Appropriately sizing a customer’s solar system is imperative for happy clients. Correct sizing is even more critical with off-grid solar systems, as the utility grid is not available to use as a power source. Needlessly oversizing an off-grid system means clients pay more money for components that are often, if not always, unnecessary. Undersizing an off-grid system means that the system will not cover their energy needs, causing them to run out of energy, often at inconvenient times. Eventually, customers with undersized off-grid systems may need to purchase additional system components and redesign their systems, adding more expense. 

In Wes Kennedy’s Comprehensive Solar Plus Storage course, he breaks down the different considerations for designing each. 

Grid-tied solar is a photovoltaic system that is electrically connected to the utility grid. In grid-tied systems, there is no battery storage thus no batteries, charge controllers, nor battery management system. Grid-tied inverters are utilized. These require frequency from the utility grid to operate. When the utility grid is down, grid-tied inverters shut down as well. 

Designing grid-tied solar systems is simple when compared to the considerations needed for off-grid solar systems. Three main factors are examined. One or all of these factors will determine the customer’s PV array size. A grid-tied inverter is then sized to invert the maximum amount of solar. 

The first factor is the site’s annual energy (or kWh) consumption. Oftentimes, utilities have regulations on how large a grid-tied solar system can be in terms of expected energy production based on the site’s historic energy consumption. These regulations vary depending on location and utility. 

The second factor is the amount of viable space available for the array. On most residential projects, this is the unobstructed sunny roof space. In the event of a ground-mounted system, this is the field space. The amount of space determines how many solar modules can be included into the solar design.

Finally, the last factor is the customer’s budget and available incentives, like tax credits or rebates. The Database of State Incentives for Renewables & Efficiency (DSIRE) is a great resource to investigate incentives in a customer’s area if located within the United States.

Photo credit: Wes Kennedy

Off-grid solar system sizing is much more complex. While ambient conditions like temperature and irradiation are always considered in solar designs, it becomes much more critical when the system does not have the utility grid as backup. Additionally, off-grid solar systems often have additional backup power sources, like diesel generators, which add complexity. Energy consumption patterns must be considered. Loads must be analyzed, including the maximum demand in kW.

In off-grid solar systems, the load is integral. It is not possible to accurately size the battery and battery inverter based solely on the PV array size. The battery energy storage system must be equal to or exceed the load. That’s why proper load analysis is critical. Designers will complete the load analysis with a client, examining current loads, forecasted growth, and any future demand considerations. Upon completion of a full load analysis, designers can then begin sizing the off-grid components. 

Put simply, the solar PV production must then be sized to produce more energy than the load demand each day. The battery bank is sized to carry the load, typically for 1 to 3 days. This is known as the days of autonomy. The battery inverter is sized to handle the peak load (kW). 

Still, there is even more complexity to consider with off-grid system design. Seasonal variances in sun hours mean that the system needs to either be oversized during periods of higher sun hours or be designed with the average amount of sun hours in mind and paired with a generator for those times with lesser sun hours. System losses like voltage drop, module soiling or snow, or battery inverter losses also affect the performance of off-grid systems. It is best to design conservatively to avoid clients being without electricity or plan on having a backup power source like a generator.

To learn more about design considerations in off-grid solar systems and so much more, join Wes Kennedy in his Comprehensive Solar Plus Storage course.