A U-value is the rate at which heat is lost through a thermal element like a wall, floor or roof. It is measured in watts, per degree Celsius (or Kelvin), per square metre (W/m²K). So, if a wall has a U-value of 0.3 W/m²K and it is 21°C inside and 15°C outside (a difference of 6°C), it will be losing heat at a rate of 0.3 x 6 = 1.8 watts per square metre.