Reply #16: Oh. Well thanks to the magic of net metering you can get away with half that.

[Statistical]

Your AC doesn't run 24/7/365. What matters is how much energy you use per year. For average home that is 800kWh. Those with gas heating & hot water heater will be less. Those with heat pumps, electric hot water, and live in high AC usage areas will use more.

Only way to know for sure if check your power bill. Total up last 12 months that is your annual consumption.

With net metering if your power output is > than consumption your meter runs backwards (rduces recorded consumption). Thus you only need an array big enough for your total annual consumption.

A 100 watt panel will generate 100watt hours for each hour in peak sunlight.
A 5KW array with generate 5kWh per hour of peak sunlight.

So you look at this map:


That number is number of hours of peak sunlight. If it is a 5 then a 1KW array will produce 5kWh of energy per day. So the number (insolation) * 365 = number of hours of peak sunlight you have per year.

Average home is 8000 kWh AC. Now there will be some inefficiencies for a PV system (wiring loss, inverter loss, voltage mismatch) so we need to derate to find out how much DC energy you need. Good starting figure is 80%
8000kWh / 0.8 = 10,000 kWh DC

Say insolation in your area of TX is 5.5. 5.5*365 = 2007.5 hours peak sunlight per year.
10,000 kWh DC / 2007.5 hours = 4.98 KW. You would need a 4.98KW array.

Lets go in reverse to check math.
A 4.98KW array will generate 5.5 hours * 4.98KW = 27.4 kWh of energy per day.
27.4kWh * 365 = 10,000 kWh DC.
80% derate: 10,000 kWh DC * 0.80 = 8,000 kWh AC. Yup math checks out.


That is the basics. There is a little more to it if you are sizing an array for real. Panels need to face directly south (true south not magnetic south). Panels need to be elevated at the latitude of the location. Panels can have no shadows (even small shadow kills output of cell). Most roofs only have part of it facing south so you need to keep that in mind. Flat roofs will require more expensive angle mounts to hold panels at angle equal to latitude. There are more advanced tools to find out exactly how big of an array you will need.