Determining what size solar array you’ll need will depend on a few factors, the main one being how much power you want to generate. Other factors include how efficient and effective individual panels are, how much sunlight's available in your location, how much space you have to spare, and of course what you can afford.
Kilowatt hours per day
The first step is to assess how many kilowatt (kWh) hours per day you want to generate. Your average daily usage is usually information that is found on your electricity bill. Take an daily average reading for a whole year’s worth of bills if you have them, as electricity usage is likely to spike in the coldest and hottest parts of the year due to additional heating and cooling being used. This will show you how much energy your home is using per day.
Sun hours
Next, you need to know how much sun exposure your panels are going to get. Not all daylight hours are equal in the amount of energy they will produce; the low-lying sun of the morning and evening won’t generate nearly as much as the face-on sun of noon. What you need is the amount of hours of sunlight that will produce the panel’s rated voltage, known as 'sun hours' or 'insolation'. This totals all the voltage produced by each hour of the day and comes up with how many hours of full power that equals. For example, a 10 daylight hour day in winter may only produce enough energy for 3-4 sun hours because not all the light can counted as full strength.
Panel efficiency
When you’ve determined your kWh target and know how much available energy there is in your area from the sun hour value, then you need to consider how efficient each panel is going to need to be. Higher wattage panels are more costly, but more efficient and can usually make better use of available space. A 300W panel is obviously going to be three times as powerful as a 100W panel over the same amount of space, for example, so to reach a target of 1.2kWh, you could use four 300W panels or twelve 100W panels. Fitting four panels onto a roof is typically a lot less of an ask than fitting twelve, however those four panels may cost more than twelve of the others, so you need to find a balance point you are comfortable with.
As with any significant investment, it's well worth shopping around!
