Planted tanks are much easier to run in the range of 24 to 26 celsius. Gaseous exchange is better as the water holds CO2 and O2 saturation better and plant metabolism is slowed down. Algae is often easier to manage in cooler tanks as well. Plant growth forms tend to be more compact.
In tropical climates, tank temperatures are often significantly higher, around 28 to 30 degrees celsius especially when hot lighting units are used. Chillers are one method to lower the temperature of tank water, the other being running the planted tanks in an air-conditioned room. The main concern is cost - a chiller is expensive, and folks are also concerned about electrical costs. This article's focus is on the electrical costs of constantly having a chiller running.
In tropical weather, setting chillers to 25°C (with a +1°C tolerance) gives optimal performance when considering Cost vs Benefits. Going any lower than 25°C results in glass condensation that could damage equipment, and exponentially higher electrical bills as the chilled temperature gap increases. 25°C is sufficient to provide 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).