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Black Hills Home Builders

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  • Sonny Rivers

Sizing a Solar Power System

In many cases, sizing a solar power system comes down to the available space to install solar panels. Systems may also be sized based on energy usage. At least one of these metrics apply in all cases.

Physical capacity

Roof mounted systems are often sized according to the the roof space, and systems mounted on the ground (usually larger systems) are typically sized for higher performance or capacity than a roof will accommodate.

In cases where space is limited, a higher efficiency panel might be offered to make the most of it. In cases where there is more than enough space, the system size is usually determined by energy usage.

First we look at the roof either by satellite image, contractors drawings, and/or getting directly on the roof in person. If suitable and shade-free, we figure the maximum number of solar panels to fit. The best fit for aesthetic appeal is also considered.

Energy capacity

To design based on energy use, a 12-month summary can be obtained from individual power bills or be requested from the utility. Some bills display a monthly bar chart, which works well to estimate average usage. (Since every year varies, precision is not critical.) Our sizing tools allow us to closely approximate the optimal system size depending on the energy needs and objectives.

For grid-tie options without batteries, solar capacity is usually optimal in the 30% - 50% range (of electricity to offset). Larger systems benefit from economy-of-scale, but also produce excess power that either sells to the grid or is stored with a battery system.


In many cases there is no single best system size, but instead a range to fit the combination of usable space, preferred layout, a percentage of total energy use, economy-of-scale, or fit a budget. It is also optional to add additional solar panels and/or batteries over time.

From an investment perspective, smaller systems generate a faster return-on-investment, while larger systems have a larger return in the long-term.