As loadshedding is slowly becoming a normality in South Africa, many are looking towards solar systems as a way to keep the lights on during these dark times. However, common questions such as how big of a solar system is needed or how many solar panels are required, often form the basis of this discussion. Thus, NrG has provided below a quick and simple guide on how to size a solar system:
1. Estimating your energy usage
The first step in sizing a solar system, is estimating your current energy usage. This is usually done by looking at your electricity bills and identifying the kilowatt-hours (kWh) that you consumed. Since, energy usage varies and fluctuates throughout the year and its seasons, taking a years’ worth of electricity bills, will help provide a more accurate picture of your energy profile. When sizing an industrial or commercial solar system it would be wise to take into account the hours of the day the site is in operation to avoid oversizing the system.
To size the solar system a daily kWh value is needed. Therefore, to calculate this value, one should sum and add all the monthly kWh consumption values, then divide it by the number of days in a year, thus 365. This will result in your daily kWh usage value. If sizing a commercial or industrial site you would divide this figure by the number of hours the site is in use.
2. Determining peak sun hours
The second step in sizing a solar system, is determining your peak sun hours. “One peak sun hour is one hour’s worth of sunshine at an irradiance of 1 kilowatt per square meter (kW/m²). Peak sun hours, measured as kilowatt-hours per square meter (kWh/m²), are influenced by the time of day, the season, the presence of clouds, and geographic location.”[1] South Africa averages from 4.3 to 6.3 peak hours per day. The table below, shows a breakdown of daily peak sun hours per province.
Table 1: Summary of Daily Peak Sun Hours per Province[2]
| Province | Average Daily Peak Sun Hours (kWh/m²) |
| Eastern Cape | 4.3 – 5.6 |
| Free State | 5.3 – 5.7 |
| Gauteng | 5.3 – 5.5 |
| Kwazulu-Natal | 4.3 – 5.3 |
| Limpopo | 4.7 – 5.3 |
| Mpumalanga | 4.7 – 5.5 |
| Northern Cape | 5.7 – 6.3 |
| North West | 5.5 – 6.1 |
| Western Cape | 4.7 – 6.0 |
3. Calculating solar system size
The final step is calculating your solar system size. The formula is:
This formula, however, assumes that the solar panels will work at 100% of its capacity rating, however, we know that is never the case. An infographic below, taken from Solar Empower, describes potential losses that can account to roughly 25%.
Figure 1: Infographic from Solar Empower.[3]
This means that our formula above needs to be multiplied by rough efficiency factor value of 1.25 to account for the potential losses. Our final formula becomes:
This calculated value will determine the solar system size needed in kilowatts (kW). Since solar panels come in all different sizes and ratings. This value can help you determine the number of panels needed and thus the square meterage needed for the system. Furthermore, the calculated value can help you determine your inverter and battery sizes and needs. Extra constraints and considerations to evaluate are space and budget constraints, as solar systems and especially back-up batteries are expensive. Orientation, roof tilts and mounting of the panels, as different facing directions and roof angles can affect the efficiency of the solar panels and the mounting on different types of roofs and panel weight requirements can become an issue.
Should you require more information or are ready to make a step in a renewable future, feel free to contact us.
[1] (Africa Solar Industry Association, 2022)
[2] (ClimateBiz, n.d.)
[3] (Bruce, 2022)
References
Africa Solar Industry Association. (2022, March 8). Peak Sun Hours. Retrieved from AFSIA – Africa Solar Industry Association: http://afsiasolar.com/glossary/peak-sun-hours/
Bruce, J. (2022, November 22). Solar Panel Sizing – How To Calculate Home Solar System Size. Retrieved from Solar Empower: https://www.solarempower.com/blog/solar-panel-sizing-calculate-solar-system-size/
ClimateBiz. (n.d.). AVERAGE PEAK SUN HOURS (SOUTH AFRICA). Retrieved from ClimateBiz: https://climatebiz.com/average-peak-sun-hours-south-africa/
GoGreenSolar. (n.d.). Sizing Solar Systems: A Step-By-Step Walkthrough. Retrieved from GoGreenSolar: https://www.gogreensolar.com/pages/sizing-solar-systems
SunPower. (n.d.). How Many Solar Panels Do You Need: Panel Size and Output Factors. Retrieved from SunPower: https://us.sunpower.com/solar-resources/how-many-solar-panels-do-you-need-panel-size-and-output-factors#:~:text=You%20can%20calculate%20how%20many,generate%2011%2C000%20kWh%2Fyear).
Unbound Solar. (2020, July 14). How to Size a Solar System: Step-by-Step. Retrieved from Unbound Solar: https://unboundsolar.com/blog/how-to-size-solar-system