Fill power is an indicator of quality in the down that fills your comforter, and the higher the fill power, the better. A high fill power indicates large down clusters and good quality down, which means your comforter will be both lightweight and warm. A down comforter of 750 fill will be warmer than a 650 fill down comforter of the same weight.
The weight of the down in your comforter determines not only how heavy it is, but also its warmth. A 650 fill, 44 oz. down comforter will be warmer than a 650 fill, 28 oz. down comforter. Buying a heavier comforter is a cheap way to add warmth, but it also makes your comforter less light and fluffy. While adding fill power increases both the quality and warmth of your comforter, adding weight adds warmth without improving quality.
The most concrete consideration when choosing the warmth of your down comforter is the temperature of your bedroom. Consider whether you intend to use your down comforter all year round, or only seasonally. Either way, determine the average temperature of your bedroom on the nights you intend to use your comforter. A level 1 comforter is a lightweight option; if you keep the air conditioner on high or intend to use your comforter in the winter, you will most likely need a higher warmth level than level 1. A level 1 down comforter is most appropriate for bedrooms between 60 and 65 degrees.
The warmth of your body while sleeping determines how much warmth you need from your comforter. There are "warm," "cold" and "average" sleepers, and each needs a different warmth level in a room of the same temperature. A person who is always warm might like a level 1 down comforter in a room as cool as 55 degrees, or possibly even cooler. However, a person who is always cool might use a level 1 comforter in a room as warm as 70 degrees.
Depending on how "warm" or "cold" you are, there may be several warmth levels that would be appropriate for the temperature of your bedroom. Also, at different retailers, the warmth of a level 1 down comforter may be slightly different, or this warmth may be achieved with a variety of fill power and weight combinations.