The default alt power bar has a tooltip with information on what it's currently showing.

function UnitPowerBarAlt_OnEnter(self)
local statusFrame = self.statusFrame;
if ( statusFrame.enabled ) then
statusFrame:Show();
UnitPowerBarAltStatus_UpdateText(statusFrame);
end
GameTooltip_SetDefaultAnchor(GameTooltip, self);
GameTooltip:SetText(self.powerName, 1, 1, 1);
GameTooltip:AddLine(self.powerTooltip, nil, nil, nil, 1);
GameTooltip:Show();
end