I'm thinking of getting a dehumidifier for the garage space and running it year-round connected to a hose that would drain to a sump pit. Any advice? BTW, I'm not sure how to figure out how much electricity it would use, but electricity is pretty expensive here, about $.21 per kWh.
Unless the unit upstairs is adding moisture to the air of the garage (shower, washing machine, cooking, etc), then it is unlikely that you'd need to run the dehumidifier in the winter. Though the garage will still be cool, it will be warmer than the cold air outside, so its RH will be lower. Plus, mold, rot, mildew, etc won't progress very much at temps below 50 even if the RH is high.
Summer: If you really need to run the dehumidifier, it's gonna cost some appreciable electricity. The label on my unit says it draws 7 amps (so, about 770 watts), so if it is running 1/2 of the time, it will cost you about $2 per day (interestingly, the label says it removes 1.77 liters of water per KWH of electricity used, so if you knew the air leakage into the garage and the water content of the air, you could get a pretty close estimate of the power you'd use). And, a dehumidifier will warm up the garage, adding to the AC load of the unit above. So, if you decide to go with a dehumidifier, definitely take the time to air-seal the garage as much as possible to reduce the amount of moist air that comes in.
Another option is a small window AC unit. It could be cheaper to purchase than a dehumidifier, and it will cool the garage while getting the moisture out, so it won't add to the heat load upstairs. The down-side to this is that most of these units don't have a humidistat, so it will only run when it is hot in the garage (not when it is moist).
There are "smart" ventilators that sense the temp/humidity inside a space and the temp/humidity outside, then run the fan only when it will serve to dry out the inside area. These are much more energy efficient than an AC unit or a dehumidifier, but they work best in places that have significant swings in daily outside humidity. If you are near the coast and it is humid outside whenever it is hot, they won't be much good to you.
If the concrete floor of the garage is wet, it could be due to condensation (esp if the floor is cold and it is exposed to warmer, moist outside air). Or, it could be due to wicking of ground moisture up through the slab (esp if there's no plastic barrier over the gravel and below the slab). To tell what you've got, securely tape a sizeable (e.g. 2' x 2') sheet of foil or plastic down in the middle of the slab. Wait a few days, then on a day when the slab isn't wet elsewhere pull off the tape and look underneath the foil: if it is visibly wet compared to the rest of the slab, then the water is coming up from below. Tape it back down, and then wait for a day when the slab is wet inside the garage: Is it less wet under the plastic? Then that wetness elsewhere is due to condensation. You can apply sealers to a concrete floor which will do a fair job of reducing the water vapor from coming up, and that might be worth a try if this looks like an appreciable source of the humidity in the garage. There's a good chance it is if the garage is more than a few decades old and the soil is not very well drained (putting plastic under the slab of a garage is a relatively new building practice).
