When designing solar panel systems for homes how do you convert the watts generated by the 12V DC solar panels to watts available in a 110/120V AC system? When I do the math it seems to defy logic.
Watts = amps * volts (which also means W/V=A) is the basic formula. So what is wrong with the following: The solar system generates 100 watts at 12V DC = 8.33 amps. Now I want to use that power that has been generated so I feed the power into a 12V DC to 110V AC inverter. When I multiply the amps produced of 8.33A by 110V I get 916.3 watts. I always thought that a watt was a watt (minus power loss due to resistance and inefficiencies of your circuit design of course) so do I really turn 100 watts DC into 900 watts AC? That seems to defy logic. Or do I still have only have 100W 110V AC and why? Am I overlooking a formula that should be included when Inverting DC to AC?
If my math is right then why do they say that I would need a 5kW 12V DC system for my home? That would convert to 416 amps and a 45.8kW 110V service in my home when the solar is at peak performance. (I know that I would get no power at night). My average hourly usage is 2kWh so it seems that 46kW is WAY overkill even if I save some to batteries for night time use.
Best Answer
Your maths is wrong.
100 Watts generated by solar power at 12V.
Your 8.33 amps is the correct calculation.
The mistake you have made is assuming that this current stays the same value when added to the 110V source. You cannot just connect the systems. To feed the 12V (DC) into the 110V (AC) you need to convert them to the same thing - usually the 110V AC. This means using some form of converter (basically a transformer with the DC turned into AC by transistor switches). The POWER OUT will always be less than the POWER in so that at 110V the current will be less than 0.909 Amps NOT 8.33 Amps. (Vin * Iin >= Vout * Iout)