Solar & Battery Storage: A Procurement Manager’s Practical Sizing Checklist

I’ve negotiated over 50 solar-plus-storage contracts in the last 6 years, tracking every line item in our cost system. When I see someone spec a system without checking SMA string sizing or calculating storage ROI first, I cringe. That’s where the hidden costs live.

This checklist is for solar installers, commercial developers, and utility buyers who want to size a system that actually delivers on its promise—without blowing the budget. I’ll walk you through 5 steps, including one most people skip.

Step 1: Lock Down Your Load Profile (Before You Touch a Panel)

First, grab your last 12 months of utility bills. Not the summary—the interval data (every 15 or 30 minutes). You’re looking for three things:

• Peak demand (kW) – The highest 15-minute usage block.
• Daily energy (kWh) – Total consumption per day.
• Time-of-use patterns – When does your facility hit max draw?

I once worked with a warehouse that assumed they needed a 50 kW system based on their bill total. When I pulled the interval data, their actual peak was 38 kW. We sized it at 42 kW (with a 10% safety margin). Saved $12,000 on hardware up front.

One thing most people skip: Check for demand spikes from equipment startup currents (e.g., HVAC compressors, industrial motors). They’re brief but can trip inverter limits if you size too tight. SMA inverters (particularly the Sunny Tripower series) handle short-term overloads well—up to 120% rated power for a few seconds—but you still need to account for it in string sizing.

“People think biggest solar array equals best savings. Actually, the right system matches your load shape. Oversizing just exports to grid at pennies per kWh.”

Step 2: SMA String Sizing—Get the Voltage Right

This is where I see procurement managers get burned. They buy inverters and panels separately, then discover the string configuration doesn’t work.

Here’s the rule: your string’s open-circuit voltage (Voc) must stay within the inverter’s MPPT range, cold-temperature adjusted. For SMA inverters:

• Minimum string voltage: Must exceed the inverter’s startup voltage (typically 125–150 V for residential Sunny Boy, 200 V for commercial Sunny Tripower). Below that? No power output.
• Maximum string voltage: Can’t exceed the inverter’s absolute max (typically 600 V for residential, 1000 V for commercial). Exceed it, and you trip the protection circuit—no generation, and potentially void warranty.

I built a simple spreadsheet after getting burned on a project in 2023. I’d assumed a 12-panel string with 400W panels would work. Nope—Voc at -10°C was 498 V. My inverter’s max was 500 V. That’s 2 V of runway. In practice, one cold snap and the system shuts down. We re-strung to 11 panels.

Pro tip: Use SMA’s online string sizing tool (via SMA Portal) or Sunny Design. It handles temperature coefficients and cable losses. I always double-check with a manual calc—the tool once gave me a result I didn’t trust. Turned out I’d entered the wrong panel model.

Step 3: Sizing Battery Storage—Start with the Critical Loads

How much battery storage do I need for solar? This is the most common question I get from project developers. My answer: start with what you need to keep running during an outage, then scale up for savings.

Here’s a method I use on every project:

1. Identify critical loads – Lights, refrigeration, servers, pumps. Not the whole facility. Typically 10–30% of peak load.
2. Estimate backup duration – 2–4 hours is enough for most commercial applications. Residential can go 6–8 hours.
3. Calculate kWh needed – Critical load (kW) × backup hours. Add 20% for inverter losses and battery degradation over 10 years.

Example: A small office with critical loads of 8 kW needs 4 hours backup. Base: 32 kWh. Add 20%: 38.4 kWh usable battery capacity.

If you’re pairing with a Sunny Boy Storage (SMA’s battery inverter) or a hybrid inverter, check the battery’s usable depth of discharge. Most lithium-ion batteries (like SMA’s partners, e.g., BYD or LG) offer 90–95% DoD. Lead-acid? Closer to 50%—you’d need double the capacity. That’s a procurement trap I’ve seen multiple times.

Step 4: Use SMA Portal for Monitoring and Tuning (Not Just Reporting)

Most installers set up SMA Portal for basic monitoring. That’s like buying a Ferrari to drive to the grocery store. The real value is in the portal’s diagnostic and tuning features:

• String-level current monitoring: When one string drops, you’ll see it in the portal before the site manager notices. I’ve caught failed MPPT channels and dirty panels this way. Average downtime reduction: 3 days.
• Historical data export: Pull CSV files and compare output to modeled performance. If actual < 90% of modeled for two consecutive months, investigate. I’ve found shading from a new building and a misaligned panel string using this method.
• Firmware updates: New features (like self-consumption optimization) can improve yield 3–5% without hardware changes. I schedule updates quarterly via the portal. One client’s system gained 4% annual yield after a firmware patch in early 2024.

I’ll admit: When I first deployed SMA Portal, I thought it was overkill for a small system. After tracking 18 sites for two years, I changed my mind. The portal’s alert system saved me from a $2,000 overcharge on a failed inverter—the warranty claim was processed within a week because we had logged data.

Step 5: Run the Total Cost of Ownership (TCO) Calculation

Here’s where procurement managers earn their paycheck. Don’t just compare equipment quotes. Use this TCO checklist:

• Upfront hardware: Inverters, panels, racking, battery, cable, combiner boxes, disconnects. Beware of “savings” from undersized wire—it’ll cost you in losses and potential fire hazard.
• Installation labor: Includes commissioning and SMA Portal setup. A complex string sizing config adds 4–6 hours. Budget for it.
• Warranty period and coverage: SMA’s standard is 5–10 years (extendable). Battery warranties vary (10 years/70% capacity retention). Factor replacement cost after warranty.
• Operation & maintenance (O&M): Annual cleaning, inverter fan replacement (every 5 years), battery management system updates. Estimate 1–2% of upfront cost per year.
• Decommissioning cost: Yes, plan for it. Solar panels and batteries have a finite life (25–30 years for panels, 10–15 for batteries). Disposal fees vary—$50–100 per panel.

I compared two 10 kW proposals earlier this year. Vendor A quoted $45,000, Vendor B $38,000. I ran the TCO:

• Vendor A: 10-year warranty, $600/year O&M, $2,000 decommissioning = $53,000 total
• Vendor B: 5-year warranty, $900/year O&M (inverter failed after 6 years), $2,500 decommissioning = $51,500 total

Vendor B looked cheaper, but only saved $1,500 over 10 years—and that’s assuming inverter holds up. If it fails after 7 years (common for budget inverters), total jumps to $55,000. We went with Vendor A.

“The assumption is that lowest bid saves money. The reality is that low-bid projects often have higher TCO because of hidden costs in warranty and O&M. I’ve seen $10,000 ‘savings’ turn into $15,000 extra over 8 years.”

Common Mistakes to Avoid

• Mismatching inverter and panel voltage specs – I’ve seen a 600V inverter paired with panels that hit 620V at cold temps. System doesn’t start. Recheck your string sizing (Step 2).
• Battery storage sized for backup only, ignoring self-consumption – If you’re on net metering but time-of-use, battery can shift solar from low-export hours to peak rates. That’s a 15–30% ROI improvement. Many developers skip this because they assume “backup first.”
• Not using SMA Portal’s advanced features – It’s not just a dashboard. Set alerts for string imbalance. I missed a 6-day generation loss because I didn’t configure the alert threshold—learned that one the hard way.
• Forgetting the inverter’s CEC efficiency rating – SMA inverters are typically 96–98% CEC efficiency. But cheaper inverters? 94–95%. That 2–3% difference translates to 200–300 kWh/year lost on a 10 kW system. Over 25 years, that’s 5,000–7,500 kWh—$750–1,125 in lost revenue (at $0.15/kWh).

One last thing: Don’t trust a vendor who says “guaranteed 25-year lifespan” on any inverter. Electronics fail. Period. Budget for replacement at year 10–15. SMA’s reliability is excellent (I’ve had two failures out of 200+ units deployed), but I still plan for it in my procurement spreadsheet.

This checklist works when you follow it from step 1—not backwards. I’ve seen teams design the system, then check load profile as an afterthought. That’s how you end up with a $50,000 system that exports 40% of generation at wholesale rates. Run the numbers first, size later.