yes i calculated using 250 days because the average person works 5 days a week 50 weeks of the year. That leaves 115 days where there would be extra power generated that isnt used. The problem with trying to use that extra power is the cost of a grid tie inverter is typically 2-3k$. So you would be almost doubling the cost of the system in order to get that 115 days. You also have to form a paralleling agreement with your power company, which they have a minimum charge for - my area the service charge minimum is 76$ and im paying only on average 100$ a month average right now so its not really worth connecting to the grid as i would need to produce 176/.13 = 1353kwh per month to make that zero. which means the system size would need to be 6 times as large (10000w in panels) to produce that.
So since a grid-tie inverter would cost a fortune to implement because of the minimum service charge, i could go with an off-grid which would be hooked up to the appliances. The inverter would pay for itself in:
1049$/{(7kw/day)*(13c/kwh)*115days/year} = 10 years.
This model for example would work:
5000 Watt Pure Sine 24 Volt Inverter by AIMS Power
Then the problem is that it runs on 24v and the car battery is probably running at much higher than that, so you would need a dc-dc converter as well, not sure where you can get a 5000w dc to dc converter.