The next chapter: solar energy

On may 13th we had our solar PV panels installed; 12 monocrystalline black panels with a total system size of 3720 watt-peak. The inverter is a StecaGrid 3600 Coolcept, a german brand, not commonly seen.

The array of panels nicely fits below the flat-plate thermal collector that came with the house and had already been located all the way at the top deliberately.

Our roof's azimuth is 146° (south-east) and its tilt 45°. There's no shadow, except for a short moment at the end of summer afternoons when the sun is just about to move to the front of the house. Then the chimney casts a shadow across the entire surface; we'll actually see this effect in our graphs later.


In the weeks before installation I had already been monitoring the values of the electricity meter, visualizing this in separate "power consumption" and "power generation" gauges, the latter - of course - had always shown 0 watt. As mentioned in an earlier post, this interpretation had worked fine until the moment the inverter was activated on may 13th.

Let's begin with the basic situation:

We use electricity in our home appliances, and their requested power is pulled from the grid. What we measure at the smart meter is the amount of energy flowing into the house and in this scenario this happens to be exactly equal to our energy consumption. So far so good.

Now when the inverter is activated and it starts pumping amps into our house, we suddenly get the following situation:

I remember seeing my virgin "power generation" gauge go up to 2500 watt, while the "power consumption" value dropped down to 0... Hmmm... In this scenario a portion of the generated power is directly consumed while the rest is pushed out into the power grid.

Now imagine - and this could be just several seconds later - that we turn on the microwave while a cloud blocks the sun:

Our generated power is no longer enough to fulfil the request and the remaining power is drawn from the grid.

It becomes clear that with our P1 telegram measurements we only measure the net energy that passes through the meter, and as we see, this could be in either direction throughout the day. A sad result is that we have lost all insight in our actual consumption (or generation, for that matter).

Fortunately our electricity meter does have separate counters for the two directions in which energy can flow through it, so we can at least distinguish those; let's call these export and import from now on.

Note that at any moment in time you can either import or export energy, but never both simultaneously. Now you may think that our P1 monitoring will always have one value at 0, but remember that we're actually measuring averages over 5 minute periods. And in one such period both import and export could have occurred.

We can toss our variables in a formula to see how they relate to each other:

      Import + Generation = Export + Consumption

This formula makes sense if we assume that we don't store any energy in batteries for later use: all the energy that comes into the house (either generated or imported) must have gone somewhere (either consumed or exported).

Since we're already measuring import and export periodically we only need the value of one additional variable for each period, and we would have a complete picture again. An obvious candidate is generation; both the inverter itself and a separate kWh meter are already keeping track of this.

We'll have to figure out a way to obtain this additional measurement automatically and get it at the right place in order to apply the formula. Well, this turned out to be a proper challenge, but fun adventure...