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A down sleeping bag should be compared by its complete temperature rating, fit, total weight and packed volume. Fill power is useful information about the down, but it does not tell you the warmth of the finished bag by itself.
Fill power and fill amount answer different questions
A higher fill power describes greater loft per amount of down. The amount of down used and its distribution also matter. Patagonia's explanation of fill power helps interpret that number. Compare two complete bags at a similar temperature rating rather than declaring an 800-fill bag warmer than a 650-fill bag without other information.
Inspect the shoulder and foot dimensions as well. A bag that meets your target weight but feels too narrow may be a poor match for your sleeping position. Check the hood, collar and zipper arrangement if you expect colder nights.
Packing and damp conditions
Check the manufacturer's compressed-volume specification and try it against the space available in your pack. Down can be a useful choice when compact packing matters, but plan to protect the bag from rain and wet surfaces. Water-resistant treatment is not the same as a waterproof sleeping bag.
If the trip is likely to be persistently damp and you cannot manage drying, compare a synthetic alternative at the needed rating. Avoid deciding from material slogans alone.
Buying and caring for the bag
If animal-welfare sourcing matters to you, look for a documented certification on the exact product rather than assuming all down from a particular brand has the same status. Follow the bag's cleaning and storage guidance, using the manufacturer's care label to choose the method. Let it dry fully and avoid long-term storage under heavy compression. The useful purchase is the bag that fits your trip and care routine, not simply the one with the highest advertised fill power.
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