it on wooden spacers with 1/2" air space in between will make a huge difference. Also, take the temperature inside your hood to see if the air in there is much hotter than room air, and use that temp (heat difference) in step 1) for the top surface of the tank. If it's hot in your hood, an exhaust fan can fix that. None of my animals use light for food, so I use dim cool lights, and so can't advise you much about how much heat bright lights will add (maybe a lot) but the 25% guess might be close enough.
Get a total for the number of Watts of heat you are adding to your water from all sources. You can convert that to BTUs/hr by multiplying it by 3.4, so if your total watts added to the water from all sources is 250, that is about 850 BTU per hr (BTR/hr).
Chiller cooling capacity is given in BTU/hr (ignore the horse power rating), and you since you don't want your chiller running all the time, look for a chiller with a capacity of about three or four times the BTU/hr you calculated above (so it can deal with hot days, and not beat itself to death running more than 50% of the time). So for the above example, a chiller with a capacity of 2500 to 3500 BTU/hr would be perfect.
My tank is a 60 gallon with no sump, it's super well insulated (double pane on front and top, 1.5" - 3" Styrofoam on 4 sides) and my pump and lights add a minimum of heat, so my 1/4 hp chiller is probably larger than I need, and it only runs about 10-25% of the time (depending on room temp and tank target temp). Even so, it adds a very noticeable amount of heat to the room, and I have to run the AC to keep the room temp down to 72 much of the year. Don't enclose your chiller, or any other air cooled thing (pumps, lights) in a poorly ventilated place. You need lots of cool air to carry the heat away or the pumps and chiller will wear out prematurely.
If the external surface temperature of your tank is cool enough, and the air is humid, you could get water condensation ("sweating") on your tank, obscuring the view. Acrylic insulates better than glass, if the water temp is 60 degrees, the outside surface of a glass tank will be 60-61, while the outside of an acrylic tank will be 63-66 (WAG), so an acrylic tank will be less likely to sweat. If you have AC to keep the humidity and temp down in the room, and/or your water temp isn't too low (65?) you probably won't have any trouble with sweating, but think about it. One way to predict sweating is to look at weather reports for where you live, and note the "Dew Point", which is the temp at which dew will form (water condensing from the air) on a given day (based on the temp and humidity that day). Whenever the surface temp of your tank is below the dew point, you will see sweating on your tank (except for the effects of AC or heating in your house).
Cold water tanks have little or no evaporation (I almost never need to add fresh water to my 62 degree tank) so that's an advantage. Cold water dissolves more oxygen, and cold water animals tend to be more robust, so there are some nice advantages, especially if your animals don't need big expensive lights (what you save in bulbs and electricity in five years might pay for a chiller?)
You don't have to pay $900 to have a cool tank. Get a used chiller, no larger than you need, with a good reputation, don't let dust build up on the radiator, and you can probably get the job done for $300.
