We calculate the annual energy output of a proposed solar installation using:
- The user-specified system efficiency parameter
- The number, type and placement of solar panels
- Predicted sun positions
- Local solar irradiance and temperature data
This article explains the source of each of these inputs, and how we combine them to calculate the Annual output value shown in kilowatt hours (kWh).
It is essential for you to enter an appropriate value for the System Efficiency, and to understand what this number should and should not include. This field is not calculated by Nearmap.
System Efficiency should take into account, if possible:
- Inverter AC-to-DC conversion efficiency, if applicable
- Battery conversion efficiency, if applicable
- Loss from shading, if structures, hills, or other panels will block available light some of the time
- Wiring losses
- Dirt or dust on the panels
- Lost power due to outages for grid-tie systems or maintenance periods
- Manufacturer tolerances
- Any voltage mismatch among system components
System Efficiency should not account for:
- The panel temperature
- The sunlight available at the location
Number, type and placement of solar panels
The number, location, orientation and tilt of the solar panels you place in your design will impact the output of the solar job as a whole.
The panel type and the associated parameters of your chosen PV unit will also affect the output of your solar job.
Manufacturer and Model
We have a wide range of PV units to choose from, representing solar panels with various attributes and power outputs. If your model of interest is not listed, select a similar one by the manufacturer, or add a custom panel.
Nominal power at standard test conditions. This is what the panels are capable of producing under 1000 W/m² irradiance at 25 C. It will almost always considerably exceed real-world performance.
Nominal Operating Cell Temperature. This is the temperature that the manufacturer expects the panels to heat up to during operation in nominal operating conditions, which are 800 W/m² irradiance, 20 C, a 45 degree tilt angle, and 1 m/s wind. It is typically between 45-48 C.
Temperature coefficient of power
This corresponds to the percentage of power lost for each degree above 25 C the panel temperature rises.
We predict instantaneous outputs of the solar cells in a solar job, which are then integrated over the course of a year for an annual value. To predict the annual output of a given solar cell, we first need to predict the positions of the sun; we use the PSA (Plataforma Solar de Almerýa) algorithm to do this.
Our calculations also take into account the average solar irradiance and temperature in the location of a solar job. In the continental USA, irradiance data comes from hourly satellite observations over a nationwide grid.
Temperature data in the USA, and irradiance data for Hawaii and Alaska, comes from ground-based weather stations.
In all cases, we average the data for each location to produce a typical year for irradiance and temperature. These values feed into our calculation of the output for a solar job in kWh.
For solar irradiance data in the continental USA, we use hourly gridded data with 0.1 degree latitude/longitude spacing from the National Renewable Energy Laboratory’s (NREL) National Solar Radiation Database (NSRDB) SUNY model. This model uses geosynchronous satellite measurements of solar irradiance taken between 1998 and 2005.
For Hawaii and Alaska, solar irradiance data is taken from the output of ground-based weather stations. These measurements were taken hourly between 1991 and 2012.
To calculate thermal derating of solar cells in the USA, we use temperature data from ground-based weather stations across the nation. These measurements were taken hourly between 1991 and 2012.