Calculating the power to the heat exchanger or boiler, isn't very diffecult.
You only must have 2 tempsensors, and a flowmeter.
1 litre water heated up 1 degree, has a caloric value of 4,19 kiloJoule.
4,19 kJ = 0,001164 kiloWatthour
So 1 litre water heated up 1 degree = 0,001164 kWh
So you have your boilerwoodstove, 2 tempsensors. 1 is the cold water going into your woodstove heat exchanger (before the mixing valve). The other is the warm water out of your woodstove.
(see picture attached). The watermeter (flowsensor) must be installed in the circuit of the tempsensors.
So now you're gonna check the power to the heat exchanger (boiler)
Took a stopwatch. Write down the temp in and temp out. Look how many litres
passed in a minute. Write down the temp in and out again.
So for example: Start measuring temp in = 20,0 temp out = 60,0 litres passed in 1 minute = 2,2 End measuring temp in = 20,6 temp out 60,4
Average temp out (60,4 + 60,0 / 2 = 60,2) - average
temp in (20,6 + 20,0 / 2 = 20,3) = 39,9
tempdifference x water passed in 1 minute x 0,001164 = 39,9 x 2,2 x 0,001164 = 0,1022 kWh
So in 1 minute, the energy given to the water is 0,1022 kWh.
But what is the power?
kW = kWh / h
So kW = kWh / (1/60), because we only measured 1 minute!
So kW = 0,1022 / (1/60) = 6,13 kW