My system is a 10.5 kw system and I have very good conditions(facing south, very little shading)- it produced 45.52Kwh yesterday. The thing to remember is that on an ideal sunny day with no clouds- it generates tons of power - but you can only store so much - so say you have your two batteries - they will be fully charged by noon. Now all the power you are creating is lost - you have filled your storage container. You need to store your days worth in the battery since your most of the consumptions happens after the sun goes down or before it comes up.
For example - I just checked my usage yesterday.
Between midnight and 6am I used a total of around 1.5kwh. Then comes 6AM - that was 1Kwh, 7AM jumps to 3Kwh, 8AM goes to 4Kwh - so for the first part of the day we used 9.5Kwh. At 8Am we have only produced 3Kwh. We then use virtually no power again until 3pm when the kids get home. But the panels would power this consumption. The power production drops at 4pm - which is right about the time power consumption starts jumping. By 5pm we are producing no power - it is all being fed from the grid. In the next four hours we burned 24Kwh. So all told - we used about 33Kwh yesterday during times when the panels were not producing.
Essentially when you go on battery you pretty much need to store the bulk of your electricity daily in the battery unless you work from home or something like that. Then you need to consider that it is quite easy to go two or three days with crappy production. Then factor in winter shortages.
I guess in the end, what I was trying to say was that 20.00 per month is really not bad. It prevents a large outlay of capital and provides 100% efficiency - since I get 100% credit for all power produced.