Watts measure power, and Joules measure energy. Watts are a rate, while Joules are a total amount.
Consider a pot of water on the stove. Joules would measure how much energy it would take to make the water boil. Watts measure how quickly you're putting energy into the water.
Power = (Energy) / (elapsed time). If your house is losing 21000kW through the walls, you're losing 21,000kJ per second. How long would it take to lose 429000kJ? Elapsed time = energy / power = 429000kJ/21000kW = 20s. I'd say one of your figures is not correct.
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