The shield is effectively an extension of hp and has to be brought to 0 for the character to suffer any actual hp damage. Like actual hp, it benefits from your armor. An improved version is Energy shield MK2.
It takes 4.167 seconds to recharge from 0 to 50 hp. Multiple Shields recharge simultaneously.
The maximum shield recharge rate is ([number of energy shields] * 50 / 4.167) units per second, if enough power is provided by batteries, solar panels, or a fusion reactor. The maximum rate is only reached when all shields are recharging at once, i.e., all shields are depleted except the last one (or all shields are depleted including the last one).