U-Value is a measure of heat transfer coefficient, describing the
rate of heat transfer through a building element over a given area under standardised
conditions. The lower the U-Value, the better the thermal performance of the building
envelope. U-Value is measured in Watts per square metre Kelvin (W/m²K).

We can simplify it to:

Heat Transfer Rate per Degree Celsius (W/°C) = Q / ΔT

To calculate you can put X Watt heater in the room, run it and measure how temperature
changes and when it stabilized. Then you can calculate U-Value as:

For example if outdoors 17C and you reached 23C indoors with 1000W heater:

U-Value = 1000W / (23C - 17C) = 1000W / 6C = 166.67 W/°C