The Simple Formula to Calculate Your Solar System Size
Every solar installer uses a variation of the same core calculation. You can do it yourself in 60 seconds with your electricity bill:
System Size Formula
kW needed = Monthly kWh ÷ (Peak sun hours × 30 × 0.80)
Where: Monthly kWh = from your electricity bill | Peak sun hours = from NREL PVWatts for your zip code | 0.80 = real-world efficiency factor
System Size by Home Type and Usage
| Home Type | Monthly Usage | System Size (Avg Sun) | Panels (400W) | Est. Cost After ITC |
|---|---|---|---|---|
| Small apartment / condo | 400 kWh | 3.2 kW | 8 panels | $6,700 |
| Small home (1,200 sq ft) | 600 kWh | 4.8 kW | 12 panels | $10,100 |
| Average home (1,800 sq ft) | 900 kWh | 7.2 kW | 18 panels | $15,100 |
| Large home (2,500 sq ft) | 1,200 kWh | 9.6 kW | 24 panels | $20,200 |
| Large home + 1 EV | 1,500 kWh | 12.0 kW | 30 panels | $25,200 |
| Large home + 2 EVs + pool | 2,000 kWh | 16.0 kW | 40 panels | $33,600 |
System Size by Location — Same Home, Different Sun
Your location dramatically affects how many panels you need. A home using 900 kWh/month needs very different system sizes across the US:
| City | Sun Hours | System Size for 900 kWh/mo | Number of 400W Panels |
|---|---|---|---|
| Phoenix, AZ | 6.5 | 5.8 kW | 15 panels |
| Los Angeles, CA | 5.8 | 6.5 kW | 17 panels |
| Dallas, TX | 5.4 | 6.9 kW | 18 panels |
| Denver, CO | 5.3 | 7.1 kW | 18 panels |
| New York, NY | 4.7 | 7.9 kW | 20 panels |
| Boston, MA | 4.5 | 8.3 kW | 21 panels |
| Seattle, WA | 3.9 | 9.6 kW | 24 panels |
Should You Size Solar for 80%, 100%, or 110% of Usage?
Most installers recommend sizing for 100% of your annual electricity consumption. Here's the logic for each approach:
| Offset Target | Pros | Cons | Best For |
|---|---|---|---|
| 80% offset | Lower upfront cost, less excess production | Still pay electricity bills year-round | Budget-constrained buyers |
| 100% offset | Eliminates electricity bills, optimal ROI | Some excess production in summer | Most homeowners |
| 110–120% offset | Covers EV charging, future usage growth | Higher upfront cost, excess may earn low export rate | Homes planning EV or battery addition |
What Your Electricity Bill Tells You
The most important number for solar sizing is your total annual kWh usage. Find it on your electricity bills — many utilities show 12-month usage on each bill. Divide by 12 for your monthly average. Key tip: use a full year of data, not just one month. Summer and winter usage often differ significantly, and annual average is the right input for solar sizing.
If you're adding an EV soon, add 300–400 kWh/month to your baseline. If you're planning a home addition, hot tub, or pool, add estimated usage for those loads. Solar sized today should reflect where your usage will be in 2–3 years, not just today's bills.
Roof Space: Do You Have Enough?
Each 400W panel takes approximately 18–19 sq ft of roof space. Here's how much roof area different system sizes require:
| System Size | Panels (400W) | Roof Space Needed | Typical Roof Available |
|---|---|---|---|
| 6 kW | 15 panels | 270–285 sq ft | ✅ Most homes |
| 8 kW | 20 panels | 360–380 sq ft | ✅ Most homes |
| 10 kW | 25 panels | 450–475 sq ft | ⚠️ Larger homes |
| 12 kW | 30 panels | 540–570 sq ft | ⚠️ Needs good south-facing area |
If your roof can't accommodate your ideal system size, high-efficiency panels (22%+ efficiency, like SunPower Maxeon or REC Alpha) produce more power per square foot — letting you fit a larger system in the same space, at a 30–40% price premium per watt.