You don't have Solar Panels and want to verify if Smappee's measured value is correct. Follow this procedure..
If you have Solar Panels, follow the same procedure, but at night or when your panels do not produce any energy.
We will calculate your instant power based on indications found on your Energy Meter. How? This meter does not show power values in watt? Correct! It shows only kWh consumption values.
Let's compare this with your car: you want the speed in km/h but suppose your speedo meter is broken and it shows you only the distance in km. How would you calculate your speed (equivalent of the power). if your car would only indicate total distance travelled (equivalent of the total energy consumed)?
Look at your energy meter and search for something like: NUMBER r/kWh (tr/kWh, omw/kWh, ..). This meter shows a value of 600 r/kWh, which means that the disc will rotate 600 times for each kWh! Compared to a car: 600 rotations of your wheel for a distance of 1 km. Note that the the disk has a a mark to help you count the rotations: generally black, sometimes red and mostly one mark per rotation.
If we count the number of seconds required for 10 rotations then we could extrapolate this to an hour which counts 3600 seconds. Let's take an example for this meter:
600 rotations per kWh, suppose you measured 200 seconds for the 10 rotations. Then in one hour (=3600 seconds) we would have seen 18 x more ( 3600/200) rotations (if you drive at the same constant speed - if you consume at the same constant power). This results in 18 x 10 = 180 rotations of the disc (of the wheel) per hour. 180 rotations corresponds to a consumption (distance) of 180/600 kWh or 0,3 kWh. To consume 0,3 kWh (drive a distance) in one hour require an average power (speed) of then 0,3 kWh / h = 0,3 kW or 300 W.
Smappee should show you the same 300 W value. Of course Smappee's values are updated every 5 seconds and the one you observed in this example is the average over 200 seconds (10 rotations). Thus, while you verify Smappee's measurement, try to keep your home's consumption stable. This means that Smappee's value should be stable as well and not change too much around the 300 W average value. If your Smappee indicates 280 W in the above example, consider it as correct, but a value of 200 or 400 would be wrong.
Rotations / kWh : r
Time in seconds, measured for 10 rotations of the disc: t
Power in kW = 36000 / ( r . t )
in previous example: r= 600, t= 200s then Power in kW = 36000 / ( 600 . 200 ) = 36000 / 120000 = 0,3 kW = 300 W
Apply the formula now, with your meter's r and the measured t value for 10 rotations of the disc. Hold the power steady and compare with Smappee!
How to verify the Green Bubble?
You should also check Smappee's Solar Power values by verifying this directly with the display on your inverter. If these values are close, then you know that Smappee measures the solar correctly.