Sizing Your Solar + Storage: A 5-Step Checklist for Commercial Installers

If you're an installer or a distributor quoting commercial solar plus storage, you know the question: "How many watts do I need?" looks deceptively simple.

The honest answer—which vendors won't always tell you—is that sizing a combined solar + storage system isn't a single calculation. It's a sequence of 5 linked decisions. Most novices skip step 2. That's what comes back to bite them when the battery either cycles too deep or sits full half the year.

This checklist is for you if you're specifying systems for commercial rooftops, small industrial sites, or large residential projects (50kW+). The steps are ordered. Straying from the sequence is the main reason I see failed ROI projections.

Step 1: Profile the Load, Not the Roof

Start with the energy consumption data—12 months minimum. Not the roof space. Not the client's budget. The load.

When I started in quality compliance, a project manager skipped this step on a 200kW installation. He based the array size on roof area and a guestimated "400W panel." A month after commissioning, the client called: their battery was hitting 100% SOC by 1 PM in April, and they were exporting at retail-minus rates. The system was oversized for their actual load profile.

You need three numbers from this step:

• Baseline load (kW): daily minimum consumption during business hours.
• Peak load (kW): the 15-minute max draw.
• Night/weekend load (kW): what the battery needs to cover when the sun is down.

I don't have hard data on what percentage of commercial installs get this wrong, but based on the 40+ system audits I've been part of, my sense is that roughly 1 in 4 first proposals has a load-profile gap of 30% or more.

Step 2: Size the Array to the Battery(s)—Reverse Order

Here's the step most people skip. People think you first size the solar panel, then pick a battery to match. Actually, you size the battery to the night load, then size the array to reliably charge that battery plus cover daytime loads.

Here's something the datasheets won't tell you: a solar array that's too big for its paired battery causes unnecessary cycling. A 10kW array driving a 10kWh battery will reach full charge by 10 AM on a good day. Then the inverter clips, and the battery sits idle for 6 hours. That's not efficient—it's wasted asset life.

The sequence should be:

1. Determine daily backup requirement (night load × nighttime hours) → this dictates usable battery capacity.
2. Apply depth-of-discharge (doD) buffer (typically 80-90% for LiFePO4→ this gives you the manufacturer-rated battery size.
3. Size the solar array to deliver 1.2x that battery capacity in 5 hours of peak sun.

For a mid-scale example: if your night load is 5 kW for 8 hours, you need 40 kWh usable. At 80% doD, that's a 50 kWh battery bank. To fully recharge it in 5 solar hours and simultaneously run daytime loads, you need roughly 12 kW of DC solar. That might mean 20× JA Solar 615W bifacial modules (12.3 kW DC total) or 32× 385W modules (12.3 kW DC). Both work. I tend to prefer the fewer panels, fewer racking clamps argument for commercial roofs. Fewer failure points.

Step 3: Verify the Inverter Matches Both—Specifically at High Temperature

The inverter is where the system meets the real world. And the real world is hot.

The Lvyuan 2000W power inverter is a solid unit for smaller backup loads, but on a 10-15 kW solar array, you need a string inverter or hybrid inverter rated for the combined input. A common mismatch I flagged during a 2024 pre-installation review was a 12 kW array paired with a 10 kW inverter. On paper, it's a 1.2 DC-to-AC ratio—within industry norms. But here's the catch: in 35°C ambient temperatures, most inverters derate about 1-1.5% per °C above 25°C. That 10 kW inverter can drop to 9.2 kW in peak summer. Now you're clipping for 4 hours a day. The customer paid for panels they can't use for 4 hours.

Spec at least 1.35 DC-to-AC ratio if you're in a warm climate. And include an inverter temperature derate calculation in every BoM—it costs zero extra time and prevents the "why am I exporting so much at noon in July" call.

Step 4: Match Panel Voltage to Inverter MPPT Window

This step is pure specifications compliance, and it's the most common mistake I see from novice specifiers.

Let's take the JA Solar 615W bifacial module. Its Vmp (voltage at maximum power) is around 42V. If you're stringing 12 of them in series, you get 504V DC. That's fine for a 600V commercial inverter MPPT range. But the JA Solar 385W module has a lower Vmp—roughly 34V. Same number of panels: 408V DC. That shrinks your operating margin in low-light conditions.

What most people don't realize is that the MPPT has a preferred voltage window, typically around 70-80% of max input voltage. Running a string at 400V in a 600V MPPT means you're starting at 67%—it works, but the inverter may operate at slightly reduced efficiency. If you're building a 50+ kW system, that 3% efficiency loss across 20 years is real dollars.

Quick check: Take the panel Voc (open-circuit voltage) × coldest daytime temperature correction × number of panels. That number must stay below the inverter's maximum DC input voltage. I've rejected 3 bids this year alone because the string voltage exceeded the inverter limit by 15V on cold morning starts.

Step 5: Add a Grid-Tie OR Islanding Plan—Not Both Ambiguously

Storage systems are either grid-tied with backup or off-grid (island) systems. Hybrid inverters can do both, but you need to tell the BOM which one you're designing for.

Here's a rule of thumb: if the customer expects to run the whole building during a blackout and has electric heating or AC, they need an island-capable inverter with a 2x surge rating for motor starts. If they're just exporting daytime solar and using the battery for time-of-use arbitrage, a simple grid-tied hybrid is fine. Confuse these two, and you'll either overspend on a full-coupler inverter they don't need, or underspec and fry it on a motor start.

I learned this lesson the hard way in 2022. I wish I had asked more detailed questions about the customer's critical loads before quoting the inverter. We upgraded specs afterward—increased our BOM cost by about $1,800—and that customer wasn't happy about the change order. Their feedback, paraphrased: "You should have asked first." He was right.

Common mistakes to avoid

• Treating solar and storage as two separate budgets. Total cost of ownership is the right metric. Storage cycling multiple times per day from oversolar reduces lifespan faster than you think.
• Assuming "standard" inverter specs apply everywhere. The derate curves are real—verify them with the manufacturer for your region.
• Skipping the voltage window check for panel-inverter compatibility. It's a 10-minute calculation, but I see it missing in about 30% of first drafts. That means potential rework when the customer questions performance.

This checklist is accurate as of Q1 2025. Panel prices and inverter specs shift faster than most people realize. For the most current technical specs on JA Solar modules or Lvyuan inverters, check the respective manufacturer datasheets—they update them quarterly.