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How do you turn an abundant energy source like the sun into a reliable power supply? The key is proper knowledge on when and where solar energy is available. Learn more about solar radiation and the role of satellites in predicting our resource from the sun.

The good to know guide to solar energy. This FAQ area gives answers to questions on solar performance and what influence weather and climate have on good results.

Please click on a question to see the answer.

Do you have another question that hasn't been answered? Please send us your question.

What is the role of satellites in measuring solar energy?

Every 15 minutes the weather satellite takes a picture of the earth from a distance of approximately 20 thousand miles. These pictures display the clouds and the land just like the familiar images from TV weather shows. The innovation of focus solar is that these images are combined with a model of solar energy. From the observation of clouds, the amount of solar energy is being calculated. This movie shows a solar day over Europe. You can see the intensity of the sun in different colors ranging from blue to red. In the morning, when the sun is rising, there is a rapid change of colors until the sun reaches full intensity. Eventually, the sun will settle down again and the night begins. In the meantime, the clouds are jiggling around quite a bit which you can see as moving color patches.

solar_day.gif

Before solar energy arrives on earth it has a long way to travel through space - millions of miles. The sunrays are really good at traveling though. They go with the speed of light and it takes them just a couple of minutes to get here. The role of the satellite is to observe the final part of the sunray's journey to earth: the path through the atmosphere. That's the hardest part. While it's easy to travel through empty space, the earth atmosphere is made up of lots of stuff that sunrays can collide with. There are clouds, air molecules, and air pollution - all big obstacles for a little sunray. Those sunrays passing through can be collected by solar panels and solar power plants. By studying the incoming flux of solar energy, we get a better idea how this flux can contribute to solve our energy needs.

What are the most sunny cities in Europe and the US?

What cities are the most sunny? Here is the result ranking 50 selected cities from Europe and the US in solar energy. The most sunny city is Phoenix in Arizona, USA, and the least sunny city is Oslo in Norway.

Rank
City
Country, State
Annual Solar Radiation (in kWh/m²)
1
Phoenix
USA, AZ
2169
2
Las Vegas
USA, NV
2132
3
Albuquerque USA, NM
2093
4
Santa Fe
USA, NM
2024
5
San Diego
USA, CA
1966
6
Los Angeles
USA, CA
1909
7
Miami USA, FL
1909
8
San Jose
USA, CA
1883
9
Salt Lake City
USA, UT
1804
10
New Orleans
USA, LA
1793
11
Atlanta USA, GA
1780
12
Denver
USA, CO
1749
13
San Antonio
USA, TX
1734
14
Izmir
Turkey
1724
15
San Francisco
USA, CA
1713
16
Athens
Greece
1710
17
Houston
USA, TX
1698
18
Lisbon
Portugal
1683
19
Madrid
Spain
1666
20
Valencia
Spain
1666
21
Marseille
France
1634
22
Naples
Italy
1622
23
Washington
USA, DC
1573
24
Rome
Italy
1565
25
Istanbul
Turkey
1486
26
New York
USA, NY
1477
27
Chicago
USA, IL
1438
28
Boston
USA, MA
1437
29
Sofia
Bulgaria
1398
30
Bucharest
Romania
1389
31
Belgrade
Serbia
1348
32
Ljubljana
Slovenia
1274
33
Budapest
Hungary
1274
34
Bratislava
Slovakia
1226
35
Vienna
Austria
1222
36
Munich
Germany
1220
37
Zurich
Switzerland
1206
38
Seattle
USA, WA
1195
39
Prague
Czech Republic
1113
40
Paris
France
1096
41
Warsaw
Poland
1015
42
Berlin
Germany
1002
43
Amsterdam
Netherlands
981
44
Brussels
Belgium
971
45
London
United Kingdom
952
46
Copenhagen
Denmark
952
47
Dublin
Ireland
894
48
Helsinki
Finland
894
49
Stockholm
Sweden
872
50
Oslo
Norway
831

How is the performance ratio defined?

The performance ratio is defined as the electrical output per kW peak system size and per unit of solar energy input:

kWh produced
-------------------------------------------------------
(kW peak) * (kWh/m² solar energy)

where the unit of solar energy is kWh per square meter. It expresses how much of the available solar energy is converted into electrical energy. The performance ratio was introduced to compare PV systems independent of size, mounting, and location.

Let's look at a typical example: In California, a customer might have a 3 kW peak system and the annual solar energy is 2000 kWh/m². If the system were perfect, it could potentially produce 3 x 2000 = 6000 kWh per year. When the actual production is 5000 kWh, then the performance ratio is 5000 kWh / 6000 kWh = 83%.

The performance ratio can also be calculated for time periods other than a year. Interesting time scales are for instance a month or even a day. It's only important that the time scale is the same for both, the generated electrical energy and the incoming solar energy. Otherwise it's like comparing apples with oranges.

Why is my performance ratio not 100%?

We can distinguish two types of losses: "hard" and "soft" losses. Hard losses are losses with a physical cause and which can potentially be removed. Soft losses are also real losses, but they are influenced by human perception. Soft losses are actually much harder to remove than physical losses. In practice the owner of a PV system can't do anything about them.

Hard losses:
The most important physical loss factors are: losses in the inverter (typically 4%, some brandnew inverters reach 2%), losses from shading (0-100%) and soiling (0-25%) which simply means that PV panels have accumulated dust and soil. Shading and soiling are site-dependent conditions. Dry and desert-like locations with little rain have larger soiling problems. Some locations in California have up to 25% soiling losses. There are also minor losses from cabling, diode and connection losses, mismatching, inefficiencies in the tracking of the maximum power point, etc, which typically add up to less than 5%.

Soft losses:
Soft losses relate to the definition of the standard that is applied in the nameplate rating of PV modules. To say it politely, the standard was chosen "optimistically". Soft losses occur whenever a PV module is operated at weather conditions that result in a lower efficiency than the efficiency at standard test conditions (STC). Unfortunately this is almost always the case because standard test conditions are defined for the lab environment and not for the real world. STC defines the irradiance as 1000 W/m² and the module temperature as 25°C. At these conditions PV modules reach their highest efficiency. In a sunny location, however, the temperature is typically much higher than 25°C because PV panels heat up in the sun and in a colder location, the illumination level is typically lower than 1000 W/m². In either case, the operating conditions are not the ideal conditions from the lab environment. STC performance can only be achieved if PV modules are cooled during operation. In the lab calibration process, modules are only briefly flashed by so-called solar simulators, too fast for modules to heat up.

In summary, soft losses are weather-related losses in which PV modules do not reach the nameplate efficiency. They are highly site-dependent and it is not unusual to have soft losses of 10% or even higher. When we add up all losses, we arrive at a typical number of approximately 20%. Therefore a good performance ratio should be 80% or better.

By the way, soft losses could be reduced if the industry would decide to change the standard for nameplate rating. This would require, however, to relabel a 100 W module into a 90 W module, for instance. Obviously, PV sales people will not be too happy about this prospect.

What can the performance ratio tell me about my PV system?

It is important to keep in mind that the performance ratio can identify the existence of a problem, but not the cause. The cause of the problem requires further investigation, which may include a site visit by service personnel. Performance ratios are useful for determining if the system is operating as expected and for identifying the occurence of failures. Large decreases in performance ratio indicate events that significantly impact performance, such as inverter faults, circuit-breaker trips, or loss of a whole module due to connection failures inside a PV junction box. Small or moderate decreases indicate a less severe problem. Intermittent problems such as shading, snow coverage and soiling show up as fluctuating performance numbers with an on/off characteristic. Long-term degradation appears as a small continuous decline in performance.