Sizing a Solar Power System

In many cases, sizing a solar power system simply comes down to the space available, but systems are also sized based on energy usage and the type of system and functions the homeowner is most interested in.

Sizing a solar system doesn't have to be complicated. There are generally three main factors to consider when sizing a system. We just need to learn about your interests, the home, and your electric bill to offer a recommendation.

Type of system

Homeowners are choosing solar for a variety of reasons, and their specific situation helps to determine the best approach to sizing the solar power system.

  • Off-grid homes need maximum solar power (and usually ground-based arrays).

  • Power bill reduction (for return on investment) works out best with smaller power systems that only cover only the average daytime power usage (and do not include batteries).

  • Back-up power applications can be small or large, depending on the anticipated duration of the power outage and the number of loads to back up (essential loads only or whole-home).

  • Net-zero systems generate as much electricity as they consume per year, and are usually large systems (benefit of economy-of-scale rather than return on investment).

  • Vehicle charging systems can be sized based on the daily mileage of the vehicle plus the home's electric usage (the vehicle must be charging during the daytime).

Other specific scenarios exist, but most cases fall into one of the above categories.

Maximum capacity (square footage)

Sizing for a roof begins with a look at the roof by satellite image, contractor's drawings, or seeing the roof in person in order to get dimensions. We generally start by measuring the width of the roof to see how many solar panels will fit as one row across, and then how many rows vertically. This is ideal when the homeowner may start with a small system and add additional rows of panels in the future. We will determine the maximum amount of solar that can fit, select the preferred areas, and then select the most suitable layout option(s) for the situation (it is not necessary to fill to maximum capacity).

More complex roofs need more careful attention to dimensions, while locations with numerous pipes and chimneys may limit space as well and need to be seen in person. It is also important that the layout of the solar array is aesthetic, and not simply to fit the maximum number of panels possible. In some cases, a plumber may be hired to relocate nuisance pipes to a northern side of the roof to eliminate gaps or awkward layouts of the solar panels.

Sizing for a ground mount is usually the alternative option when the roof is not favorable for any reason, but ground-mounts are obviously limited to suitable space as well. Most urban properties don't have this option due to smaller lots, but when possible the location must be a wide-open space free of shading. The benefit is that when ground-mount is an option, it is often possible to size for the maximum capacity the home needs.

Energy capacity

We also want to match the solar capacity with the energy needs. It is not particularly desirable to generate more solar power than the home can use (unless the utility offers a desirable rate of compensation for powering the grid).

To design based on energy use, details can be obtained from individual power bills or a by requesting a 12-month usage and billing summary from the utility (or checking the online account summary). Some bills also display a monthly bar chart, which works well to quickly estimate monthly use.

For new construction or new homeowners, the energy requirement is estimated according to the total loads expected (especially major appliances), the size of the home, and the number of occupants. In any case, precision is not necessary but we want to be accurate if estimating specific returns or benefits.