SolarVeri Data Team
Irradiance data and off-grid sizing
For a typical boondocking day of about 1,200 Wh, you need roughly 250 W of flat roof solar in a Phoenix July, but about 500 W in a Phoenix January and 1,800 W in a Seattle January. Where and when you camp matters more than any generic "watts per amp-hour" rule.
This page applies the method from our off-grid solar sizing guide to a vehicle roof: flat panels, limited space and a small battery. All output figures are from the PVGIS model for each location. For storage, see how many batteries you need; for why January is so much harder than July, see solar panels in winter by location.
electric_boltStart with your daily watt-hours
RV loads are smaller than a house but add up fast. A 12 V compressor fridge, roof fan, lights, water pump and device charging make up most of a typical off-grid day. The example below lands on 1,200 Wh, which is also the load used in the simulations later on this page.
| Example RV load | Wh per day |
|---|---|
| 12 V compressor fridge | 480 |
| Roof vent fan (about 25 W for 8 hours) | 200 |
| LED lights | 120 |
| Phones, laptop, camera batteries | 250 |
| Water pump | 30 |
| Furnace blower or other small loads | 120 |
| Total | 1,200 |
Rooftop air conditioning is a different scale
wb_sunnyWhat 100 W of flat roof solar makes per day
Most RV panels are mounted flat. That works well in summer when the sun is high, and poorly in winter when it is low. The table shows average daily output per 100 W of flat panels, after typical system losses.
| Location | January (Wh per 100 W per day) | April | July | Flat array for 1,200 Wh/day in January |
|---|---|---|---|---|
| Quartzsite, AZ | 274 | 563 | 539 | 450 W |
| Phoenix, AZ | 265 | 553 | 523 | 500 W |
| Las Vegas, NV | 242 | 543 | 535 | 500 W |
| Austin, TX | 239 | 430 | 500 | 550 W |
| Miami, FL | 287 | 497 | 474 | 450 W |
| Denver, CO | 197 | 463 | 513 | 650 W |
| Atlanta, GA | 219 | 457 | 461 | 550 W |
| Nashville, TN | 174 | 410 | 452 | 700 W |
| New York, NY | 148 | 397 | 452 | 850 W |
| Chicago, IL | 119 | 380 | 481 | 1,050 W |
| Bozeman, MT | 90 | 377 | 539 | 1,350 W |
| Seattle, WA | 68 | 360 | 497 | 1,800 W |
The winter column explains why snowbirds head to the desert Southwest. Around Quartzsite, 100 W of flat solar makes about 274 Wh on an average January day, versus 119 Wh in Chicago and 68 Wh in Seattle. In July the differences mostly disappear: every location in the table gives about 450 to 540 Wh per 100 W.
query_statsWhat a 400 W system actually delivers
Averages hide cloudy streaks, so we also simulated a common setup hour by hour with PVGIS: 400 W of flat panels, a 2,560 Wh battery (12 V, 200 Ah LiFePO4) and a 1,200 Wh daily load. The share of the load covered is the energy the system actually delivered, averaged over the month.
| Location | Share of load covered, January | April | July |
|---|---|---|---|
| Quartzsite, AZ | 80% | 100% | 100% |
| Phoenix, AZ | 78% | 100% | 100% |
| Las Vegas, NV | 69% | 100% | 100% |
| Austin, TX | 68% | 99% | 100% |
| Miami, FL | 84% | 100% | 100% |
| Denver, CO | 56% | 99% | 100% |
| Atlanta, GA | 61% | 99% | 100% |
| Nashville, TN | 49% | 98% | 100% |
| New York, NY | 41% | 96% | 100% |
| Chicago, IL | 33% | 94% | 100% |
| Bozeman, MT | 26% | 95% | 100% |
| Seattle, WA | 21% | 95% | 100% |
In July, 400 W is enough nearly everywhere in the US. In January it covers about 80% of the load in Quartzsite and about 84% in Miami, but well under half in the northern states. That is why winter RVers north of the Sun Belt usually combine solar with alternator charging while driving, a generator, or campgrounds with hookups.
buildWays to close the winter gap without more roof
- check_circleTilt a portable panel. At a 30° tilt, January output in Denver rises from 197 to 355 Wh per 100 W per day. A ground panel aimed at the sun also lets you park in shade.
- check_circleDC-DC charging from the alternator. A few hours of driving can put back a day of use.
- check_circleCut the biggest loads. The fridge and heating fan dominate; a more efficient fridge or better insulation often beats adding panels.
- check_circleA bigger battery for short trips. It does not raise daily production, but it lets you ride out two or three dark days. See battery bank sizing.
ruleA quick sizing rule for RVs
For summer camping almost anywhere in the US, the break-even is about 250 to 300 W of flat solar per 1,200 Wh of daily use; with a 20 to 30 percent margin, 300 to 400 W is a comfortable target. For winter camping in the desert Southwest, break-even is about 450 to 500 W, so plan more or tilt the panels. North of about 40° latitude in winter, roof solar alone is unlikely to keep up, and the main sizing guide explains how to plan around that.