4kw solar power system produce how much kw output per day?

I have a 3.74 kw/dc system in the summer I was able to generate around 40 kwh per day and in the winter about 10 kwh per day. The location, angle and number of hours of daylight make a big difference. My system should pay for itself within 6 years, maybe sooner if you take into account the 20% rate increase that was just approved for my city.