How to Verify Your Solar System Is Performing Correctly

How can I tell if my solar system is performing correctly?

Many homeowners install solar and trust monitoring without knowing what to look for. Here's a complete framework: what numbers matter, how to compare to expectation, and red flags that something's wrong.

The key metric: Performance Ratio (PR)

Performance Ratio = Actual production ÷ Expected production

• Healthy system: PR = 0.75 to 0.85
• Good system: PR = 0.85 to 0.92
• Excellent: PR > 0.92 (rare; usually means modeled expectation was conservative)
• Problem: PR < 0.70 (something is degrading or shaded)
• Critical: PR < 0.50 (major problem — investigate immediately)

Why PR isn't 1.00

"Expected production" is what your panels would produce in a lab. Real-world losses:

• Module mismatch: 1-3%
• DC wiring losses: 1-2%
• Inverter conversion: 2-4% (98-99% peak; lower at low loads)
• AC wiring: 0.5-1%
• Soiling: 2-5%/yr (varies by climate)
• Temperature: 5-15% lower output in summer in hot climates
• Shading: 0-30% depending on site
• Inverter clipping: 0-3%
• Annual degradation: 0.5%/yr typical

Total: 13-25% loss from STC nameplate. PR of 0.75-0.85 is "as designed."

How to calculate your expected production

Free tools (do this monthly)

• NREL PVWatts (pvwatts.nrel.gov): plug in your address, system size, tilt, azimuth, derate factors. Get monthly + annual expected kWh.
• Google Project Sunroof: shade-aware satellite estimate for your specific roof.

Your installer's design model

Your install contract probably included a "design model" giving expected first-year production (e.g., 13,500 kWh in year 1). Compare:

• Year 1: actual vs modeled. Within 90-100%? OK. Below 85%? Problem.
• Year 2-5: account for 0.5%/yr degradation (so year 5 = 97.5% of year 1).
• Year 10: 95% of year 1 typical. Below 88%? Investigate.
• Year 25: 87% of year 1 (12 years of degradation).

Daily monitoring routine

What to check daily (or every few days)

• Today's production matches recent days (controlling for cloud cover).
• Inverter status: green/operational. No persistent fault codes.
• Battery state of charge if applicable.
• Module-level production uniformity (Enphase Enlighten or SolarEdge per-optimizer).

Red flags during day

• One module/string consistently underproducing vs neighbors.
• Unexpected zero production during sunny periods.
• Inverter restart loops.
• Battery not charging or discharging as expected.

Weekly review

• Total kWh week vs same week last year (or vs PVWatts forecast for the week).
• Net consumption from grid: imports rising? Loads going up?
• Battery cycles: reasonable depth (30-90% typical for daily cycling)?

Monthly review

• Compare monthly kWh to PVWatts forecast for your zip code. Should be within +/- 10% in moderate-cloud months.
• Check utility bill for net consumption / export credits.
• Verify monitoring is reporting consistently: WiFi/cellular link OK? No data gaps?

Annual review (the big one)

Step 1: Total annual production check

• Last year's total: ___ kWh
• Original modeled (year 1) - 0.5%/yr degradation = expected ___ kWh
• Ratio = ___% (should be 90-110%)

Step 2: Module-level analysis

• (Enphase) Look at "Site" view: any individual module producing <90% of neighbors?
• (SolarEdge) Per-optimizer view: same check.
• (String inverter) Visit the inverter, log fault codes, look for IGBT/transformer issues.

Step 3: Visual inspection

• Walk around the array. Look for: cracked glass, brown/burn spots, dirt/debris, vegetation overgrowth, vibrating racking, cracked sealant, exposed wiring.
• (For roof installs, use binoculars from ground — don't climb on roof unless you must.)
• Check inverter LED status, time/date display, fault history.

Step 4: Battery health (if applicable)

• Capacity test: full discharge then charge, observe usable kWh.
• Check warranty cycle/throughput remaining.
• Most batteries report battery health % in app (e.g., Tesla shows 85% remaining capacity at year 5).

Step 5: Compare to baseline

• Year 1: lock in your baseline.
• Year 2: compare to year 1 - 0.5% degradation. If higher than 1% drop, investigate.
• Year 5: should be ~97.5% of year 1 baseline.
• Year 10: ~95%.
• Year 25: ~87%.

Common production shortfalls and what they mean

5-10% shortfall (year 1+)

• Optimistic install model.
• Higher-than-expected soiling or shading.
• Single module underperforming 10-20%.
• Microinverter or optimizer drop-out (1 of N missing).
• Compare your shading at install time to current (tree growth?).

10-25% shortfall

• Inverter clipping (DC oversize relative to inverter capacity).
• Major shading change (tree canopy expanded, new neighbor structure).
• Multiple module failures.
• String inverter fault.
• System hasn't been commissioned correctly.

25%+ shortfall

• Major system issue: inverter failure, multiple module damage, electrical fault.
• Service call needed promptly.

What to do when you spot a problem

1. Document: screenshots of monitoring, photos of any visible damage, write down dates and observations.
2. Compare: use PVWatts to confirm the shortfall is significant (not weather).
3. Contact installer first. They diagnose under workmanship warranty (typically 5-10 years).
4. If installer is unresponsive or out of business: see defunct installer playbook.
5. If issue is at component level: contact manufacturer directly (Enphase, SolarEdge, Tesla, etc.) for warranty. See O&M / RMA guide.
6. Production guarantees: if your contract had one, file claim in writing.

Annual maintenance recommendations

• $200-500/yr: hire a service technician to check the system once or twice/yr. Cost-effective vs lost production.
• Visual inspection: 1-2x/yr by you (binoculars from ground).
• Cleaning: usually unnecessary; rain handles it. Hire only if you see significant soiling and cost-justify.
• Inverter inspection: at year 10-15, expect inverter end-of-life conversation.
• Battery inspection: at year 12-15, plan for potential replacement.

Frequently asked questions