You are at the mercy of the manufacturer, only they can tell you exactly.
For a transformer, the output voltage depends to some extent on how much current you draw from it. Lower current means the voltage will be higher and vice versa. So the more you draw from it the more it struggles to keep the voltage up. Power transformers normally have a 'regulation' value, typically a few percent, which tells you how much the voltage is influenced by the current, a lower percentage means less drop per Amp. The voltage specified for a transformer is therefore at a given current, yours may be 9V at 500mA with 5% regulation for example. Obviously, there is some current limit that the transformer simply can't go beyond, usually they overheat and burn out but the manufacturer will normally leave a wide safety margin on their specification.
Solar panels are similar but are a bit less predictable. Like a transformer, their voltage drops under load but other factors such as insolation and temperature play a large part in their capabilities. Their unloaded voltage is typically twice that at their optimum load current.
Brian.