System Sizing: On Grid
There are a series of important questions that come to mind when talking about sizing a PV system.
1.What is your monthly electrical usage?
Your utility bills will show you how many kilowatt-hours of electricity you use on a monthly basis. You can calculate the watt-hours of each appliance you use, simply by multiplying many watts the appliance uses times the number of hours it runs in a day, this will give you a daily energy use.
2. What percentage of electrical needs do you want to meet with your system?
Owners begin with a small system to supplement their main supply of electricity. A small system means fewer up-front costs and the ability to gauge how much generating capacity is necessary to power the entire household. It is more cost-effective to choose a system that meets most of your needs, and rely on the grid for those times of extreme energy use than to purchase an oversized system
3. What is the amount of sunlight available at your site?
The quantity of sunlight reaching any region is also affected by the time of day, the climate (especially the cloud cover, which scatters the sun’s rays), and the air pollution in that region. Likewise, these climatic factors affect the amount of solar energy that is available to PV systems. It is important to consider that the amount of sun striking your panels will depend on their orientation, tilt and whether they can track the sun through the day.
Designing the system
1. Determine the number of panels needed:
Typical residential solar panels are about 65 inches by 39 inches ( 2 m x 1m ), with some variation amongst manufacturers. The number of panels needed to power your home will depend on the amount of power you will offset with solar energy. The amount of power offset can be anywhere from 25% to 100%.
2. Determining roof size:
Even if you know the amount of solar panels you need, it won’t help if your roof can’t fit them. In the chart below, we break down the square feet your rooftop would need for a 5kW, 10kW, and 15kW system.
| System size (kW) | Square footage needed for low efficiency (16%) | Square footage needed for medium efficiency (18%) | Square footage needed for high efficiency (22%) |
|---|---|---|---|
| 5 kW | 313 | 278 | 227 |
| 10 kW | 625 | 556 | 455 |
| 15 kW | 938 | 833 | 682 |
The above figures were calculated based on the potential efficiency of panels used. Solar panel efficiency essentially means that your solar panel is either okay, good, or great at absorbing sunlight for energy production.
Higher efficiency panels means you need less of them, and in turn, less square footage of roof space in order to produce energy.
It is more important to have a panel that produces more watts of energy versus a more efficient panel, but in order to use the smallest amount of panels on your roof, you would need the most efficient, highest-wattage producing panel.
2. Determine the size of inverters:
While solar panels are sized to meet daily energy load, an inverter is sized to meet the peak load, which is the sum of the power for all appliances that could be used simultaneously.For backup systems, the inverter must handle the peak loads of your vital appliances— the total watts you are using at any given moment. This figure is determined by summing all the wattage you use at the same time. Your peak wattage will likely occur when you are running large appliances simultaneously. Inverters should be sized to 25% more than your expected peak load to handle this situation. In addition, make sure your inverter is rated to handle surge loads, which occur when electrical devices start-up and may require six times their normal power.
3. Determining size of the battery:
This is optional, only in the case that you require a backup during times of outages.
The battery bank must be large enough to store electricity for the duration of the outage and be able to provide power to the inverter during peak demand. It is important to note that the to extend the life of your batteries you will have to discharge 60%–70% of their capacity; thus, you will need to upsize your bank accordingly.
Also, battery banks should be sized to 20%–30% greater than anticipated maximum capacity because of charging and discharging inefficiencies.
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