Solar + Storage (Commercial) Calculator

The battery's sustainable discharge rate depends on how long it needs to sustain output — a battery can discharge at full rated power for only a limited time before its usable energy runs out, so the calculator divides battery energy capacity by a 4-hour assumed window to find the power level it can hold that long. If your building's real demand peak is shorter or longer than 4 hours, the battery's effective demand-shaving contribution will differ from this estimate.

Demand charges are based on your single highest 15-minute usage spike in the billing period, and that spike doesn't necessarily occur at solar noon when production peaks — it might happen in the morning ramp-up, late afternoon, or be driven by equipment cycling unrelated to sun angle. The 30% assumption is a rough estimate of how much solar output is likely still available during a typical peak window; if your facility's actual peak reliably coincides with full solar output, real demand reduction could be higher than this projects.

It applies your entered round-trip efficiency (energy out divided by energy in) only to the portion of solar output assumed to cycle through the battery — modeled as about half of daily solar production. The resulting losses are subtracted from total annual solar production before that usable energy is valued at your energy rate, so a lower round-trip efficiency directly reduces annual energy savings, separate from the demand-charge savings calculation.

The biggest risk is a mismatch between your facility's actual peak-demand timing and the calculator's built-in assumptions about the peak window length and solar overlap — a facility whose peak occurs during an evening shift, for example, gets no solar contribution at all despite what the 30% assumption projects. Because the tool doesn't verify your interval-level demand data, it's best treated as a sizing starting point rather than a guaranteed savings figure.