Ahh, Thundarr the Barbarian is the show you were thinking of; Thundarr wields the sun sword. ..........I watched too many cartoons as a child. :-/
One thing to think about, is that the Brilliant Energy quality of some magic weapons (in the DMG) is given a market price equivalent of +4 or so, and an energy blade that ignores armor or natural armor AC bonuses should not be available any cheaper than getting the Brilliant Energy quality. Now, the BE quality may be more worthy of a +3 equivalency or, I daresay, perhaps a mere +2 equivalency, but you can houserule that if you want, and there should be no alternative any cheaper than that magic weapon quality. So if you make a magic weapon type that uses a blade of energy, make sure that it counts as a physical weapon for purposes of armor, natural armor, and the like, even if it retains other energy capabilities. It may still be able to set things on fire, cut through trolls' regeneration, bypass DR X/+X and DR X/silver or DR X/magic, etc., as normal for an energy effect.
The stuff I'm going to suggest is based on the utility of having an always-available energy weapon as opposed to a physical blade, even if it is impeded normally by armor and natural armor, et al. A +1 steel short sword is going to generally be less effective/useful than a short sword with a blade of fire. Getting energy weapons that deal greater damage than a short sword should cost a lot more because it costs quite a bit to get a magic enhancement bonus to attack and damage.
Anyway, I suppose you could make a Wondrous Item called, for instance, an Energy Hilt. Give it a market value of 3,000 GP, and caster level of 3rd. Energy Hilts have an aura of evocation magic when viewed with Detect Magic. The Energy Hilt appears to be a finely-wrought sword hilt, with a jeweled pommel and a similar gemstone embedded in the center of the handguard, but no actual blade attached. When the command word is spoken, the gemstone in the handguard glows and projects a sword-shaped beam of energy 2 feet long, which is semisolid but spongy and penetrable (like a Flaming Sphere spell effect). The type of energy can be either acid, cold, electricity, fire, or sonic, which can be identified by what sort of gemstone is built into the Energy Hilt; a green emerald projects acidic energy, a blue sapphire projects cold, an amber projects electricity, a red ruby projects fire, and a clear topaz projects shimmering sonic energy. Replacing the original gemstone with a different type, or with no gemstone, prevents the Energy Hilt from functioning. Speaking the command word again dissipates the energy beam.
However, as long as the energy beam is active, the Energy Hilt may be wielded exactly as though it were a short sword, in every way except as follows. Its normal damage is the appropriate energy type instead of being physical damage, so it is neither bludgeoning, nor slashing, nor piercing. As immaterial energy, the Energy Hilt's blade ignores any Strength modifiers to damage, Power Attack modifiers to damage, and any modifiers that are dependent on (or specifically function like) a Strength or Power Attack damage modifier. The Energy Hilt, as with many other magic items, resizes to fit the user, so a wielder who is Medium-sized uses the Energy Hilt as a Medium short sword, whereas a Gargantuan wielder uses the Energy Hilt as a Gargantuan short sword. While the blade itself is weightless, the Energy Hilt itself weighs as much as a full, normal short sword of the appropriate size.
Though the Energy Hilt projects a blade of energy to strike foes and objects with, the blade of an Energy Hilt is still partially physical, semisolid energy, so it does not normally bypass any particular bonuses to the Armor Class of a victim, unlike a Brilliant Energy magic weapon. However, as normal for certain energy types, a blade of fire or electricity can set objects aflame, a blade of acid or electricity or fire deals half damage to objects, and a blade of cold energy deals one-quarter damage to objects, for instance. The blade of an Energy Hilt of fire or electricity generates light equivalent to a torch, while a sonic blade from an Energy Hilt generates a quiet humming noise equivalent in intensity to a human talking normally. Damage from the Energy Hilt's blade is treated as energy for other intents and purposes, such that an Energy Hilt with a blade of fire is ineffective against a creature with Fire Immunity, and Sonic Resistance applies against any sonic blade from an Energy Hilt, while Damage Reduction 5/silver does not hinder the blade of an Energy Hilt.
An Energy Hilt can receive enhancements, special abilities, and spell effects as though it were originally a masterwork short sword (it is not otherwise masterwork, however). The normal rules for such matters apply, and since the Energy Hilt does not deal the same types of damage as an actual short sword, it cannot benefit from the Keen or Vorpal qualities, for instance. Energy Hilts can receive and benefit from the Briliant Energy quality as normal for a magic weapon (they simply do not gain its benefits for free, despite their blades of energy). Likewise, since the Energy Hilt has a blade of only semisolid energy, it does not significantly benefit from the hilt being made with special materials like adamantine, alchemical silver, cold iron, darkwood, mythril, or the like; however, the hilt itself still benefits from such materials if made of them, though it provides no benefit for use in combat (aside from the hilt being harder to break if made of adamantine, for instance). If the Energy Hilt itself is made of mythril or darkwood, it weighs appropriately less. The Energy Hilt's default size is that of a short sword's hilt for a Medium-sized wielder, and it reverts to this size if unsheathed and unattended for 1 minute; its default weight is equal to that of a Medium shortsword as well, and these defaults are used for purposes of determining how expensive it is to make the hilt out of a special material.
Energy Hilts can also be built or found as Greater or True versions, which are larger and more powerful, though more rare. A Greater Energy Hilt is effectively like a longsword instead of a short sword, projecting an energy beam of 3 feet in length, while a True Energy Hilt is effectively like a greatsword instead of a short sword, projecting an energy beam 4 feet long. Greater Energy Hilts have a market price of 10,000 GP and a caster level of 6th, while True Energy Hilts have a market price of 22,000 GP and a caster level of 9th.
