Solar System Sizing Guide: Load Analysis, Peak Sun Hours & Worked Examples
Sizing a solar PV system right is what separates a productive array from an underperforming one. This guide walks through the full methodology — load analysis, peak sun hours, DC/AC ratio optimization, battery sizing, and NEC conductor sizing — then applies it to three worked examples: 5 kW, 10 kW, and 20 kW. Follow the steps and you'll have a defensible design before you touch a datasheet.
Step 1 — Load Analysis Worksheet
List every load the system must serve, with its daily run-hours. Multiply watts × hours for daily Wh, sum for total daily Wh, then divide by system voltage if sizing a battery bank in Ah.
| Load | Watts | Qty | Hours/day | Daily Wh |
|---|---|---|---|---|
| LED lighting | 10 | 10 | 5 | 500 |
| Refrigerator | 150 | 1 | 24 (cycle) | 1,200 |
| Heat pump (heating) | 3,000 | 1 | 6 | 18,000 |
| EV charger (L2) | 7,200 | 1 | 3 | 21,600 |
| Electronics/misc | 400 | 1 | 8 | 3,200 |
| Total | 44,500 Wh (44.5 kWh) |
Step 2 — Peak Sun Hours by US Region
Peak sun hours (PSH) is the equivalent hours/day of 1000 W/m² irradiance. Size the array to produce your daily kWh in your location's PSH.
| Region | Representative city | Annual avg PSH/day (fixed, south, 30° tilt) |
|---|---|---|
| Pacific Southwest | Phoenix, AZ | 6.5 |
| Mountain West | Denver, CO | 5.5 |
| South Central | Austin, TX | 5.2 |
| Pacific Northwest | Portland, OR | 3.7 |
| Southeast | Atlanta, GA | 5.0 |
| Northeast | Boston, MA | 4.2 |
| Great Lakes | Chicago, IL | 4.1 |
| Mid-Atlantic | Baltimore, MD | 4.4 |
Use PVWatts (NREL) for site-specific numbers, and design against the winter PSH for off-grid systems (worst month).
Step 3 — Array Sizing & DC/AC Ratio
Array kW (STC) ≈ daily kWh ÷ (PSH × performance ratio). Performance ratio accounts for inverter efficiency, wiring, soiling, temperature — assume 0.80 for residential.
DC/AC ratio (array DC kW ÷ inverter AC kW) optimizes inverter utilization and cost. Typical sweet spot 1.15–1.35.
- 1.15–1.25 — minimal clipping, conservative, higher inverter cost per watt.
- 1.25–1.35 — best $/W; small clipping on the clearest summer days.
- >1.35 — risk of significant clipping and MPPT instability in cold weather.
Step 4 — Battery Sizing Methodology
- Determine backup load (Wh/day) — what must run during an outage (fridge, lights, well pump, comms).
- Choose autonomy days — 1 day for grid-tied backup, 2–3 for off-grid.
- Account for inverter efficiency — divide by ~0.90.
- Select DoD — 90% for LFP, 50% for lead-acid.
- Battery kWh = backup Wh × days ÷ 0.90 ÷ DoD
Example: 5 kWh/day backup, 2 days autonomy, LFP at 90% DoD, 90% inverter efficiency → 5,000 × 2 ÷ 0.90 ÷ 0.90 = 12.3 kWh battery. Round up to a 14 kWh module.
Step 5 — Conductor Sizing per NEC
- DC string current = Isc × 1.56 (690.8). Select PV wire ampacity ≥ this after derating.
- Temperature derate (310.15(B)(1)) and conduit fill (310.15(C)(1)).
- Voltage drop — keep ≤3% on DC home runs, ≤2% on AC feeders.
- OCPD ≥ conductor ampacity, ≥1.56 × Isc.
Worked Examples
5 kW residential (Phoenix, 6.5 PSH)
- Daily target: 30 kWh (PR 0.80 → 30 × 0.80 = 24 kWh usable). Array ≈ 24 ÷ 6.5 × 1.25 ≈ 4.6 kW → use 10 × 500W = 5.0 kW.
- Inverter: 5.0 ÷ 1.25 = 4.0 kW AC string inverter (or 4.0 kW microinverter set).
- Battery: optional 10–14 kWh LFP for TOU shifting.
- String: 10 × 500W panels, Voc ~50V → 500V string (under 600V limit), Isc ~13A × 1.56 = 20.3A → #10 PV wire, 25A string fuse.
10 kW residential (Atlanta, 5.0 PSH)
- Daily target: 40 kWh. Array ≈ 40 × 0.80 ÷ 5.0 × 1.25 ≈ 8 kW → use 16 × 500W = 8 kW... round to 20 × 500W = 10 kW for full offset.
- Inverter: 10 ÷ 1.25 = 8.0 kW AC (two 4 kW or one 7.6–10 kW string inverter).
- Battery: 14 kWh LFP for self-consumption + backup.
- Conductors: two 10-panel strings, each #10 PV wire, 25A fuse; combiner output #6 to inverter.
20 kW commercial (Denver, 5.5 PSH)
- Daily target: 88 kWh. Array ≈ 88 × 0.80 ÷ 5.5 × 1.25 ≈ 16 kW → use 40 × 500W = 20 kW.
- Inverter: 20 ÷ 1.30 = 15 kW AC (commercial 3-phase string or two 10 kW).
- Battery: 30–40 kWh LFP for demand-charge management.
- Conductors: four 10-panel strings; combiner 100A; #2 Cu to inverter; AC feeder per 705.12 busbar calc.
Tools & Calculators
- PVWatts (NREL) — production by ZIP/tilt/azimuth.
- System Advisor Model (SAM) — detailed financial + production modeling.
- Inverter manufacturer string designers — SolarEdge Designer, Enphase IQ Combiner calculator, Fronius Solar.configurator.
Size Your System with PES Supply
PES Supply stocks panels, inverters, batteries, BOS, and conductors sized for the systems above. 50,000+ SKUs from 169 authorized brands; standard delivery 7–10 business days. Need a kit? Our solar starter kits bundle matched components.
Related Collections
Further Reading
- String Inverter Sizing Guide: How to Match Inverters to Your Solar Array
- How Many Solar Panels Do I Need? System Sizing Guide with Calculator
- Microinverters vs. String Inverters: Which Is Better for Your Solar Array?
- Hybrid Inverter vs. Off-Grid Inverter: Choosing the Right System
- Solar Battery Sizing: How to Calculate Storage Needs for Off-Grid Living
Need the right gear for your next install?
PES Supply stocks 50,000+ SKUs from 169 authorized brands — solar panels, inverters, batteries, racking, BOS, and electrical components from one B2B warehouse. Standard delivery is 7–10 business days.
Browse the Full Catalog Request a Bulk Quote