Solar Panels for For A Duplex

Duplex Solar Panel Count by System Size (2026 Reference Table)

The number of panels you need depends on the wattage of each panel. Most residential installs in 2026 use 400W–430W modules. Here’s how the math plays out across common system sizes for a duplex:

Duplex Solar Panel Count by System Size (2026)

*Output estimates assume 4.0–5.0 peak sun hours/day, averaged nationally. Southern states and Southwest locations land at the high end; Pacific Northwest and New England at the low end.

A duplex with average combined consumption (~1,700 kWh/month) in a state with 4.5 peak sun hours needs roughly a 10–12 kW system — about 25 to 30 panels at 400W each. Before installation day, confirm your roof has 600–750 sq ft of usable south-facing area; each panel occupies roughly 18–22 sq ft. Panel degradation averages 0.5% per year per NREL data, so a system producing 14,000 kWh in year one will produce about 11,700 kWh by year 25 — size your system with that decline in mind.

One Meter vs. Two Meters: Which Solar Setup Works for a Duplex?

This is the question most duplex owners skip — and it’s the one that causes the most complications after installation. Duplexes typically have one of three metering configurations, and your solar system design must match it precisely.

Single shared meter (owner pays utilities): One solar array, one inverter, one interconnection agreement with the utility. All net metering credits flow to one account. This is the simplest setup from a permitting standpoint and works well when the owner occupies one unit or pays both electricity bills directly.

Two separate meters (tenants pay their own bills): You have two options — install two separate smaller systems (one per unit, each with its own interconnection agreement), or install one larger shared array with a sub-panel arrangement that allocates production credits to each unit proportionally. The two-system approach is cleaner legally and avoids tenant disputes. The shared-array approach requires a more complex inverter setup and utility approval, but cuts permitting and equipment costs.

Virtual net metering: A handful of states — including California, New York, and Minnesota — allow multi-tenant buildings to allocate solar credits across separate meters without physical sub-metering hardware. Check DSIRE for your state’s specific virtual net metering rules before signing any installation contract, as program eligibility varies by utility territory.

For most owner-occupied duplexes where the owner controls both meters, a single 10–12 kW system on a string inverter is the path of least permitting resistance. For rental duplexes with independent tenant billing, ask your installer about a split-inverter design or virtual net metering eligibility before committing. People frequently ask whether a duplex qualifies for net metering the same way a single-family home does — in most states, yes, as long as the system is behind the meter and sized to the building’s annual load.

How to Size a Duplex Solar System Step by Step

If you want to size a duplex solar system yourself before getting installer quotes, here’s the four-step method solar engineers use. Running these numbers gives you an independent benchmark — quotes that land more than 15% above or below your result are worth questioning directly.

Step 1 — Calculate combined monthly kWh. Add 12 months of electricity bills for both units. Divide the annual total by 12 for a monthly average. Example: 18,000 kWh per year ÷ 12 = 1,500 kWh per month.

Step 2 — Find your location’s peak sun hours. NREL’s solar resource maps show daily peak sun hours by ZIP code. Most of the U.S. falls between 3.5 and 6.0 hours. Phoenix averages 5.8; Boston averages 4.0; Seattle averages 3.5.

Step 3 — Calculate raw system size. Divide your daily kWh need by peak sun hours, then add 20% for inverter losses and temperature derating. Formula: (1,500 kWh ÷ 30 days) ÷ 4.5 peak sun hours × 1.20 = 13.3 kW.

Step 4 — Convert to panel count. Divide system wattage by your chosen panel wattage. 13,300W ÷ 400W = 33 panels. At 430W panels: 13,300W ÷ 430W = 31 panels.

A 13 kW system at $3.00/watt installed costs $39,000 before incentives, dropping to $27,300 after the 30% ITC — well within the range where solar delivers a positive return on a duplex over a 25-year panel lifespan. People also frequently ask how rooftop obstructions affect sizing: every 10% of shading on the array reduces output by roughly 10–15% with a string inverter, and may push your required panel count up by 3–5 panels. Microinverters or power optimizers mitigate shading losses and are worth the $1,500–$3,000 premium on complex rooflines.

Use our Solar ROI Calculator to model the full 25-year return on your duplex solar investment with your actual utility rate and incentive stack.