You can estimate your total monthly energy consumption in several ways. One method is to compare your energy consumption levels during different periods using the energy bills supplied by the utility company. You can also make an inventory of all of the electric appliances that you have at home and using websites like Energy Star or EU labelling system try to calculate the power requirement multiplied by your average daily hourly use.
When wanting to install solar PV for your home, you need to consider many factors regarding your energy demand. Do you live in a large household? Do you use more electricity in winter? Are you at home most of the time or only at night? Understanding your behaviors, geographic location and specific circumstances will provide you insights in knowing to what extent PV solar panels are a good solution for you. There are solar insolation maps that provide a summary of solar power potential across different geographic locations.
Solar maps typically use kWh-to-kWp ratio, which show how much electricity solar panels generate in a specific location compared to how much they would generate under standard test conditions (STC) that we described earlier in this guide. The acronym kWp stands for “Kilowatt peak” and represents the maximum energy output that the solar panel can generate under STC. The higher the ratio the more electricity the solar panel generates since it gets close to maximum power output that is often mentioned in the manufacturer’s product specifications.
As you can see in this snapshot taken from Global Solar Atlas, southern areas in Europe and North Africa have more intense red colors. This means that there is more solar potential than in northern countries in Europe which have more greenish and blue colors.
In practice, solar PV electricity generation is far below from standard test conditions. Weather conditions such as wind, rain, dust, shade and other environmental and modular characteristics diminish the capacity for a solar PV to transform sunlight into energy.
To resolve this, manufacturers might test their products under Nominal Operating Cell Temperature (NOCT) or Nominal Module Operating Temperature (NMOT) conditions. NOCT consider PV electricity output under different conditions:
- Air temperature of 20 °C
- Solar irradiance of 800 W/m2
- Windspeed of 1 m/s
- Open backside mounting (a thin space between the solar panel and your rooftop for example).
NOCT attempts to mirror conditions closer to what you typically find in real life.
When sunlight strikes solar PV panels, these can get overheated and lead to lower electricity conversion efficiencies. Depending on the systems installed and materials used, the efficiency of the solar panels will be higher or lower. We will discuss these details more in-depth later on.
As a result of these conditions, true power generated by a solar PV is almost always less than its nameplate capacity. Sometimes you might want to take the 60-80% range of power nameplate capacity as to how much electricity you will generate over a year. For instance, if your household’s average energy yearly consumption is 11,000 kWh and you want your panels to cover all your consumption, you would need a namesplate capacity of approximately:
11,000/0.06 =~ 18,300 kW
This is a fairly conservative measure. Your manufacturer and reviews from other buyers online should guide you as to what is the typical electricity generation from your particular model.
Another question that you may have when considering transitioning to solar PV is whether you can sell excess electricity generated to the grid. Customers that generate electricity independently are sometimes known as independent power suppliers (IPP). Depending on the jurisdiction, they might be able to sell their electricity output to the national grid.
In Spain, energy utility companies do not purchase excess electricity from customers who generate their own solar power, according to a source. Instead, households and small businesses with independent power production (IPP) systems can earn credits from utility providers to offset future energy consumption.
For instance, let’s say you own a home with a solar panel system that generates an excess of 300 kWh in February. If your utility provider credits excess electricity at a rate of 0.06 €/kWh, your credits for that billing period would amount to:
300 kWh * 0.06 €/kWh = 18 €
Now, imagine that during the same month, you consume 100 kWh of electricity from the grid at night when your solar panels are not generating power. If the electricity you purchase from the grid is charged at 0.25 €/kWh, your cost for that usage would be:
100 kWh * 0.25 €/kWh = 25 €
To calculate your total electricity bill, you subtract the credits earned from your grid consumption:
25 € (cost of electricity purchased) – 18 € (credits earned) = 7 €
In this scenario, you would owe 7 € to the utility company, plus any additional fixed charges, such as connection and maintenance fees.
It’s worth noting that if the value of the electricity you sell back to the grid exceeds the value of the electricity you purchase, your credits will be capped at the amount you owe. Additionally, the rules regarding unused credits can vary. In some countries, credits expire at the end of the billing period, leaving no option to carry them over into future months.