Sizing Solar Power for Off-grid Field Studies – Part 2

Part two of the three part series. Here is Part 1.


How Much Solar Energy is Available?

The next step is determine the amount of solar energy available at the deployment site. This consists of three main components: the angle and timing of sun (which can be calculated for any location on earth based on the latitude, longitude and date), the meteorological effects (which can be calculated for areas near a weather station based on historical data), and the shading effects of the local topography (which generally requires prior access to the deployment site and specialized equipment).

Calculating Peak Sun Hours (PSH)

The most important figure to calculate for your potential site is the Peak Sun Hours (PSH) per day. This is not a simple measure of daylight hours but rather the equivalent number hours of an amount of standardized solar radiation (1 kW/m2). In the graph below, the red line shows the amount of solar radiation received during a day. The blue line show the equivalent number of PSH. The area under both curves is the same, but since PSH is in standardized units (the same standards that solar panel manufacturers use to rate their panels) this number can be used to choose a solar panel appropriate to your application.

Peak Sun Hours - Eosense

Figure 2: Peak Sun Hours

While there are maps showing peak sun hours for various locations, they are not always useful for remote installations. They often show only highly populated areas like the continental US or Western Europe. More importantly, they generally show the yearly average peak sun hours. This can be useful for determining the economic feasibility of a long-term residential site. It would be very expensive for a solar home to size it’s panels based on the worst month of the year since it would have unused excess capacity for the rest of year; instead they generally design for the average and supplement the solar power with wind, diesel generators or a grid connection during the lean months. Since a remote autonomous installation does not have these options, we must design for the worst month of the year (or for a short term deployment, the worst month of the deployment).

Optimizing for Solar Angle

While most guides recommend either setting the angle of the solar panel to the location’s latitude in degrees or adjusting the angle several times throughout the year, neither of these are suitable for this application since the site will be unattended and we need to design for the worst month rather than the average. Set the angle to get the maximum benefit at solar noon on the winter solstice (December 22 in the northern hemisphere), or the day of your deployment that is closest to the winter solstice if it is less than a full year. This angle can be found by the formula A = L – (23.45° × sin(T ÷ 365.25 × 360°)), where L is the latitude of the installation, T is the number of days from spring equinox (March 21st), and A is the ideal panel angle for that day. Alternatively, websites like SunCalc can be used to determine the angle of the sun to the ground (referred to as the altitude) for any given location, date and time. The azimuth is the compass angle the panel should point, which is generally due south (though when doing the installation, be sure to correct for the local magnetic declination at your site). Screenshot - Eosense

Figure 3: Reading sun angles from

You may have little control over the placement of the data collection site, but ideally it will have no large buildings, trees or other obstructions blocking the view of the south-east to south-west horizon (if located in the northern hemisphere; north-east to north-west horizon for the southern hemisphere). If there are obstructions that cannot be removed, you may want to consider moving to higher ground if possible, or mounting your solar panels on a pole to minimize the effect of shading from the obstructions. It is difficult to quantify the effect of shading without specialized equipment, however there is an Android app available called “Scan the Sun” which uses your phone’s camera, GPS and compass to perform a decent site analysis including shading calculations.

ScanTheSun Android App Display

Figure 4: ScanTheSun Android App Display

The calculations for daily peak sun hours (or solar insolation) are complex to perform manually. Fortunately there are many calculators available on-line. One of the best regarded is the PVWatts Calculator provided by the U.S. National Renewable Energy Laboratory. While it was designed primarily for residential installations within the United States, it works well for any location worldwide. The calculator initially asks for a street address as a location, but if you enter an address near to your desired location, you can manually drag the marker to an “off-grid” location. Weather data from the nearest available weather station is used to compute the meteorological effects.

Location into PVWatts Calculator Screenshot

Figure 5: Entering Location into PVWatts Calculator

In the “System Info” screen, you can select a PV panel size (the minimum is 0.05 kW or 50 W) and panel type. Array should be “Fixed (open rack)”. You can accept the recommended tilt angle and azimuth or enter the optimized angles you calculated in the earlier stage. The economic parameters are not applicable and can be left blank. The “System Loses” can be refined if you know these values for your system or you can just accept the defaults. Under “Advanced Parameters”, since we are running a DC-only system, set the “DC to AC Size Ratio” to 1 and the efficiency to 99.5% (the highest the software allows).

System Info in PVWatts Calculator

Figure 6: Entering system information in PVWatts Calculator

You can then go the Results page and see a summary showing the solar radiation for each month in kWh/m2/day (also known as Peak Sun Hours per day). The column marked “AC Energy” is the useable energy produced for that month after system loses in kWh. We calculated earlier that our sample system needs 74.4Wh per day. Multiplying by 30, this system would need 2232 Wh or 2.2 kWh per month. If the energy produced in the worst month of our deployment is less than 2.2 kWh, we can go back to the System Info and select a larger panel, then check the results again. More detailed monthly and hourly results can be downloaded in CSV format.

Another on-line calculator with fewer options but easier to use can be found at the Solar Electricity Handbook website. It does not allow latitude/longitude input but has settings for most major cities worldwide.

Part 3: Setting up the system – charge controllers & wiring…