Energy upkeep reserve

Commonly this is a semi-permanent thing that causes the upkeep, such as an aura, a summon or a ward/glyph.

The immediate effect is that your maximum energy is limited, which by itself does very little. But it may with enough things that take upkeep prevent you casting more expensive spells or simply discourage prolonged conflicts where larger energy pool would come in handy (at the mercy of your potentially pitiful energy regeneration).

This is NOT for upkeep that is deduced by negative regeneration, but rather one that reduces the maximum available energy pool (not not necessarily reduce the maximum itself, though practically it does so).