How to Size Your Home Solar System: A Step-by-Step Technical Guide
Step 1: Calculate Your Daily Energy Consumption
Start with 12 months of electric bills. Find monthly kWh and divide by 30 for daily average.
Example: 12,000 kWh annual ÷ 365 = 32.9 kWh/day
If building up from appliances:
| Appliance | Watts | Hours/Day | kWh/Day |
|---|
| Refrigerator (modern) | 150W avg | 24 | 3.6 |
|---|
| HVAC (3-ton) | 3,500W | 8 | 28.0 |
|---|
| Water heater (electric) | 4,500W | 2 | 9.0 |
|---|
| Lighting | 200W total | 5 | 1.0 |
|---|
| TV + entertainment | 200W | 4 | 0.8 |
|---|
| Computers | 300W | 8 | 2.4 |
|---|
| Washer/dryer | 3,000W | 1 | 3.0 |
|---|
| <strong>Total</strong> | <strong>47.8</strong> |
|---|
For off-grid design, add 15% safety margin.
Step 2: Determine Your Peak Sun Hours
Peak Sun Hours (PSH) is the number of hours per day when sunlight intensity equals 1,000 W/m² — not hours of daylight.
Key US Cities (annual average):
| City | PSH | City | PSH |
|---|
| Phoenix, AZ | 6.58 | New York, NY | 4.08 |
|---|
| Las Vegas, NV | 6.41 | Chicago, IL | 4.08 |
|---|
| Los Angeles, CA | 5.84 | Seattle, WA | 3.73 |
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| Denver, CO | 5.76 | Boston, MA | 4.23 |
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| Atlanta, GA | 5.16 | Honolulu, HI | 6.02 |
|---|
Our Wizard has accurate PSH data for 50 US cities and uses your location automatically.
Step 3: Size Your Solar Array
Formula:
Array size (kW) = Daily energy (kWh) ÷ (PSH × System efficiency)
System efficiency factor accounts for inverter losses (~4%), wiring losses (~2%), temperature derating (~5%), soiling (~3%), mismatch (~2%) — use 0.80–0.85 as a combined factor.
Example (New York City, 32.9 kWh/day):
32.9 kWh ÷ (4.08 hrs × 0.83) = 9.7 kW
With 400W panels: 9,700W ÷ 400W = 25 panels = 10.0 kW array
Step 4: Select Your Inverter
Grid-tied without battery:
- <strong>String inverter:</strong> Lower cost, simpler monitoring. One failure takes down the system; one shaded panel reduces entire string output
- <strong>Microinverters (Enphase IQ8+):</strong> One per panel, maximum shade tolerance, panel-level monitoring. Higher upfront cost, no single point of failure
- 3-ton AC = 3,500W running, 8,750W startup → need <strong>5,000W continuous / 10,000W surge minimum</strong>
Step 5: Size Your Battery Bank
Formula:
Battery capacity (kWh) = (Daily load × Autonomy days) ÷ DoD
- LiFePO4 DoD = 0.90
- Lead-acid DoD = 0.50
(5 × 2) ÷ 0.90 = 11.1 kWh minimum
Two Renogy 12V 300Ah batteries (3.84kWh each) in 2S2P = 24V × 600Ah = 14.4kWh — covers it with headroom.
Step 6: Quick Financial Check
Payback (years) = (System cost × 0.70 after 30% ITC) ÷ (Annual kWh offset × $/kWh)
Example: $28,000 system, 12,000 kWh/year, $0.20/kWh:
($28,000 × 0.70) ÷ (12,000 × 0.20) = $19,600 ÷ $2,400 = 8.2 years
After payback, the remaining 17 years of 25-year panel life is essentially free electricity — roughly 10–15% annual return on investment.
Use our Solar System Wizard to automate all of these calculations for your specific location and usage profile in under 2 minutes.
