Heating advice, what wattage heat pad to use !

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.

If its controlled be a tstat the get the 25 so you know you will have enough heat

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Yeah have a day night temp and light controller that they will be attached too. Guess higher is safer bet as guaranteed to get enough heat.