No, you just shouldn't assume that that's the case. In the case of a computer and mains, both of those are nominal voltages and there will be a range of voltages which are expected to function, and if you want to check for sure, you should check those ranges.
The point is, voltages are usually a 1:1 match. That includes when you're working with ranges, you want the supply range to be inside the load range.
And even for amps where you see that 80% rule, that's for keeping the load smaller than the supply. Solar panels aren't a load and don't work that way.