How Many Solar Panels Do I Need? (By Home Size + Electric Bill)

Sizing by Monthly Electric Bill

The most accurate way to size a solar system is by your actual electricity usage, not your home size. Here is a quick reference for New England and Mid-Atlantic states:

Step-by-Step System Sizing Calculation

Let's walk through a real sizing calculation for a Massachusetts homeowner with a $280/month electric bill. Their utility rate is $0.28/kWh, so their annual consumption is:

The Sizing Formula

Common Sizing Mistakes: Oversizing and Undersizing

Undersizing also reduces your net metering credits in summer. If you produce 1,200 kWh in June but use 900 kWh, you bank 300 kWh in credits. In December, you produce 400 kWh and use 1,200 kWh, drawing 800 kWh from the grid. Your 300 kWh banked credits cover part of it, but you still buy 500 kWh at retail. A properly sized system would have banked 600 kWh in summer to fully cover winter shortfalls.

How Shading Reduces Effective Panel Count

A single tree shading 3 panels on a string inverter system can reduce the entire string's output by 40–60%. String inverters connect panels in series, so the shadiest panel determines the current for the whole string. If one panel produces 200W instead of 400W due to shade, all panels in that string drop to match it.

Microinverters solve this problem. Each panel has its own inverter, so shading on one panel only affects that panel. If 3 panels are shaded out of 20, you lose 15% production instead of 60%. For roofs with chimneys, dormers, or tree shade, microinverters often justify their 10–15% higher cost by recovering the shaded production loss.

When sizing a system for a shaded roof, installers should calculate usable roof area excluding heavy shade zones. If you have 600 sq ft of roof but 200 sq ft is shaded by a chimney and trees, you have 400 sq ft available. At 18 sq ft per panel, that is 22 panels maximum. If the sizing calculation says you need 25 panels, you either need to trim trees, use higher-wattage panels (440W instead of 400W), or accept an undersized system.

Plan for Future Energy Needs

Seasonal Production Variation

Solar production varies 3–4x between winter and summer. A Massachusetts system producing 1,400 kWh in June might produce only 400 kWh in December. This is due to shorter days (9 hours of daylight vs 15 hours), lower sun angle (sun is 25° above horizon in winter vs 70° in summer), and cloud cover.

This seasonal swing is why annual net metering is essential. Without credit banking, you would overproduce in summer (wasting the excess) and underproduce in winter (buying expensive grid power). With annual net metering, summer overproduction credits roll forward to cover winter shortfalls, and your year-end net consumption is what matters.

When sizing a system, always use annual production estimates, not monthly. A system sized to offset your December usage would produce 3–4x your needs in June, resulting in massive excess credits at true-up. Size for annual offset and let net metering handle the seasonal variation.

Roof Factors That Affect Panel Count

• Orientation: South-facing is ideal (100% production). East/west faces produce 80–85%.
• Pitch: 15–40 degrees is optimal for most latitudes. Flat roofs use tilted racking.
• Shading: Trees, chimneys, dormers reduce available space. Microinverters mitigate partial shade.
• Available area: Each panel needs ~18 sq ft. A 10 kW system needs ~400 sq ft of usable roof.

Real-World Example: The Johnson Family