Why isn't the post-solar bill $0 if the system offsets 100% of usage?

For many net metering tariffs, even if you have a 100% offset system, you can still end up with utility bills for a variety of reasons:

1. Monthly minimums: Many utilities have monthly minimums for residential and commercial customers. So even if you use 0 kWh, there will still be a monthly minimum charge.
2. Time-of-use rates: For tariffs with TOU rates, depending on the customer load profile, even if the customer has zero *net* usage, if the average export rate for solar during the day (off-peak) is lower than the average import rate for solar in the evening (on-peak), the costs can exceed the credits and you can be left with a bill.
3. Non-Retail Net Metering Rates: Some utilities pay a lower credit rate for kilowatt hours exported to the grid, compared to rate charged when consuming those kilowatt hours during night time hours.
4. Demand charges: for commercial rates with demand charges, the PV system may reduce peak demand but will not get it to zero, so you will be left with demand charges, which can be non-trivial.