I have no idea why lower wattage heaters have problems, but we used to get heaps of 25, 50 and 75 watt heaters returned because they stopped working or they cooked everything, but the 100, 150, 200 and 300 watt heaters hardly ever got returned. They were all the same brand, but for some reason, the lower wattage ones always had problems.
According to the suppliers, they all used the same thermostat and the only difference was in the length of the heating coil and the amount of power they used.
In the end we stopped carrying 25, 50 & 75watt heaters and only had 100+ watt heaters.
We tried a few different brands too and the lower wattage ones always had issues.
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Fluorescent light bulbs are fine regardless of wattage.