A while back I posted on this topic - here are Part I, Part II, and Part III.
With our game rolling back around to the fully-stocked city of Stericksburg, purchases of magic items are becoming a reality again. So we had to make some decisions.
I decided on this - and, for what it's worth, the only players affected by it have absolutely no problem with this. I think they prefer it, actually, because the open hole of "with Power I should do more" meant they felt compelled to use it to its limit. As written below, it's just a non-issue.
Always On Versions
Most Regular spells will have an Always On variation. The energy cost to enchant is 50% of the cost an item that can cast the spell, plus a Power enchantment to reduce the cost to maintain to 0. Spells which have an ending trigger (for example, Invisibility) do not have an Always On variation available.
Missile Shield - Costs 5/2. Item costs 400, Power 2 costs 1000. Total 400 + 1000 = 1400. Always On version costs 1400 / 2 = 700.
Dark Vision - Costs 5/2. Item costs 400, Power 2 costs 1000. Total 400 + 1000 = 1400. Always On version costs 700.
Walk on Air - Costs 3/2. Item costs 500. Power 2 costs 1000. Total 500 + 1000 = 1500. Alway On version costs 750.
Notes: This is just a way to cost Always On. The Power enchantment is not actually put on the item, nor does the item cast the spell. It merely generates a cost that is not unfair compared to other Always On item costs.
More Power Items
More items can hold power for themselves, using the rules from DF8 and DF18 and my own spin on them.
No Power Enchantment
The Power enchantment is not available for purchase. Magic items may exist with it, but you cannot pay to have items enchanted with this spell.
How does this affect the game now?
Not badly. Only one person had an item with the Power enchantment who is currently playing, and he hadn't received it yet. That is Vryce, who wanted an Always On Missile Shield item in the first place. That's the item that sparked the whole round of "How to keep this from being Missile Shield on everyone, always" discussion. Not that Missile Shield is a big issue, exactly, but imagine when it's multiple items - everyone had Shield 2, Armor 2, Blur 2, Missile Shield, Dark Vision, Walk on Air, Resist Fire, Resist Lightning, and Resist Acid, plus maybe Resist Poison and Resist Cold, just for grins. Even just 2-3 of them is pretty game-changing, even just from an annoyance standpoint of spending time putting on the layered buffs, deciding who needs Shield 3 instead of his default Shield 1, etc.
The other character affected is Galen, but Galen's player is not available to play because of real-life issues. Rolling his Iron Arm with Power 1 item back to Iron Arm only and refunding him cash is fine. It's still a useful magic item, for one. And for another, I still didn't like the effect of ruling that Blocking spells don't get a discount for skill but the mechanically similar Power spell worked.
I may still put in items which break the rules, but that's my prerogative as the GM. This way, players don't need to hunt through GURPS Magic for items with a low Maintain just to stack onto their item with the Power enchantment.