Typical PV installations won't power your house in a blackout (one during the day, that is), since they feed directly into the grid, and then your house receives power from the grid. Hence the market for something like the Tesla Powerwall, which reroutes all that enough that you can power your home from its battery, and charge it from the PV if the grid goes down (for a while - each Powerwall's battery is 13kWh, about 1/6 of a Model 3...) . But to your general question, at this point the panels themselves are a fairly small part of the overall cost, so I don't think most people would want to go small, if you're installing any kind of proper inverter system, make it big enough to cover your usage.