If you're measuring the temperature in the room currently, you could try trending it yourself. Start the heater, and see how quickly the temperature rises (e.g., degrees per hour). Call this Rate 1.

Then turn off the heat and see how quickly the temperature drops. Call this Rate 2. For the formula below, make it a positive number.

Assuming the weather conditions are similar and the room temperature doesn't change too much during data collection:

Rate of heat loss = Heater power * Rate 2 / (Rate 1 + Rate 2)

This number could be impacted by the weather: temperature, wind and insolation (affected by time of day, time of year, latitude, and cloud cover). It's also impacted by room conditions (temperature, slade position, how many times the door is opened), so you'd need to do a few trials to get a sense for thr impact of different variables.

You've probably already thought of this, but your strategy is going to result in noticeable swings in temperature in the room, because ypure going to do a lot of heating at once when prices are cheap, then turn off the heating and let the room cool. Compare that to a thermostat that tries to maintain a constant temperature.

Sounds like a fun project - good luck! I'd love to hear updates here as you go.