How to Size a Battery System for Your Commercial Property

Battery storage is one of the most capital-intensive decisions in a commercial solar project — and one of the most frequently sized incorrectly. Installers often recommend systems based on what they have in stock or what maximizes their margin. Independent consultants start from your actual load data.

Start With Your Load Profile, Not a Rule of Thumb

The most common mistake in battery sizing is using average daily consumption as the primary input. A facility that uses 500 kWh per day but concentrates most of that usage in a two-hour afternoon peak has very different storage needs than one with flat consumption throughout the day.

Proper sizing starts with 12 months of interval data — typically 15-minute readings from your utility's smart meter. This data reveals your actual peak demand windows, the duration of those peaks, and how much energy you'd need to store to meaningfully shift consumption away from high-rate periods.

The Three Goals of Commercial Battery Storage

Most commercial battery systems are sized to accomplish one or more of these objectives:

• Demand charge reduction: Many commercial utility rates include a demand charge based on your peak 15-minute interval each month. A battery can shave that peak, reducing a charge that often represents 30–50% of a commercial electric bill.
• Time-of-use arbitrage: Store solar energy during low-rate periods and discharge during high-rate evening peaks, reducing grid purchases when rates are highest.
• Backup power: Provide resilience during outages. This requires sizing for critical loads and duration, not just economics.

What Oversizing and Undersizing Actually Cost You

A battery system that's 50% larger than necessary might cost $80,000–$150,000 more for a mid-size commercial installation — capital that earns no additional return. Conversely, a system that's too small to cover your peak demand window leaves demand charges largely intact, undermining the financial case for storage entirely.

The right size is the one that maximizes your return on the marginal dollar invested in storage — and that calculation requires your actual utility rate schedule, your interval data, and a financial model that accounts for battery degradation over time.

Technology Considerations

Lithium iron phosphate (LFP) chemistry has become the standard for commercial applications due to its safety profile, cycle life (typically 4,000–6,000 cycles), and improving cost per kilowatt-hour. Systems from Tesla, Enphase, SunPower, and others all use LFP for commercial-scale applications.

Round-trip efficiency (typically 90–95% for modern LFP systems) and warranty terms (usually 10 years with a guaranteed capacity retention floor) should be factored into any financial model. An independent consultant will compare these specifications across competing proposals rather than defaulting to a single manufacturer.