When you need one you need one. All last week i was reading on cap to do a recap on my amp not listening to my inner voice telling me something. The most experience I have with Capacitors is in motors. Capacitor start motors. Last night my A/C failed to cool. So having a background in A/C I start looking into it. In the past I have had to fix freon leaks and it has been fine for a number of years. Well I assumed it was leaking again. So I put the shoes on and I was going outside to feel the supply line to see how cold it was and realized the compressor was running and the fan was not. It was getting very hot so I pulled the fuse block. I take off the panel and notice the contactor is cracked and a copper buss was dangling on one side. played with it and was unable to get it to start. Looking at the capacitor the top was no longer flat but it was swollen in such a way it symmetrical looking and i ignored it. This morning I run to the local supply and purchase a new contactor, put it in and duh, It was not starting. So I pull the cap and look closer to the bottom was swollen. The parts place has closed. I did have a spare but gave it to my son a time ago. Long story short i acquired one from a friend in the biz and put it in and everything is cool now. point coming. Keep a spare A/C capacitor. It will go bad. Mine is commonly used in most condensing units. 377-440 volts 35-5 Uf Mine was a 35-7.5 uf but this one will work just fine. On monday I am buying two. One for me and one to replace the one I got today. Stay safe. Pull the fuse block near the A/C unit and shut off the furnace power. Most are switched on the side. This is where the 24v control comes from.