How many solar panels do I need?

This question is best answered by asking, how much do you use?

By building a load profile that outlines all the power you intend to consume during a day and estimating the duration of each device and its power draw you can build a power profile. A power bill is similar but not to the same level of detail as it only shows a total for the month which you divide by the number of days in that month. That gives you a kWh value. That number is sometimes referred to as the number of units. It's what you are charged for on your power bill. i.e. 500 units for the month at 20 cents per unit (1 kWh) would be $100. 500 units divided by 31 days equals 16.12 units per day. So that's 16kWh, and in New Zealand, the peak sun hours over winter are only 2 hours. So you'd need an 8kW solar array which will produce on a clear day 8kW per hour and with the 2 hours of usable sun energy in a day you'd generate 16kWh of power.

This would generate enough power to offset your consumption, but it's not all at the right time. You use power at night from the grid so you'll be charged for importing power to your house. But during the day while you aren't at home or your power usage is low you'll be exporting to the grid which may result in you receiving a small credit to your account.

You can see how the real way to benefit from solar is to utilise the sun's energy during the day and limit your need for it at night. This is where batteries can help as they store the day's energy for you to use at night.