I don't have very much grass in my yard and even the smallest (5000 sq ft) bag of fertilizer is about 3 times too big. Is it ok to save it over winter and use it next year? Or does it somwhow lose 'power' over time?

Follow manufacturer's instructions for storage and disposal of all chemicals and fertilizers.

If you store the fertilizer with other chemicals, the storage area should be:

well ventilated- to avoid contamination with other volatile chemicals.

dry- dampness reduces shelf life of chemicals & fertilizers and causes metal and paper containers to degrade. See also Catlover's comments on the effects of humidity to fertilizers.

secure from flooding - to avoid chemicals from contaminating environment.

temperature - high temperature could cause chemical reactions.

Avoid storing fertilizers and other volatile chemicals (car additives, gasoline for lawnmowers, garden chemicals etc.) near heaters. They are fire hazards.

I also use old fertilizers and have had no problems with them.

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Amor est vitae essentia.
Love is the essence of life.

Storing fert over the winter is no problem. It is not a fire hazard. That is a very common misconception. Fertilizer alone will not burn easily. It needs to be mixed with other things in a special way in order for it to become an explosive. This can not be done by accident, even if you were to spill gasoline on the fert.
It will not loose any of its potency, but the one thing that may happen is that the fert may become hard and rock like. If that does oocur, just put on a pair of safty glasses, and pound it with a hammer to loosen it back up again. Most of the time it will just crumble between your fingers though.

As long as it is a solid fert storing it in a dry place is best. It is okay if it freezes as long as it remains dry.
If liquid you may want to keep it in your basement or a heated garage

Hope this helps

Mike

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Mike Maier
aka
The Plant Doc