The A/C unit pulls the most power during start up and this can cause the lights to flicker. It should only be for a split second, if the lights are dimming or you notice the condition is getting worse there may be an electrical problem with the unit.
This condition is usually caused by a poor ductwork design, ductwork that is in poor condition, or someone has added excessive heat producing appliances to a room. Also the A/C system could be in need of a tune up by a professional service technician.
You should never have to add refrigerant (freon) to you’re A/C system. The refrigerant runs through copper lines all soldered together. If the system does become low or run completely out this means you have a leak in your system. A special leak detector is used to locate the leak. Usually the leak can be repaired by soldering it closed but sometimes it requires replacement of the part that has the leak.
A few ways to reduce your electric bill includes adding insulation in the attic or walls, insulating the water heater, caulking around the doors or windows and using more efficient lighting. However your A/C system has the highest power usage of all appliances in your home so keeping the system serviced and clean will save the most on your electric bill.
Your A/C system has a lot to do with this because it is constantly circulating air through the house. If the filter for your A/C system is not properly installed dirt and dust can be bypassing the filter. Also a very low efficiency filter will cause the dirt and dust to go right through the filter.
During the winter the evaporator coil is inactive so some dirt accumulates on it. When the cooling is turned on for the first time and condensation starts to run down the coil a musty smell will occur until the dust and dirt has been rinsed off through normal operation. To prevent this the system probably needs a better filtering system.