Lights Flicker And Inverter Makes Clicking Noise

What could be the reason?

• http://diy.stackexchange.com/questions/44789/how-can-i-fix-flickering-and-continuous-clicking-with-a-ups-on-an-inverter-suppl

• www.tomshardware.com/forum/55733-7-turn-beeping-noise
• http://benignblog.com/2013/01/solutions-for-ups-problems.html

Additional reading:

• Taking care of A/C: http://www.makeuseof.com/tag/air-conditioner-blunders-avoid-summer/ (AC)