Who this is for: Facility managers, operations directors, or business owners looking at battery storage to reduce demand charges or add backup to a solar electric system for home or business scale. If you're trying to figure out the right size for a commercial battery storage system and want to avoid a costly overspend, this checklist is for you.
I manage procurement for a mid-sized manufacturing facility (about 200 people). We just installed a battery storage system this year. I spent six months getting the sizing right. Here is the 5-step checklist I wish I had at the start.
Step 1: Map Your Load Profile—Not Just Your Average Bill
This is where most people get it wrong. They look at their annual kWh usage and try to find a 'best battery pack' that matches. Don't do that.
You need your 15-minute interval data for at least 12 months. Your utility can give you this. I had to file a request with ours. It took two weeks.
- Look for your peak demand. Not the average. The single highest 15-minute window in the last year.
- Check the duration of those peaks. Is it a 15-minute spike when the AC kicks on, or a 2-hour production grind? A residential energy storage system might only need to shave a 30-minute peak. A commercial system often needs to cover a sustained 2-4 hour window.
When I audited our 2023 data, I found our peak demand was 30% higher than the second-highest peak. It happened exactly three times—during a heatwave in July. If we had sized for that absolute peak, our battery would be sitting idle for 362 days a year.
Step 2: Define Your Primary Use Case—Shaving vs. Backup
What most people don't realize is that a battery designed for peak shaving and one designed for backup power have very different cost profiles.
Ask this question: What is the primary reason for this investment?
- Demand Charge Reduction (Peak Shaving): You want the battery to discharge during your highest utility rate periods. This typically means a system sized for 1-2 hours of output at your peak demand level. This is usually the best ROI for commercial battery storage.
- Backup Power (Resilience): You want to run critical loads during a grid outage. This means a larger system based on how many hours of autonomy you need. My boss wanted both. I had to explain the math: a system that does both well costs about 60% more than one optimized just for shaving.
I assumed 'sizing' was just a math problem. Didn't verify the use case priority with my stakeholders. Turned out the operations team wanted backup for the server room, while finance wanted to cut the demand charge. Those are different systems. We ended up with a compromise: shaving with a 20% buffer reserved for critical loads.
Step 3: Calculate the 'Usable' Capacity—Not the Nameplate
This is an insider tip: the total kWh on the spec sheet is not what you can use. Nameplate capacity is the total energy stored. Usable capacity is what the system can safely discharge without degrading the battery too fast.
For lithium-ion systems (the standard for a solar and wind hybrid system for home or commercial use), most manufacturers recommend a Depth of Discharge (DoD) of 80-90%. That 100 kWh nameplate system? You can only use 80-90 kWh per cycle.
- Check the DoD in the warranty. Some manufacturers void warranties if you cycle past 80% regularly.
- Factor in efficiency losses. You lose about 10-15% in the charge/discharge process (round-trip efficiency). So 100 kWh in gives you 85 kWh out. Roughly speaking.
Vendor A quoted a system with a 200 kWh nameplate. Vendor B quoted 180 kWh. I almost went with A until I calculated usable capacity: A's 80% DoD = 160 kWh usable. B's 90% DoD = 162 kWh usable. The 'bigger' system wasn't actually bigger. That's a 10% difference hidden in the fine print.
Step 4: Factor in Solar Coupling (If You Have It or Plan to)
If you have a solar electric system for home or commercial use, the battery sizing math changes. A standalone system sizes for demand. A solar-coupled system sizes for excess generation.
First, check your solar array's peak export. If your panels generate 100 kW at noon but you only consume 40 kW, you're sending 60 kW to the grid. A battery that can capture that 60 kW for 2 hours needs a 120 kWh inverter capacity, not just a 100 kWh storage capacity.
The 'cheap' option—just adding a battery to your existing solar inverter—resulted in a $1,200 redo when we realized the inverter couldn't handle the combined load from the solar array and the battery discharge. We had to buy a separate inverter. Not ideal, but workable.
Step 5: Run the Total Cost of Ownership (TCO)—Not Just the Quote Price
Here's something vendors won't tell you: the first quote is almost never the final price.
I built a cost calculator after getting burned on hidden fees twice. Here's what goes into TCO for a battery storage system:
- Base equipment price. The battery and inverter.
- Installation. Sometimes quoted separately. Expect 15-25% of equipment cost. Some quotes include it. Ask.
- Grid interconnection fees. Your utility might charge $500-$5,000 to connect the system. In Q2 2024, when we switched vendors, the new one's standard quote didn't include this. It cost us an extra $2,200.
- Warranty and degradation. A 10-year warranty that guarantees 70% capacity is different from one that guarantees 80%. Calculate the replacement cost in year 12.
"The vendor who said 'this isn't our strength—here's who does it better for grid interconnection' earned my trust for everything else."
Common Mistakes to Avoid
- Sizing for the 'one bad day.' That absolute peak we found in Step 1? Size for the third or fourth highest peak. The cost to capture the absolute peak is usually not worth it.
- Ignoring future load growth. We planned for our current load. In year 2, we added a new production line. Our battery is now undersized by 20%. Consider a modular system where you can add capacity later.
- Assuming the 'standard' system is correct. Every facility is different. A solar and wind hybrid system for home might be sized perfectly by a rule of thumb. Commercial is different—it rewards custom analysis.
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