The basic calculation is to find out how many kilowatt hours of electricity you use on a monthly basis. WKh will be shown on your monthly bill. Add the number of KWh for the whole year and divide by 12. This is the number kilowatts you need your system to provide. Yes, there are a ton of other factors such as the number of sun hours your home receives, but this is the basic calculation. It is best to always round up a little to be safe, of course. If you are off grid this is particularly important as you will not have grid power to supplement you in higher usage months. In you are grid assisted you can add a number of additional panels if possible and actually sell your additional power to the electric company. They don't pay a ton, but it doesn't cost you anything to generate this monthly check other than the original additional cost of adding extra panels/equipment.

This chart is great but remember, you won't be charging anything with solar panels at this time of year unless you live in sunny Florida or Arizona. Right now the sun is at its farthest distance from earth and on a cloudy day you won't get any solar charging at all so a generator will be needed to supply enough power to charge up your batteries. As the days start to get longer and the sun gets closer to earth the solar panels will start producing and you will get a couple amps even on a cloudy day. We have had no sun here in SW Oregon for 2 weeks sraight now and I can hear generators at night off in the distance doing their thing for the families who live off grid. Also, solar panels get buried in snow and if mounted on a rooftop it is dangerous to get up there and clean them off so its always better to mount them on the ground if you have the room, for mainteance and snow removal.
Solar panels have dropped in price due to a slowdown in sales and if you are looking to purchase some there may be sales with greatly reduced prices after the first of the year. It pays to shop around for the best prices.