instrument.rb

# models/instrument.rb
class Instrument
  attr_accessor :units

  def initialize(status, units)
    @status = status
    @units = units
  end

  def turn_on
    @status = STATUS_ON
  end

  def turn_off
    @status = STATUS_OFF
  end

  def status
    @status
  end

  def power_consumption
    (@status == STATUS_ON)? @units : 0
  end
end