The real cost of running chillers in the tropics?

Planted tanks are much easier to run in the 24 to 26 Celsius range. Gas exchange is better as the water holds CO2 and O2 saturation better and plant metabolism is slowed. Algae are also often easier to manage in cooler tanks. Plant growth tends to be more compact;

In tropical climates, tank temperatures are often much higher, around 28 to 30 degrees centigrade, especially if hot lighting units are used. Chillers are one method of lowering the temperature of the tank water, the other being to operate the plant tanks in an air-conditioned room. The main concern is cost - a chiller is expensive and people are also concerned about the cost of electricity. The focus of this article is on the electrical costs of running a chiller all the time.

In tropical weather, setting chillers to 25°C (with a tolerance of +1°C) gives optimum performance in terms of cost vs benefit. Anything below 25°C will result in glass condensation, which can damage equipment, and exponentially higher electricity bills as the cooling temperature differential increases. 25°C is sufficient to achieve significant improvements in plant health.

Costs of running a chiller in Singapore (Data collected by Darren Low https://www.instagram.com/leaf_stones/ )
A local hobbyist in Singapore estimated the cost of running a chiller, by capturing 4 hours of kick-in/out time from 11am to 3pm (the hottest hours of the day). The default temperature of his tank without any cooling was ~30°C, with ambient temperatures of ~30°C. The chiller was set to 25°C (chiller turns off when 25°C / turns on when 26°C). The chiller model used was Hailea HC100a, rated at 1/20HP and 150watts, and costs about S$328. The tank is a standard ADA 60p size, 60x30x36cm (65L / 17gal).

By estimating the % of “on” time (41%), the average cost of running the chiller is S$7.94 per month (based on the local rate of $0.1798/kWH). This is perhaps much cheaper than most folks would think - the running cost is perhaps not the most significant aspect of running a chiller. There are some other downsides to consider however; a chiller slows down flow rates of filters quite a bit so larger sized filters are necessary. Chillers also warm up the room they are running in slightly. 

Side notes: Equipment heat matters
T5 and poorly-cooled LED lightings (Chihiros A-series), as well as additional equipment (oversized filters / powerheads / skimmers) contribute significantly to the heat, resulting in higher operating times (on 60%) – water takes longer to cool down and heat up faster (longer kick-in and shorter kick-out). Using heat-efficient LEDs (BuildMyLED) and removing excess equipment reduced his electrical consumption by 47%.

Chart above shows the timings that the chiller kicks in (dropping tank temperatures) or turns off (Rising tank temperatures). The comparison table below shows the difference in cost in running a 'hot' light (T5+Chihiros) and a cooler light (BML).