We actually cover this pretty common question in our getting started article;
https://aquariu.ms/article/2-freshwater-getting-st…
Below is the specific section that answers your question regarding any low power mode (there isn't one, at least that I have found in testing over >15 of these on the market for present ones).
Having an adjustable heater is something that is highly recommended since you can maintain the temperature throughout the whole aquarium by moving the switch from one degree to another. This is since many preset heaters, or always on heaters do not turn off when they reach a specific temperature (unlike adjustable ones) and you can cook your inhabitants to death. Since there are many different manufacturers, we suggest that you read the product's back packaging to know the specific watts you need for your aquarium size.
