Hi guys

I built a snake rack where I am using containers from a local butcher. The containers are quite thick, good few mm. I am currently using 30watt heat wire through the rack but here in South Africa with us coming out of winter, the thermostat is constantly reading 25 degrees Celsius on the hot spot and battles to get above unless its a warmer day. I know a heat pad would concentrate the heat in an area better and thus aid in getting the temperature I want but what wattage to use. Have the option of 9, 15 and 25 watt.