A rooftop solar power system requires significant investment. The return on investment (ROI) of this system is dependent on selecting the right size for the system. The suppliers/providers of such systems tend to over-size them because a larger system implies a higher price and therefore higher profits for the provider. Their goal is rarely to optimize the ROI for the customer. This blog post aims to guide the customer in selecting the correct size for their rooftop solar power system.
Executive Summary:
Size the system such that the yearly generation is slightly less than the yearly consumption. For most locations in India, a rooftop solar system will generate 4 kWh/kW/day, when averaged over a year. The size of the rooftop solar system in kW is calculated to be: yearly_consumption/(4 * 365). Therefore a 20,000 kWh yearly consumption will require a 13.7 kW system. If the available space cannot accommodate this system then it is recommended to go for a lower size. Never try to ‘fit’ a large system by cramping together rows of panels. This is because shadows will reduce the output significantly. The solar system needs 8 shadow-free hours in the middle of the day (8 am. to 4 pm.) for optimal generation, even during the winter months.
Assumptions:
- The discussion applies to a system in India but could be extended to systems in other parts of the world.
- The electricity utility company supports net-metering for rooftop solar systems.
- The rate at which the utility company purchases excess electricity is significantly lower than the marginal rate at which the consumer purchases electricity.
Step-by-Step Procedure to Size the System:
- Identify the application which will be supplied solar power. For instance, lifts (elevators), common lighting, water pumps, air conditioning, etc.
- Collect monthly bills for one full year and note the consumption in kWh for each month. Calculate the yearly consumption in kWh.
- The yearly consumption is the target yearly generation for the solar photo-voltaic (PV) system.
- The capacity of a solar PV system is measured in kilowatts (kW). A grid-tied system consists of solar panels and an inverter. The solar panels generate DC output while the inverter output is AC. The inverter and panels generally have the same capacity. So, a system with solar panels of 10 kW DC capacity should use an inverter with 10 kW AC capacity. However, a 10 kW DC system will rarely generate 10 kW output since the solar panel rating is specified for ideal conditions. Such conditions are rarely present in practice unless the installation is in the Sahara desert and the sun is shining directly overhead. Hence, it is perfectly safe to use an inverter that has 10 to 15% lower capacity than the solar panels.
- Calculate solar panel capacity (DC kW) using the following formula:
Capacity (kW) = (target_ yearly_generation in kWh)/(365 * 4)
In this formula, the number ‘4’ is the typical generation in kWh/kW/day for most locations in India. This generation is for a system with solar panels facing south, tilted at ‘latitude’ degrees. For instance, latitude for Pune is 18 degrees and hence the solar panels should be tilted at 18 degrees to ground. This website provides a good estimate of monthly and yearly generation for most locations in the world.
- This capacity is the target capacity for the system. Never buy a system with larger capacity.
- Measure the area (in square meters) on the roof that will be used to install solar panels. The area should be with a clear view of the south and there should be no obstructions such as trees, water tanks, elevator rooms, etc. Note that parapet walls are not a concern since the panels can be installed elevated. The idea is to avoid shadow on panels from 8 AM to 4 PM in all seasons, as far as possible.
- Multiply the area by power density to obtain the approximate capacity of panels in kW that can be installed in the area. The power density depends on several factors. If multiple rows of tilted solar panels are installed then the panels in front cast a shadow on the back row. An adequate distance is necessary between rows to avoid the shadow. The angle of tilt is generally the same as the latitude of the location. Higher the latitude, higher the tilt and hence more distance is necessary between rows, implying a lower power density.
- For small systems, say less than 4kW, a single row of panels may be sufficient (see the picture above). For such a configuration, a power density of 170 W/m2 can be used to calculate the system size. For larger systems with multiple rows of panels, following table summarizes approximate power density for different latitudes:
| Latitude | Power Density |
|---|---|
| 0 | 170 |
| 5 | 157 |
| 10 | 142 |
| 15 | 127 |
| 18 | 117 |
| 20 | 111 |
| 25 | 93 |
| 30 | 75 |
- For instance, in Pune, India, with a latitude of 18 degrees, an area of 100 m2 can support a 11.7 kW installation. If the target capacity calculated based on consumption is 15 kW, this space will be insufficient. Cramping a 15 kW system will result in shadows that will reduce generation. A low generation implies a correspondingly low ROI. In this situation, it will be best to install a system with 11.7 kW (DC) capacity. The inverter, or the AC capacity of the system, should be chosen to be 10 kW.
Note:
This blog-post will help the customer optimize solar system size for most installations. It should also help while verifying proposals from solar installers. While the information provided will apply to most situations, there are cases when deeper analysis may be necessary. For instance, at certain locations, it may be better to install panels without a tilt. The power density in this case will be the same as that for latitude 0.
Reference: Calculators for Solar Generation and System Sizing
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