init.lua

function sleep(timeout)
    checkArg(1, timeout, "number", "nil")
    local deadline = computer.uptime() + (timeout or 0)
    repeat
        computer.pullSignal(deadline - computer.uptime())
    until computer.uptime() >= deadline
end

function clear()
  local gpu = component.proxy(component.list("gpu")())
  local oldColor = gpu.getBackground( false )
  local w,h = gpu.getResolution()
  gpu.setBackground( 0x000000, false )
  gpu.fill( 1, 1, w, h, " " )
  gpu.setBackground( oldColor, false )
end

function progressBar( label, value, maxVal, show)
  local gpu = component.proxy(component.list("gpu")())
  local oldColor = gpu.getBackground( false )
  gpu.setBackground(0x000000, false)
  gpu.fill( 3, 1, 155, 2, " " )
  w = math.floor( value * (155 / maxVal) )
  p = math.floor( (w / 155) * 100 )
  gpu.set( 3, 1, label .. ": " .. tostring( p ) .. "%" )
  gpu.setBackground( 0x222222, false )
  gpu.fill( 3, 2, 155, 1, " " )
  gpu.setBackground( 0xFF0000, false )
  gpu.fill( 3, 2, w, 1, " " )
  gpu.setBackground( oldColor, false )
  if show then
    local valStr = value .. " units"
    local n = string.len( valStr )
    gpu.set( 158 - n, y, valStr )
  end
end

local cells = component.list( "energy_device" )
local gpu = component.proxy(component.list("gpu")())

clear()

local x,y = gpu.getResolution()
gpu.setResolution(x/2,y/2)

while true do
  for address, name in pairs(cells) do
    local cell = component.proxy( address )
    local val = cell.getEnergyStored()
    local max = cell.getMaxEnergyStored()
    progressBar("power", val, max, true)
  end
  sleep(2)
end