Last September, the DGO installed a digital meter at home.  No choice, you (and I, we all) must accept it.  We can argue about the relevance of doing it for the grid user not interested in delivering ancillary services nor energy sharing, but the cause is already long lost by the campaigns depicting the PV owners as profiteers on the back of the others.  Since then a real debate is no longer possible and the owners, connected through an analogue meter, were already taxed whether they injected their production or not.  So, don’t waste any more time on that.

A digital meter brings 3 main consequences in comparison with the analogue meter:

• the offtake power is measured on 15’ basis and its monthly maximum affects the grid part of your bill
• your injection does not compensate your offtake anymore, at any moment
• the injection will be remunerated at a much lower rate than the one used to charge your offtake, ask your supplier to get both figures

Let us examine each point and then draw conclusions and define possible actions.

Offtake power measurement is achieved through the metering of the energy offtaken during each quarter of hour (in kWh), then multiplied by 4 (there are 4 quarters of hour in each hour, so 1 kWh offtaken during 1/4h = 4 kW (power) × ¼h). Each month the maximum value, reached during the month, is registered and you pay the average of the 12 latest monthly values, each with a minimum of 2,5 kW.

So, there is no advantage at all of diving below 2,5 kW because you will always get this as minimum, whether are you present or not, did you offtake or not during the month.  Each kW is charged at 56 €/kW (VAT exclusive), thus nearly 68€/kW yearly. The yearly minimum (related to 2,5 kW) is thus around 170 €.  People who don’t pay attention to their power usage, will easily reach a range between 5 and 7,5 kW, sometimes even higher. Thus, an extra of 170€/year up to an extra of 340€/year, for a total respectively between 340 and 510€/year.  And this only for the power term of your grid costs.

Next, your injection does not compensate your offtake anymore, at any moment, also within the same quarter of hour.  Thus, at any very moment you are offtaking energy, the offtake meter increments, similarly for the injection.  In the same quarter of hour, you can have an offtake and an injection.  That means that if during every moment in a quarter your offtake exceeds your injection, this excess is taken into account at energy level. When in a quarter of hour both offtake and injection are present, the offtaken power will normally not peak (offtake during less than 15’, but average over 15’).  This thus mainly affects the energy part of your bill, with an offtake charged commonly charged at 0,18 – 0,25 €/kWh (VAT inclusive, but excluding all taxes, certificates and all grid costs), while your injection is commonly bought by your supplier at 0,02 €/kWh.

This brings us to the last point: the difference between the offtake charged rate and the injection remunerated rate.  Extending the explanation of the non-compensation during a quarter of hour, to a whole year, the metered offtake over the year will be charged at much higher rate than the metered injection will be remunerated.  The reason is that solar production occurs mainly in the summer, between sunrise and sunset, when the electricity prices can become negative, sometimes very negative.  There are quarter of hours where the production excess cost money instead of bringing revenue.  Globally on a yearly average, you can still get something around 0,02 €/kWh for your injection.  But don’t sign a dynamic contract, which applies a price per 15’, because you will get negative prices for your excess, i.o.w. you will be charged for your injection instead of being remunerated.  A variable price contract is not a problem, if it is driven by a monthly averaged price.  A fixed price will also be O.K. if the higher offtake price and possibly lower injection price do not turn out worse.

So, with compensation, the offtake and injection energy prices were equal and, without it, prices differ. With an offtake energy (charged) price of 0,25€/kWh and an injection energy (remunerated) price of 0,0,2, the difference is:

– [Offtake (kWh) × 0,25 – Injection (kWh) × 0,25]             in compensated mode
Offtake (kWh) × 0,25 – Injection (kWh) × 0,02              in uncompensated mode
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0                 + Injection (kWh) × (0,25 – 0,02) = Injection (kWh) × 0,23

An additional cost of 0,23€/kWh injected, for the part that does not exceed your annual offtake, and a remuneration of 0,02€/kWh for the remaining.

If you have 5 kVA of solar panels, good for an annual production of around 5.000 kWh and an annual consumption of 3.500 kWh, these 3.500kWh were compensated but not any more, so the extra energy cost is of 3.500 × 0,23 = 805€/year, minus a remuneration of (5.000 – 3.500) × 0,02 = 30€/year, thus a net extra cost of 775€/year.

On top you must add the taxes (defined per DGO territory), certificates costs (regionally defined) and energy part of the grid cost (also defined per DGO territory): something around 0,12 €/kWh (VAT inclusive) depending on your situation, thus 350€/year on top.

A huge difference.

What can well do to counter it ?

Of course you can try to shift your consumption to quarter of hours your PV are producing a lot (and/or push the clouds back during your consumption 😊) and impede injection, but it does not seem really obvious.  What you well can do is lower your consumption, but it will not be sufficient, surely not for the peak power term of the grid cost.

The only way to reduce your peak is to use a buffer between the grid connection and your loads.  The buffer will load at a (user) defined power and the loads will offtake from the buffer at their necessary power rate.  This buffer can also be used to store not instantly consumed production so that the injection will be minimized (target = 0).  Such a buffer is a battery steered by an EMS.

So, we have now defined the headlines for the sizing of the battery system:

• reduce offtaken power downto 2,5 kW, or conversely size your battery system to limit your grid offtaken power downto 2,5 kW, over the whole year
• benefit to the maximum from your PV production, however, depending on the power of your PV installation, it is unlikely that you would be able to avoid injection, over the whole year,
• subsidiarily try to shift your offtake (for battery loading) into the off-peak time window, if you have a bihourly tariff (which is recommended in any case with a digital meter)

Sizing the battery and choosing the EMS will be treated in another article, … coming soon.