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.
Interestingly, the peak voltage of 240V AC (typical) is 330ish+ volts.
Typical house wiring is required to handle 600v due to voltage transients and wear and tear.
So any component rated to 240V dc will fail immediately on AC, and even 400-500V DC is not a good idea.
I guess there is a reason there is a whole category of engineers for this kind of thing.
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.