The panel count is the last step of a sizing calculation, not the first. It falls out of the energy you need, the sun your site receives, the losses your system carries and the wattage of the module you buy.
This guide works through the chain and then gives illustrative module counts from 3 kW to 1 MW.
Work out your own number with the PV System Size Calculator or, if you already know the capacity you want, the Number of Panels Calculator.
The Four Numbers You Need
| Input | Typical source |
|---|---|
| Energy consumption | Twelve months of utility bills |
| Peak sun hours (PSH) | TMY / NASA POWER data for your coordinates and tilt |
| Performance ratio (PR) | Loss chain, or 0.75–0.82 as a stated assumption |
| Module wattage | The module SKU you will actually buy |
Step 1 – Load Calculation
An 850 kWh/month household wanting a full offset needs 27.9 kWh/day from PV.
Step 2 – Required DC Capacity
At 5.0 PSH and PR 0.78: 27.9 ÷ (5.0 × 0.78) = 7.15 kWp.
The PR term is what separates this from the widely quoted kWh ÷ PSH shortcut. Skip it and you undersize by roughly a fifth. The individual loss terms are explained in the performance ratio guide.
Step 3 – Module Count
For 7.15 kWp with 550 Wp modules: 7,150 ÷ 550 = 13.0 → 13 modules → 7.15 kWp installed.
Rounding up always overshoots slightly. On a 3 kW system one extra 550 Wp module is an 18% overshoot; on a 500 kW plant it is 0.1%. Smaller modules give a finer fit; larger modules give a cheaper install per watt and fewer mounting points.
Step 4 – Area Sanity Check
A 550 Wp module at 21% efficiency is about 2.62 m². Rooftop layouts add roughly 15% for walkways and setbacks; ground-mount arrays need about 2.2–2.6× module area for inter-row pitch. Check the real figure with the Roof Area Calculator and shadow-free spacing with the Inter-row Spacing Calculator.
Step 5 – Inverter Sizing
At a 1.20 target, 7.15 kWp needs about 5.96 kW AC — in practice a 6 kW inverter, giving an actual ratio of 1.19. A ratio between 1.00 and 1.40 is the normal conceptual band; above 1.40 you must check clipping, MPPT limits and thermal derating. Detailed string and MPPT checks belong in the Inverter Sizing Calculator.
Illustrative Examples
All rows assume 5.0 peak sun hours, PR 0.78, 550 Wp modules and a 1.20 target DC/AC ratio. They are illustrative only — your location, tilt, shading and losses will change every number.
| Target DC | Modules (550 Wp) | Installed DC | Theoretical AC | Example inverters | Daily generation | Annual generation |
|---|---|---|---|---|---|---|
| 3 kW | 6 | 3.30 kWp | 2.75 kW | 1 × 3 kW | 12.9 kWh | 4,698 kWh |
| 5 kW | 10 | 5.50 kWp | 4.58 kW | 1 × 5 kW | 21.5 kWh | 7,830 kWh |
| 10 kW | 19 | 10.45 kWp | 8.71 kW | 1 × 10 kW | 40.8 kWh | 14,871 kWh |
| 25 kW | 46 | 25.30 kWp | 21.08 kW | 2 × 12 kW | 98.7 kWh | 36,013 kWh |
| 50 kW | 91 | 50.05 kWp | 41.71 kW | 1 × 50 kW | 195.2 kWh | 71,241 kWh |
| 100 kW | 182 | 100.10 kWp | 83.42 kW | 2 × 50 kW | 390.4 kWh | 142,492 kWh |
| 500 kW | 910 | 500.50 kWp | 417.08 kW | 4 × 110 kW | 1,952 kWh | 712,412 kWh |
| 1 MW | 1,819 | 1,000.45 kWp | 833.71 kW | 3 × 320 kW | 3,902 kWh | 1,424,041 kWh |
Read the annual generation column as an order of magnitude, not a guarantee: the same 1 MWp plant produces roughly 1.35 GWh in a cloudy temperate climate and 1.8 GWh in a high-irradiance desert site.
How Consumption Maps to Panel Count
Working the other way, at 5 PSH and PR 0.78 each 550 Wp module produces about 2.15 kWh/day:
| Monthly bill | Daily load | Required DC | Modules (550 Wp) |
|---|---|---|---|
| 150 kWh | 4.9 kWh | 1.26 kWp | 3 |
| 300 kWh | 9.9 kWh | 2.53 kWp | 5 |
| 600 kWh | 19.7 kWh | 5.06 kWp | 10 |
| 1,000 kWh | 32.9 kWh | 8.43 kWp | 16 |
| 3,000 kWh | 98.6 kWh | 25.29 kWp | 46 |
| 10,000 kWh | 328.8 kWh | 84.31 kWp | 154 |
Off-grid Systems Need More Panels
At 3.2 PSH in December, PR 0.78, 90% round-trip and 95% battery-inverter efficiency, the same 27.9 kWh/day load needs 13.1 kWp — nearly double the grid-tied figure. Size the storage with the Battery Bank Calculator and check runtime with the Battery Backup Time Calculator.
Practical Constraints That Change the Answer
- Roof area. Available shade-free area may cap the count below the energy requirement.
- Sanctioned load and net-metering caps. Utilities often limit exportable AC capacity.
- Structure. Wind uplift and dead load govern rooftop feasibility as much as area does.
- Module availability. A 545 Wp module in stock beats a 550 Wp module on backorder.
- String layout. Module count must divide sensibly into strings that sit inside the MPPT voltage window.
Once You Know the Panel Count
- Check DC cable size with the Solar DC Cable Sizing Calculator or the DC Cable Size Calculator
- Check AC side cable with the AC Cable Sizing Calculator
- Verify percentage drop with the Voltage Drop Calculator
- Estimate returns with the Solar ROI & Payback Calculator
Every figure here is a preliminary estimate. A financeable design needs a site-specific hourly simulation with real meteorological data and a 3D shading scene.