startup.lua

reactor = peripheral.wrap("back")

maxEnergy = reactor.getEnergyCapacity()

while true do

    EnergyStored = reactor.getEnergyStored()
    print("Energy (RF) = " ..(EnergyStored/maxEnergy)*100)
    if ( EnergyStored <= maxEnergy * 0.03 ) then
        reactor.setActive(true)
    elseif (EnergyStored >= maxEnergy * 0.9) then
        reactor.setActive(false)
    end
    sleep(1)
end