Little Test Code

bridge = peripheral.wrap("left")

bridge.clear()

width = 200
x = 0
capacity = 40000000
amount = 0
msg = ""
message = ""
value = 0
mfsu0e = 1
mfsu2e = 1

local storageUnits = {
   {
                 ["id"] = "mfsu_0",
                 ["name"] = "Main Energy"
   }
}
local offset = 0
for key, storageUnit in pairs(storageUnits) do
  pxOffset = offset * 20
  storageUnit["label"] = bridge.addText(4, 4 + pxOffset, storageUnit["name"], 0x000000)
  storageUnit["bar"] = bridge.addBox(4, 14 + pxOffset, 0, 5, 0xCC0000, 0.9)
--  storageUnit["bar"].setZIndex(2)
  storageUnit["bg"] = bridge.addBox(4, 14 + pxOffset, width, 5, 0x000000, 0.5)
  offset = offset + 1
end

while true do
  for i=#storageUnits,1,-1 do
            widht = 200
            storageUnit = storageUnits[i]
                  storageUnit["bar"].setWidth(width / capacity * amount)
                  write("W"..tostring(widht))
                  write("C"..tostring(capacity))
                  write("A"..tostring(amount))
  end
end