2014-02-08 10:29:11 -0500
This post is a wiki.
Anyone with karma >75 is welcome to improve it.
There are a few things at play here and a few of them could be 1) system size - are they exactly the same 2) where the system is installed 3) are they taking into account the same amount of shade 4) are they using the same equipment (micro-inverters will yield much more electricity - especially when shade is nearby).
We like to be conservative in our estimates and to beat them. It makes for happy customers. So one of the dealers may be doing the same.
There is a button on the pvwatts labeled "derate". Here you can adjust the factor by changing the parameters. For instance micro-inverters will have a derate of .819 or .832 while central inverters may stay at .77. If you use the adjusted derate on the pvwatts page you may see your production kWh go up.
You can also do this - adjust the derate up until you get the output that matches the dealer quote - then ask them how they justify using the derate?