How are the savings obtained with photovoltaic at home calculated? A practical example


Emma Potter

We have decided to buy a photovoltaic systemperhaps with a niceaccumulation. We are sure that everything is fine, because the app proves it and we see that we save money. How much?

Before moving on to some simple countinghere are some useful tips for saving with a photovoltaic system.

How to reduce consumption: general advice

That's the facility produces during the day we all know it, we also know that, if we have accumulation, it must not be an alibi to continue with our nocturnal habits. In other words, you have to maximize instant self-consumption because, if we store the energy produced in the battery, when we have reached 100% of the charge, we will be forced to feed the rest into the grid, and we have seen that it is of little benefit to us to do so.

So the home appliances which we previously used in the evening or at night, but which we can also use during the day, or if we want to program during the day, we use them in this range. We replace obsolete appliances with low energy consumption ones.

We can think, especially in summer, of cool your home with a heat pump during the sunny hours, thus, we will remove heat from the house without drawing from the grid and the walls will remain cool, helping to keep things cool in the evening with less energy.

Let's use LED lamps instead of incandescent ones (or, worse, halogen ones: spotlights that can even reach 350 W!).

The monitoring it can be very useful to us because we can understand, based on production and input, whether it is appropriate to turn on another household appliance by anticipating its operation or postponing it. It is clear that from now on we will have to read up on how much each device we have at home consumes, because this will help us understand whether it will be included in the power produced by the photovoltaic system or not.

Another piece of advice, if we have a small photovoltaic system or we find ourselves in winter, let's avoid turning on various appliances all at oncebecause we would probably exceed the value of the power produced, it is better to spread them out over the day (always checking from the monitoring what is best to do).

Remember the famous clean inverter contacts? Well, if we have devices, such as heat pumps or water heaters, which can be controlled with a consensus, based on the values ​​of energy input into the grid by the photovoltaic system, let's have them connected to the inverter, they will easily allow us to save and the almost certainty of using the device when it suits us.

About theelectric car, its on-board battery has a capacity that varies, but is often around 40 kWh or more, so your 5 kWh storage (which becomes 4 kWh with DOD at 80%) will run out very quickly: it can do little for your car, you will be able to recharge it for a maximum of 10% if we charge at night. Still better than nothing, but if we do it during the day, we can hope for something much more interesting, especially in summer, because photovoltaic is used, as long as we have slow charging, otherwise the inverter will not be able to provide the required power from car charging. However, the system will only be able to provide a percentage of the car's energy capacity, it depends on the time of year, how much we use electricity at home at that time, how large the photovoltaic energy storage is and the peak power of the photovoltaic system itself.

Savings calculation: how to do it? A practical example

Let's see, instead, the savings that the system allows us to have thanks to failure to withdraw and input.

We have, for example, a 3 kWp system with 5 kWh storage. Let's start from here and build an example that is always clearer than generic calculations.

We pay on the bill 22 euro cents for each kWh (each of you will calculate its value, this is an example). The first calculation that is done is precisely this. It's very simple, just read the amount to be paid on the bill, subtract the TV license fee, if any, subtract the fixed costs not dependent on consumption (to be checked carefully) and divide the value by the energy invoiced to us, i.e. the total kWh that we pay in that bill (usually referring to the two-month period), the value that will emerge will be euros for each kWh.
To remain consistent with the example, I pay 200 euros for the two-month period considered, of which 130 euros are the value dependent on energy, and they billed me 590 kWh: the unit value of energy will be 130/590 = 0.22 approximately euros, or 22 cents.

From the monitoring, or from the counter, I can see that in a year (let's make the annual case, but it also applies to a month, just change the reference values) I have withdrawn 1,200 kWhI have entered 700 kWh and I have produced 3,000 kWh, so what does this mean? Am I able to know just from this data how much I have self-consumed? Obviously yes, I self-consumed about 3,000 – 700 = from the system 2,300 kWh. Monitoring, however, usually also provides self-consumption.

In these values, lThe savings portion due to accumulation is already presentIn fact, self-consumption is over 75% which, if we remember what was said previously, is quite high and can often be achieved only with accumulation. So my total consumption will be 2,300 kWh (self-consumption) + 1,200 kWh (withdrawal from the grid) = 3,500 kWh. This value is always useful, because it makes us understand if we have increased our consumption after purchasing the photovoltaic and to keep them under control.

The 2,300 kWh that I self-consumed gave me a saving, given that I didn't pay for it (because it was never counted by my meter), of around 2,300 kWh x €0.22/kWh = 506 euros. If I had to pay them, that would have been the approximate cost. But I also have another saving, which, in truth, is an amount of pure profit; in fact, it derives from the placing on the network which we are paid for. How come they pay us? Because the vast majority of residential photovoltaic systems have the so-called net metering (SSP), an agreement with the GSE (Energy Services Manager, in other words the State), signed immediately after the activation or connection of the photovoltaic system, which establishes that we will receive a certain exchange account contribution directly transferred to an IBAN communicated before signing . This contribution is valid for each kWh produced and fed into the grid, therefore not used by the user at the moment, but directly fed until it equals the energy withdrawn. What I input, but which exceeds the energy withdrawn, will be called surplus and will be paid to me at a lower value.

How much does GSE pay per kWh? Good question, to calculate it, just go to the GSE website and calculate the value which changes over time, does not remain fixed, through the various terms of the formula. It would be time-consuming and extremely tedious to calculate the final value and detail all the terms of the formula, which in turn are composed of other terms, some of which refer to national energy markets. I therefore refer those most interested in knowing the real value to the GSE website or other sites on the internet that dedicate entire articles to this very topic.

Let's go back to the example.

Let's assume a value of the exchange contribution around 10-15 cents per kWh and something less for the share of the surplus. We were saying, I saved 2,300 kWh with a value of 506 euros, to these we should add exchange contribution plus surplus. Having withdrawn 1,200 kWh and injected 700 kWh, the entire injection will be paid to me as an exchange account, therefore 700 kWh x €0.10 4 = 70 euros. So the total would be 506 + 70 = 576 euros.

We don't have a surplus because we put in less than we took out. Let's say that the excess kWh is remunerated at 8 c€ (we are simplifying, without using the GSE formulas). Modifying the previous example, if, instead, we had entered 1,500 kWh (assuming we had a larger photovoltaic system that produced more), we would have received 1,200 x 0.1 = 120 euros for the exchange account, and the remaining 300 kWh entered which exceed the withdrawal, evaluated as follows: 300 x 0.08 = 24 euros, so instead of the 70 euros we would have had 144 euros.

How long does it take to pay for the system?

Well, we did a simple count of how much we saved in a year. At this point it comes naturally to try to understand as we will pay for the system.
Far from considering myself an accountant, I can, however, venture some predictive calculations. Assuming you have purchased the system with storage for 8,000 euros, saving around 600 euros per year, I would take about 13-14 years. But the situation is rosier, because if I had purchased it with the discount on the invoice, I would have paid only 4,000 euros and I would have paid it back in half the time.

In the case of a 50% tax deduction over 10 years, it would still be convenient, but it would pay for itself in a slightly longer time, because we would see 4,000 euros divided into savings of 400 euros per year to be added to the photovoltaic savings, bringing the return to around 8 years.

Since the panorama of concessions and incentives is quite changing in this historical period and taking into account the importance that this renewable energy has assumed and will assume in the future, it will be necessary to seek, at the time of purchase, information on what the government of the day will have placed in place to reach the share of renewable energy desired or set at national level. Hoping that more and more will be done to guarantee zero kilometer energy for everyone.