I'm only half kidding. I'm a bit of a prepper and I have lots of powerbanks and devices that charge from USB but besides idling my truck I really had no other way to charge any of them in case of a long-term power outage which seemed a bit of an oversight on my part.
Not like this solves the issue. 30 watts (under ideal conditions) isn't much but it's a start.
It's pretty easy to cry about bad math, but it's a lot harder to figure out the right math.
Don't worry, I'll try to do it for you again a second time.
Power consumption varies. Use the average monthly power draw from the solar array, let's assume for demonstration purposes 1,000 kWh/month.
Multiply that by the cost of 1 kWh from the power company, let's say 20 cents.
In one month, that means you saved $200.
Let's assume the solar equipment costs $1,000.
The answer is 5 months, or 5,000 kWh.
Sorry, I'll make sure the free work I do for you is better quality next time.