Here's the math I and my installer arrived at for my grid tie system I mentioned above, with the goal to offset my entire bill.
First thing is see if your power company has detailed information on your monthly/yearly usage. fpl has a very nice website with an energy dashboard I could use to get the following graph for the past year:
Based on this I performed the following calculation:
Average of 1275 kWh month / 30 = 42.5 kWh day
42.8 kWh day / 6 hours (general good sun hours per day) = 7.08 kWh
I'm going to assume 75% efficiency for the entire system, so I need an extra 25%. 7.08 kWh * 1.25 = 8.85 kWh
Using 365 watt mission solar panels this is 8.85 kWh * 1000 = 8850 watts / 365 = 24.2 panels
The installer used a different calculation and ended up with 23 panels. I think it's always better to have a little extra so we decided to round up to 24 panels.
Just turned it on today! Here's a screen shot from my sense energy monitor that I installed a couple months ago to understand the usage of the various devices in my house.
Sense monitor on amazon
I added the solar clamps in my panel today before the grid tie system was turned on. Screenshot from the sense monitor iphone app:
Solar is producing 5,687 watts and my house is using 3,561. It's nice because you get a realtime constantly updated display along with the historical information and what devices in your house are doing. With the sense I was able to see that my ac is using 55.8% of my total electric bill. I had assumed it was high, but now I have precise data which always helps with planning.
, I'd think I'd need about twice that size to cover my needs but before I'm generating too much power (even though my coop will buy it back at 4.5 cents/kwh)
