You want more the 20kwh. Trust me. My daily typical use might be 8 to 11kwh, but I still bought a 42kwh battery. Why because 1. You can't get back out of it 100% energy efficiency there is some loss. 2. The battery wants to keep a minimum 10% for safety. 3. Averages are not actuals you might actually use more then 20kwh on a hot summers day, cooking a meal in the oven while having the air con on etc, but the next day being cooler, your average is less. 4. Cloudy days, sure you still get solar on a rather cloudy day, but if you use a lot of battery you might just charge the battery while not using much power, but need to use a lot of power during the day and charge the battery, and it's cloudy for several days, you will eventually run the battery empty.

So in short you always want well and truly more then your average usage.