Buying guide
How many solar watts do you need?
Size an array to refill your battery from daily sun.
Published · Updated
Panel watts needed ≈ daily Wh ÷ (peak sun hours × system efficiency). This site uses 0.77, the PVWatts V1 total derate in NREL's PVWatts manual, which covers controller, wiring, temperature and dirt losses. Example: 1,200 Wh/day ÷ (4 sun hours × 0.77) ≈ 390 W of panels. Use your location's worst-month sun hours if you need year-round power. Check the controller's or power station's voltage and current limits before wiring panels in series or parallel.
- Solar watts ≈ daily Wh ÷ (peak sun hours × derate).
- A derate of 0.77 (the PVWatts V1 total DC-to-AC derate in NREL's PVWatts manual, Table 6) covers heat, wiring and controller losses.
- Plan for your darkest month if you are off-grid.
The sizing formula
Panel watts ≈ daily Wh ÷ (peak sun hours × efficiency)
| Input | Where to get it |
|---|---|
| Daily Wh | Load list. See how to size a battery bank |
| Peak sun hours | NREL maps or PVWatts |
| Efficiency | 0.77, the PVWatts V1 total derate in the NREL PVWatts manual (Table 6) |
Why efficiency is less than 1
Panel temperature (hot panels produce less), controller and wiring losses, dust, angle and battery charging losses all add up.
Peak sun hours by season
Peak sun hours vary a lot by location and season. In much of the continental U.S., summer values are higher and winter values much lower. Use PVWatts for monthly values at your address.
| Planning goal | Use |
|---|---|
| Summer cabin | Summer-month sun hours |
| Year-round off-grid | Worst-month (often December) sun hours |
| Backup with grid | Average or seasonal as needed |
Panel specs to understand
| Spec | Meaning |
|---|---|
| Rated power (Pmax) | Output at standard test conditions |
| Voc | Open-circuit voltage; rises in cold weather |
| Vmp / Imp | Voltage and current at max power |
| Isc | Short-circuit current |
| Temperature coefficient | % power change per °C |
| Efficiency | Power per area |
Cold-weather voltage
Voc rises as temperature drops. Many designers add a margin for the coldest expected temperature so series strings stay under the controller's maximum input voltage. The panel's temperature coefficient for Voc gives the exact correction.
Matching panels to controllers and stations
Check these against the controller or power station spec sheet:
| Check | Rule |
|---|---|
| Max input voltage | String Voc (cold-corrected) below the limit |
| Max input current | Parallel current below the limit |
| Max input watts | Extra panel watts are clipped |
| MPPT vs PWM | MPPT harvests more, especially with higher-voltage strings. See charge controllers |
Worked examples
Example 1: weekend camping
600 Wh/day, summer, 6 sun hours, 0.77 derate: 600 ÷ (6 × 0.77) ≈ 130 W. A 150–200 W portable panel works.
Example 2: outage backup, average conditions
1,200 Wh/day, 4 sun hours: 1,200 ÷ (4 × 0.77) ≈ 390 W.
Example 3: winter off-grid cabin
2,000 Wh/day, 2.5 worst-month sun hours: 2,000 ÷ (2.5 × 0.77) ≈ 1,039 W. Many cabin owners add a generator for long cloudy stretches instead of oversizing panels. See off-grid cabin planning.
Example 4: series string check
Two panels with Voc 22 V each in series make 44 V at standard conditions. On a cold morning that can rise several volts. If the station's max input is 50 V, that still fits. A third panel (66 V+) would exceed it.
Placement and angle
- Face panels toward the equator (south in the Northern Hemisphere).
- Tilt near your latitude for year-round use. Steeper tilts favor winter.
- Avoid shade. Even partial shade cuts output sharply, especially on series strings.
- Keep panels clean and allow airflow behind them for cooling.
Common mistakes
- Using summer sun hours for winter needs.
- Assuming rated watts in real conditions.
- Exceeding input voltage in cold weather.
- Shaded panels.
- Undersized wiring for long runs.
Safety notes
- Panels produce voltage whenever light hits them. Cover them or disconnect before working on wiring.
- Use properly rated connectors, fuses and wire gauge.
- Roof-mounted and home-connected systems fall under the NEC and local codes, and should be installed by a licensed professional with permits. See solar and battery safety.
- See panels and controllers on the deals page.
FAQ
What are peak sun hours?
The daily solar energy at a site, expressed as the number of hours at 1,000 W per square meter. A location with 5 peak sun hours receives 5 kWh per square meter per day. See the peak sun hours article.
Will a 400 W panel produce 400 W?
Rarely in practice. Ratings use standard test conditions. Heat, angle, dirt and partial shade reduce output. Real-world output is often well below the rating.
Series or parallel?
Series raises voltage (good for long wire runs and MPPT controllers) and parallel raises current. Shade on one panel affects series strings more. Stay within the controller's limits.
Do I need a permit?
Roof-mounted and grid-tied systems usually need permits and inspection. Portable panels for a power station generally don't. Check local rules.
Where do I go next?
Try the power planner to put numbers to your own situation, or see the product picks on the deals page.
Terms used here
- Watt (W)
- Rate of power use or production.
- Watt-hour (Wh)
- Energy: one watt for one hour.
- Kilowatt-hour (kWh)
- 1,000 watt-hours.
- Charge controller
- Regulates charging from panels to battery.
- MPPT
- Maximum power point tracking controller.
- PWM
- Pulse-width modulation controller.
- Peak sun hours
- Hours of equivalent 1,000 W/m² sunlight per day.
- Derate
- Factor for real-world losses in solar output.
- Open-circuit voltage (Voc)
- Panel voltage with no load; rises in cold.
- Temperature coefficient
- How much panel output changes per degree.
Sources
This guide is informational and based on published specifications and general references, not hands-on testing. Check the manufacturer's current specs before you buy.