local cube = peripheral.wrap("ultimate_energy_cube_0")

function getEnergy()
    local amount = cube.getEnergyStored()         -- save the energy amount in variable
    local capacity = cube.getMaxEnergyStored()    -- save the total amount in your cube in a variable
    return math.floor((amount/capacity)*100)      -- returns how much that is in percent, and floors the value removing decimals
end

print (cube.getEnergyStored())

while true do
    local energy = getEnergy()                    -- save the percent in a variable so you only calculate that once every time
    if energy < 10 then                           -- if its less than 10% in your cube start reactor
        print("1")
    elseif energy > 95 then                       -- else if its greater than 95% stop your reactor
        print("2)
    end
    sleep(3)                                      -- then sleep for 3 seconds and do everything again.
end