I agree that looking at a year's worth of production is the way to go. In the meantime, something relatively easy to do is use PVWatts. Even if it doesn't give you the same number as Tesla, it can give you a good sense of how you are tracking. For example, if Tesla provided an annual estimate of 5,000 kWh and PVWatts said 6,000 kWh, you can take each monthly total from PVWatts and multiply by 5,000/6,000 to get the rough Tesla equivalent for the month. This will give you an earlier indication of if you are tracking towards the annual number.
PVWatts also nicely illustrates how dramatic the difference can be at different times of year - my south-facing shingles are estimated to produce about 66% of their June number in December, while the north-facing ones only produce 11%. So, I can imagine with NW-facing panels that 20%-25% is not unreasonable, and I would assume it was factored in to the annual calculations.
With all that said, it might be an interesting option to offer billing based on month/season - rather than $50/month, vary from $25-$75/month so you do not have months where you are paying significantly more than you produce.
