Peak sun hours for RV travel

A peak sun hour is one hour of sunlight at 1,000 watts per square meter, the intensity solar panels are rated at. A location’s daily solar energy in kWh per square meter equals its peak sun hours. RVWatt uses NASA POWER monthly averages for 2001–2025 on a flat, horizontal surface, which matches panels mounted flat on a roof.

That number changes a lot through the year. In Hailey, Idaho a flat panel averages 7.69 peak sun hours in July and only 1.54 in December. Sizing to a yearly average would leave you short in winter. The calculator sizes to the weakest month among the months you actually travel. Uncheck months you do not travel, or pick a different place, and the design month changes.

These figures are climatology at about 1° (roughly 100 km) resolution, so mountain valleys, coastlines and local weather can differ. If you know a local value, choose “Custom peak sun hours” in the calculator. Tilting portable panels toward the sun produces more than these numbers in winter. Portable panels in shade produce less.

What this means for panel size

With the default system factor of 0.757 (MPPT controller and LiFePO4 batteries), a van that uses 1,000 Wh a day needs 1,000 ÷ (peak sun hours × 0.757) watts of solar. At 5 sun hours that is about 264 W; at 2 sun hours it is about 661 W. The camper van and fridge guides walk through real presets with these same NASA numbers.

Open the calculator in Moab, UT Denver, CO Hailey, ID Phoenix, AZ Seattle, WA

Monthly peak sun hours, 74 locations

December, March, June and September are shown as a snapshot, plus each place’s weakest and strongest month. Click a name to open the calculator with that location selected. Values are for a flat, horizontal panel.

Location	Dec	Mar	Jun	Sep	Weakest	Strongest
Calgary, AB	0.88	3.49	5.82	3.64	Dec 0.88	Jul 6.30
Anchorage, AK	0.14	2.41	5.32	2.39	Dec 0.14	Jun 5.32
Birmingham, AL	2.30	4.32	6.20	4.90	Dec 2.30	Jun 6.20
Little Rock, AR	2.28	4.15	6.48	4.96	Dec 2.28	Jun 6.48
Flagstaff, AZ	2.80	5.71	8.40	5.93	Dec 2.80	Jun 8.40
Phoenix, AZ	3.08	5.89	8.50	6.16	Dec 3.08	Jun 8.50
Quartzsite, AZ	3.11	5.94	8.49	6.24	Dec 3.11	Jun 8.49
Vancouver, BC	0.68	2.56	5.52	3.60	Dec 0.68	Jul 6.19
Joshua Tree, CA	3.01	5.83	8.55	6.35	Dec 3.01	Jun 8.55
Los Angeles, CA	2.86	5.38	8.10	6.26	Dec 2.86	Jun 8.10
Sacramento, CA	1.97	4.64	8.29	5.95	Dec 1.97	Jun 8.29
San Francisco, CA	2.13	4.62	7.28	5.12	Dec 2.13	Jun 7.28
Denver, CO	2.29	4.73	7.22	5.37	Dec 2.29	Jun 7.22
Durango, CO	2.48	5.12	7.84	5.38	Dec 2.48	Jun 7.84
Hartford, CT	1.54	3.73	5.91	4.37	Dec 1.54	Jul 6.02
Dover, DE	1.75	3.83	6.17	4.42	Dec 1.75	Jun 6.17
Miami, FL	3.44	5.53	5.68	4.96	Dec 3.44	May 6.25
Orlando, FL	3.12	5.22	5.78	4.93	Dec 3.12	May 6.48
Atlanta, GA	2.37	4.38	6.19	4.82	Dec 2.37	Jun 6.19
Honolulu, HI	3.93	5.48	7.09	6.22	Dec 3.93	Jun 7.09
Des Moines, IA	1.67	3.82	6.29	4.56	Dec 1.67	Jul 6.38
Boise, ID	1.39	3.97	7.54	5.21	Dec 1.39	Jul 7.74
Hailey, ID	1.54	4.08	7.51	5.14	Dec 1.54	Jul 7.69
Chicago, IL	1.41	3.58	6.10	4.40	Dec 1.41	Jul 6.16
Indianapolis, IN	1.56	3.66	6.28	4.58	Dec 1.56	Jun 6.28
Wichita, KS	2.23	4.36	6.95	5.01	Dec 2.23	Jun 6.95
Louisville, KY	1.67	3.69	6.30	4.60	Dec 1.67	Jun 6.30
New Orleans, LA	2.74	4.89	6.08	5.11	Dec 2.74	May 6.46
Boston, MA	1.49	3.69	5.72	4.27	Dec 1.49	Jul 5.96
Baltimore, MD	1.75	3.81	6.20	4.35	Dec 1.75	Jun 6.20
Portland, ME	1.37	3.65	5.64	4.27	Dec 1.37	Jul 5.90
Detroit, MI	1.24	3.54	6.16	4.26	Dec 1.24	Jul 6.22
Marquette, MI	0.88	3.24	5.94	3.72	Dec 0.88	Jul 6.01
Minneapolis, MN	1.31	3.68	6.04	4.15	Dec 1.31	Jul 6.40
St. Louis, MO	1.81	3.76	6.42	4.75	Dec 1.81	Jun 6.42
Jackson, MS	2.43	4.42	6.56	5.14	Dec 2.43	Jun 6.56
Bozeman, MT	1.33	3.83	6.63	4.56	Dec 1.33	Jul 7.15
Missoula, MT	1.17	3.69	6.49	4.46	Dec 1.17	Jul 7.26
Asheville, NC	2.28	4.27	6.20	4.68	Dec 2.28	Jun 6.20
Charlotte, NC	2.29	4.36	6.46	4.71	Dec 2.29	Jun 6.46
Bismarck, ND	1.30	3.79	6.54	4.29	Dec 1.30	Jul 6.76
Omaha, NE	1.70	3.81	6.45	4.59	Dec 1.70	Jul 6.55
Manchester, NH	1.49	3.69	5.72	4.27	Dec 1.49	Jul 5.96
Newark, NJ	1.62	3.73	5.95	4.30	Dec 1.62	Jul 6.03
Albuquerque, NM	2.83	5.58	8.10	5.82	Dec 2.83	Jun 8.10
Las Vegas, NV	2.66	5.48	8.52	6.16	Dec 2.66	Jun 8.52
Reno, NV	2.02	4.77	8.27	5.92	Dec 2.02	Jun 8.27
Buffalo, NY	1.09	3.27	5.88	4.13	Dec 1.09	Jul 5.89
New York, NY	1.62	3.73	5.95	4.30	Dec 1.62	Jul 6.03
Columbus, OH	1.48	3.58	6.11	4.55	Dec 1.48	Jun 6.11
Oklahoma City, OK	2.40	4.48	6.92	5.14	Dec 2.40	Jun 6.92
Toronto, ON	1.17	3.36	6.08	4.16	Dec 1.17	Jun 6.08
Bend, OR	1.26	3.57	7.11	4.91	Dec 1.26	Jul 7.72
Portland, OR	0.98	2.99	6.00	4.30	Dec 0.98	Jul 6.82
Philadelphia, PA	1.75	3.83	6.17	4.42	Dec 1.75	Jun 6.17
Pittsburgh, PA	1.30	3.37	5.86	4.29	Dec 1.30	Jul 5.87
Providence, RI	1.55	3.83	6.07	4.47	Dec 1.55	Jul 6.16
Charleston, SC	2.49	4.60	6.52	4.82	Dec 2.49	May 6.54
Rapid City, SD	1.58	3.97	6.82	4.68	Dec 1.58	Jul 6.99
Nashville, TN	1.92	3.88	6.34	4.74	Dec 1.92	Jun 6.34
Austin, TX	2.74	4.50	6.70	5.20	Dec 2.74	Jul 6.73
Big Bend, TX	3.57	6.01	7.60	5.74	Dec 3.57	Jun 7.60
El Paso, TX	3.40	6.10	8.13	5.99	Dec 3.40	Jun 8.13
Houston, TX	2.64	4.47	6.29	5.03	Dec 2.64	Jun 6.29
Moab, UT	2.32	5.15	8.30	5.75	Dec 2.32	Jun 8.30
Salt Lake City, UT	1.86	4.51	7.81	5.40	Dec 1.86	Jun 7.81
Richmond, VA	2.07	4.10	6.35	4.47	Dec 2.07	Jun 6.35
Burlington, VT	1.10	3.41	5.62	4.05	Dec 1.10	Jul 5.71
Seattle, WA	0.85	2.85	5.73	3.91	Dec 0.85	Jul 6.33
Spokane, WA	0.86	3.07	6.47	4.37	Dec 0.86	Jul 7.42
Milwaukee, WI	1.16	3.47	5.95	4.15	Dec 1.16	Jul 6.11
Charleston, WV	1.59	3.61	6.14	4.50	Dec 1.59	Jun 6.14
Cheyenne, WY	1.98	4.45	7.13	5.05	Dec 1.98	Jun 7.13
Jackson, WY	1.53	4.17	6.96	4.84	Dec 1.53	Jul 7.36

Sun data: NASA Langley Research Center POWER Project, all-sky surface shortwave irradiance (ALLSKY_SFC_SW_DWN), monthly climatology for January 2001 through December 2025, horizontal surface.

Frequently asked questions

What is a peak sun hour?

One peak sun hour is one hour of sunlight at 1,000 watts per square meter, the intensity solar panels are rated at. A location’s daily solar energy in kWh per square meter equals its peak sun hours. RVWatt uses NASA POWER’s 2001–2025 monthly averages for a flat, horizontal surface, which matches panels mounted flat on a roof.

Why not use a yearly average?

Because batteries run flat in the worst month, not the average one. In Hailey, Idaho a flat panel gets 7.69 peak sun hours in July and only 1.54 in December. Sizing to the annual average would leave you short for months. Uncheck the months you do not travel and the calculator sizes to your weakest remaining month.

Is this the same as hours of daylight?

No. A long summer day can be bright for 15 hours and still add up to only 6 or 7 peak sun hours, because early morning and evening light is weaker than the 1,000 W/m² rating. Winter days are both shorter and weaker, which is why December numbers in the north are often below 2.

Related guides

Safety: high-current DC is dangerous. A 12 V battery bank can deliver thousands of amps into a short circuit, enough to melt tools, weld rings and watches to terminals, ignite insulation and start a vehicle fire in seconds. Lithium and lead-acid batteries can also vent, burn or explode if overcharged, damaged or charged outside their temperature limits. Remove jewelry, use insulated tools, disconnect the battery before working, fuse every positive conductor as close to the battery as practical with a fuse rated for the battery's short-circuit current (often a Class T for lithium banks), use properly crimped lugs and pure copper cable, and secure batteries against movement. If you are not confident, hire a qualified RV or marine electrician. RVWatt provides estimates only; see our Terms of Use.

These figures are planning estimates from typical appliance ratings and NASA monthly sun averages for a flat, horizontal panel. Your real use will differ with heat, shade, dirty panels and how you actually live in the rig. Follow your equipment manuals and have a qualified installer check high-current wiring.