Luther's Evolved ability is to convert energy into different forms. Energy of one form is gathered from the environment, converted into a different type of energy and then discharged in its changed form. Though this gives him the appearance of being able to generate energy from thin air, in truth he only draws upon the energy of the environment around him.
Though he doesn't have to be conscious of his ability to gather the energy, he does have to focus upon the ability to perform actual conversion. Some feats he has been able to do at an almost reflexive level now; for example, his daily routines have included going out to take in solar energy before sunset, and then having that energy slowly dissipate via light from his hands so he can read at night for a couple of hours. Because the absorption of energy is subconscious, though, he must take precautions to release the energy before sleeping, or else his body will automatically release the energy in the forms of heat and light - rather inconvenient for someone hiding his power to be glowing in his sleep.
Usually, he discharges the energy immediately after conversion, but has been able to temporarily store raw energy (as natural biochemical energy) within himself for up to two hours. Past that point, the energy discharges on its own through his body, sometimes in one uncontrolled gush and sometimes in a slow release - the type depends on the stress he is experiencing at the time. If he is placed under stress, the energy is far more likely to release as a one-time high-powered discharge; if he is asleep or resting, he undergoes a slower, steadier release of the energies. In all cases of involuntary discharge of energy, the exuded energy is light and heat and its emission point is not focused. He winds up glowing all over and possibly, if the energy released is hot enough, burning things.
He can convert from within a limited distance up to 30 feet (around 10 meters) radius around him. Luther can focus the emission point of the energy to be from any point within that limited distance, so if he wanted to heat up a car's gas tank to cause the gas within to explode, as long as it's within 30 feet he could. Not that he would — he had better have protection from the flying shrapnel and such.
The discharged "output" energy is always less than the "input" at around a ten percent loss at its most efficient level. What this generally means is that his power can only work with what is available, and never generates more power than what is taken in.
Radiation, heat and solar energies are what he has been able to interact with and interchange so far by way of external energy sources; his own body's energy (biochemical energy) could also be accessed in a pinch, but that drains him depending on how much internal energy he has (if he hasn't had anything to eat, he isn't going to have much natural energy).He is afforded temporary immunity to the damaging effects of the energies that he converts, so while he's receiving or giving off high amounts of heat or radiation or light, he doesn't get burned - but everyone and everything else around him might.
Luther's ability causes him to subconsciously absorb energy. In the morning, when the sun rises, his body takes in the light, heat and radiative sun rays — like a plant. This is often enough to wake him up from sleep, giving him that "morning person" spark of energy. Throughout the day, he can continue to take in the energy, convert and release it in a slow fashion, such as in the form of body heat - but too much makes him seem feverish. He has also gone into irradiated areas to gather radiation, then changed it to heat a piece of wood to reach flash point and light on fire; particularly handy on cold nights. Using solar power converted to heat, he has boiled water in a coffee can without having to use fire. He even microwaves his own frozen dinners without an actual microwave — if he could afford buying one, that is.
To speak of the upper echelons of Luther's ability, he has managed to survive some pretty incredible situations. He was near Ground Zero when the Bomb went off. Upon emerging from a protective basement, he encountered severely high levels of radiation that should have killed a normal person, but instead this overloaded his ability and caused it to go haywire. His body gave off such a huge amount of heat and light energy in attempt to compensate, which almost instantly combusted his clothing and nearby flammables. And he must've run a marathon in the process without tiring until the very end, at which point he passed out for several hours. He won't explode like Ted or Peter, but could release enough energy to set buildings in proximity on fire if there was an even stronger environmental source; don't shoot him with a ion beam or stick him in the electric chair. Getting unexpectedly hit by large amounts of any energy, like being struck by lightning, generally causes this overload.
Over recent years, Luther has managed to train himself so that he does not need to focus overly long to change smaller amounts of energy externally, like the heat of a candle into a small "keychain light" intensity glow while he reads at night. He has learned that while sleeping, his body absorbs very small amounts of energy, and if he wants to take in more he needs to focus upon the act.
When the energies get to a more harmful-to-humans level, he needs to concentrate in order to keep his absorption and ability in check. Not doing so risks overloading himself very quickly, usually within a half hour's use at this level of power at high level. Overload causes side effects of having increased energy levels to the point similar to overdosing on stimulant drugs (speed, E, drinking nine-shot espressos…), followed by the crash after an energy discharge; hence, he does not keep energy within himself for too long.
Overload can also happen when he has reached his limit of conversion, which is usually the levels at which a peak-condition human can take before it becomes fatal. While hitting these energy levels is not fatal for Luther, it is not a pleasant position to be in and he could experience any number of side effects and vulnerabilities such as mental and physical drain, weakness, seizures and falling into a comatose state of unconsciousness until medical help could be obtained.
Any excess energy that Luther does not convert remains in its original state, though its intensity may be lowered after he's drawn part of the energy away. In example, a fire won't burn as hotly while Luther takes the heat energy from it, but once he's stopped, the fire can return to its original temperature provided it has enough fuel to burn as hotly as before.
As a final note: Luther might be temporarily immune to the damaging effects of energies while his ability works its magic, but he is still a "normal" human being. He still has to eat, breathe, and drink at some point to stay alive. He's not impervious to falling down, bullets, knives, punches and kicks, slaps to the face, or pies to said face. As Michael Westen says, "A broken rib is still a broken rib."
