Yes, Kavik's and Davenn's posts summed it up well. If you have a source capable of supplying 10kW of POWER, and you want to run two devices from this, one requiring 1kW of power and one requiring 9kw of power, then all you do is hook the two devices in parallel, each device will pull the respective wattage. If the devices have different voltage requirements, then you'll need to bring in transformers to place between the source and the device, but then of course your have loss in the transformers - a whole different discussion. Also, just like a 15A breaker at 120V can supply fractions of a watt, it can also provide all the way up to around 1800W. You can wire ten 100W bulbs and twenty 40W bulbs, all in parallel. 120V will go to each bulb, with approximately 40W to the 40W bulbs, and 100W to the 100W bulbs. if you want to say only two devices, then suppose it's a device needing 1kW and a device requiring 800W, same thing, wire them in parallel.
Bottom line, a source only provides as much wattage as the device needs, up to the current limit of the source (I won't go into what determines the limit). For example, even though a car battery can provide hundreds of amps of current, it would only "provide" 55W for a single halogen headlamp. but if you hooked up 10 of these bulbs, the battery would "provide" 550W of power. You're generally only concerned with the maximum amount of power a source can provide without having too much of a voltage drop below the required voltage (because then you end up having a drop in power as well, for example, a car battery couldn't provide nearly enough POWER in kW to run (10,000) 55W car headlamp bulbs in parallel (this would be over 45,000 amps). If you connected 1,000 55W bulbs, the voltage would drop well below 12V and you would not get the power required through the bulbs to make them come on. You would essentially be placing a "dead short" across the battery.
Power (wattage) that "comes out of" a source is all dependent on the device connected to that source. Another example is with dry hands, touching 120V house mains (i.e. voltage from a single "hot" wire to either ground or neutral) on a 15A (single) breaker vs. a 40A (also single pole) circuit breaker will give you the same exact shock, because the dry resistance is low enough that it doesn't even come close to exceeding the 15A current limiting device, which is really only there to protect wiring - YES, CBs are to protect the WIRING in your house, (to limit the wiring from becoming heaters!) and are not intended to protect anything you're plugging into it, and definitely not to protect a person. However, having a 240V shock WILL be worse because it would be roughly double the amount of current through your body because the resistance of your skin is the same whether it is 120V vs. 240V (the same for this example anyhow, different things come into play in theoretical problems and much higher voltages). Since it's your body's resistance that determines how much current goes through you - until your body gets to be a low enough resistance that you're approaching 15A through your body, then the CB would trip (theoretical differences could actually come into play, but I'm ignoring those as they are not relevant in my example).
I have rambled long enough, but if it causes any light bulbs to come on in your head (without being shocked of course :) ) or if it prompts you to ask another question, then my rambling was worth it :) It is a wonderful thing (I think) when answers to science questions raise more questions from the person asking the question!
P.S. BUT, do be very careful if you're doing anything with 500V... Doing anything that could remotely result in you being shocked should be treated like a loaded gun without the safety on, and knowing very little about guns either. 500V is easily enough to cause a severe shock even even with dry hands, enough to kill you, and if you had sweaty hands and grabbed bare large conductors, you can almost certainly say goodnight... I have been shocked with 600V with dry skin and my vision went out for a split second, I am not exaggerating and I was very lucky! Your body's resistance depends on things such as perspiration, gender, age, body size, AND how hard you grip and how large of a surface area. Once you get to higher voltages, burns come into play, and once your skin is burned, you've lost the skin resistance which is considerably higher than "internal" resistance (skin resistance can be thousands of times higher than internal, or "blood to blood" sort of thing). If your 500V source is current limited and can only provide less than a few milliamps, then you might be okay, but I still wouldn't try it on purpose! Where I work (aerospace), we have devices that run on something like 13,000V at >3A, and would easily kill you.
