SolarGuru.net
Frequently Asked Questions (FAQ)
- How is the electricity generation estimated?
Electricity generation is calculated based on following parameters:
- Monthly solar insolation at the given location as reported by NASA
- Temperature data for each month as reported by NASA
- Tilt of solar panels
- Loss due to dust/snow
- Inverter Efficiency
- Wiring losses
- Age of solar panels
- Loss due to mismatch between panels
- Shading
- Grid Availability
- What are the assumptions for this calculator?
Values assumed for various parameters:
- Loss due to dust/snow: 5%
- Wiring losses: 2%
- Inverter Efficiency: 96%
- Age of solar panels: < 1 year
- Loss due to mismatch between panels: 2%
- Loss due to grid availability issues: 2%
- Loss due to shading: 0%
- In the northern hemisphere solar panels are oriented towards exact south and in the southern hemisphere, orientation is towards exact north.
The calculations are valid for a grid-connected system.
- How accurate are the calculations?
The predictions of this calculator were tested against actual generation by few systems in three countries: USA, Australia, and India. The results were found to be quite accurate. For most systems, the yearly generation was within 5% of the predictions of the calculator. In the case of monthly generation, there was more variation.
- What are the limitations of this calculator?
The predictions of this calculator are based on long term averages. However, on a given day, or month, weather could be significantly different than the long term averages. This results in actual monthly generation to vary significantly when compared with predicted generation. A variation of up to 20% is certainly possible. Therefore, it is advised that a variation of single month should not be taken to indicate a design flaw.
- Why does the calculator require DC-kiloWatts(DC-kW)?
The DC-kilowatts parameter is simply the total peak power of all the solar panels. The calculator takes into account the inverter efficiency as well as the wiring losses to estimate generation.
- How do I find the DC-kW for my system?
Simply add up the peak power rating of all solar panels that you are using.
- What exactly is the parameter "tilt"?
In most installations, solar panels are not placed horizontal. The panels are tilted to the horizontal plane while facing either the south direction or the north direction, depending on the hemisphere. This angle of tilt must be known to use the calculator. In many cases, this angle equals latitude of the location, since the generation is close to optimal at that angle. In other cases, the angle may simply depend on the angle of tilt for the roof on which the panels are installed. For the purpose of this calculator, positive values for tilt imply a tilt towards south while the negative values imply a tilt towards north. Value 0 for tilt implies a horizontally placed panel.
- Should the tilt always equal latitude?
The tilt need not equal the latitude in all cases. There could be locations where a different tilt may yield better output. You can play with different angles of tilt and estimate the generation.
- How do I specify parameters for a location in southern hemisphere?
It is necessary to enter latitude and longitude only if the place of installation is not found in the database. Latitude of places in the southern hemisphere must be entered as negative numbers. For instance, latitude for Hobart, Australia is entered as -42.8. For locations in the northern hemisphere, latitude is entered as a positive number.
- How do I specify parameters for a location in the western hemisphere?
Longitudes for the locations to the west of Greenwich, England (longitude 0) are entered as negative numbers. Longitudes for locations in the eastern hemisphere are entered as positive numbers. For instance, longitude for Cupertino, California is entered as -122.0.
- Why does the tool report wrong latitude/longitude for my "place"?
There are cities with same name in different countries and states. In this case, the first entry in the database is used to find latitude and longitude. You may add state or country information to city name for the search to yield the correct result. For instance, you may enter "Sunnyvale, California" instead of "Sunnyvale".
- How do I use the generation data to audit my system?
Please take a look at this usage note.
- Generation for my system is low. How do I improve it?
Please take a look at this usage note.
- When should I use the advanced version of the calculator?
Advanced version of the calculator is useful if you need to change any of the parameters from their default values.
- What are the other such calculators on the web?
PVWatts from NREL is a useful calculator. The output from this website may not match with NREL for some locations.
- How is the system size calculated?
First, yearly generation (kWh) for a 1 kW-DC solar PV system is estimated for the given location for different values of tilts. The algotithm for calculating generation is described here. The value for expected generation provided by the user is divided by the estimated generation-per-kW to calculate the system size.
- How accurate is the calculated system size?
The accuracy of calculation depends on the accuracy of calculator for generation by a solar PV sysetm. Check this answer for details.
- Why is the calculation for only a few values of tilt?
For most systems, a tilt that equals the latitude yields the highest output and hence the smallest system size. However, there are locations where other values for tilts are optimal. This calculator estimates the system size for 8 or 9 values of tilt. This is more than adequate for almost all locations in the world.
- Why does the system size change with tilt?
The solar radiation incident on the solar panels changes with the angle of tilt, which results in the yearly generation changing with the angle of tilt.
- Which system size and tilt should I choose?
In most cases, the smallest system size and the corresponding tilt is the obvious choice. However, for large systems, high values of tilt imply a need for more distance between the rows of panels for avoiding shadows, requiring a larger area for installation. In such a case, it may be optimal to choose a smaller for value for tilt. If the system size does not change much between different values for tilts then it may be best to choose the system with lowest tilt.