Solar · charging
What Size Solar Panel to Charge a 12V 100Ah Battery?
To refill a 12 V 100 Ah battery (1,200 Wh) from empty in one sunny day you need about 310 W of solar panels in June, and about 500 W in December, in Tennessee (the middle state for sunshine). A 200 W panel takes about 1.5 days in June and 2.5 days in December.
Your state, your battery
Your situation: charging a battery or power station with solar. Different situation?
Your power station or battery
Where and how
Month by month in Tennessee
| Month | Days to full | Lying flat |
|---|---|---|
| January | 2.3 days | 3.4 days |
| February | 2.1 days | 2.8 days |
| March | 1.7 days | 2.1 days |
| April | 1.6 days | 1.6 days |
| May | 1.6 days | 1.6 days |
| June | 1.5 days | 1.5 days |
| July | 1.6 days | 1.6 days |
| August | 1.6 days | 1.6 days |
| September | 1.6 days | 1.8 days |
| October | 1.7 days | 2.3 days |
| November | 2.0 days | 3.0 days |
| December | 2.5 days | 3.9 days |
Every state
| State | Best month | Worst month | Panel for 1 day, worst month |
|---|---|---|---|
| Alabama | 1.5 days | 2.2 days (Dec) | 450 W |
| Alaska | 1.5 days | 11 days (Dec) | 2,170 W |
| Arizona | 1.2 days | 1.6 days (Dec) | 330 W |
| Arkansas | 1.5 days | 2.2 days (Dec) | 450 W |
| California | 1.3 days | 1.9 days (Dec) | 380 W |
| Colorado | 1.4 days | 1.8 days (Dec) | 360 W |
| Connecticut | 1.5 days | 2.5 days (Dec) | 500 W |
| Delaware | 1.5 days | 2.4 days (Dec) | 490 W |
| District of Columbia | 1.6 days | 2.3 days (Dec) | 470 W |
| Florida | 1.4 days | 1.9 days (Dec) | 380 W |
| Georgia | 1.5 days | 2.1 days (Dec) | 430 W |
| Hawaii | 1.4 days | 1.9 days (Dec) | 380 W |
| Idaho | 1.2 days | 3.2 days (Dec) | 650 W |
| Illinois | 1.5 days | 3.0 days (Dec) | 600 W |
| Indiana | 1.5 days | 3.1 days (Dec) | 630 W |
| Iowa | 1.5 days | 2.8 days (Dec) | 570 W |
| Kansas | 1.5 days | 2.1 days (Dec) | 430 W |
| Kentucky | 1.5 days | 2.8 days (Dec) | 570 W |
| Louisiana | 1.5 days | 2.1 days (Dec) | 430 W |
| Maine | 1.6 days | 2.8 days (Dec) | 560 W |
| Maryland | 1.6 days | 2.4 days (Dec) | 480 W |
| Massachusetts | 1.6 days | 2.6 days (Dec) | 530 W |
| Michigan | 1.5 days | 3.9 days (Dec) | 780 W |
| Minnesota | 1.5 days | 3.1 days (Dec) | 620 W |
| Mississippi | 1.5 days | 2.2 days (Dec) | 450 W |
| Missouri | 1.5 days | 2.4 days (Dec) | 480 W |
| Montana | 1.3 days | 3.2 days (Dec) | 640 W |
| Nebraska | 1.5 days | 2.4 days (Dec) | 490 W |
| Nevada | 1.2 days | 1.7 days (Dec) | 340 W |
| New Hampshire | 1.6 days | 2.6 days (Dec) | 530 W |
| New Jersey | 1.6 days | 2.5 days (Dec) | 510 W |
| New Mexico | 1.2 days | 1.6 days (Dec) | 320 W |
| New York | 1.5 days | 2.5 days (Dec) | 500 W |
| North Carolina | 1.5 days | 2.1 days (Dec) | 420 W |
| North Dakota | 1.4 days | 3.2 days (Dec) | 640 W |
| Ohio | 1.5 days | 3.5 days (Dec) | 700 W |
| Oklahoma | 1.4 days | 2.0 days (Dec) | 400 W |
| Oregon | 1.3 days | 4.3 days (Dec) | 870 W |
| Pennsylvania | 1.6 days | 2.6 days (Dec) | 530 W |
| Rhode Island | 1.6 days | 2.5 days (Dec) | 500 W |
| South Carolina | 1.4 days | 2.0 days (Dec) | 410 W |
| South Dakota | 1.4 days | 2.6 days (Dec) | 520 W |
| Tennessee | 1.5 days | 2.5 days (Dec) | 500 W |
| Texas | 1.5 days | 2.2 days (Dec) | 440 W |
| Utah | 1.3 days | 2.6 days (Dec) | 530 W |
| Vermont | 1.5 days | 4.0 days (Dec) | 800 W |
| Virginia | 1.5 days | 2.1 days (Dec) | 430 W |
| Washington | 1.3 days | 4.9 days (Dec) | 990 W |
| West Virginia | 1.6 days | 3.1 days (Dec) | 630 W |
| Wisconsin | 1.5 days | 3.1 days (Dec) | 630 W |
| Wyoming | 1.4 days | 2.1 days (Dec) | 430 W |
FAQ
What Size Solar Panel to Charge a 12V 100Ah Battery?
To refill a 12 V 100 Ah battery (1,200 Wh) from empty in one sunny day you need about 310 W of solar panels in June, and about 500 W in December, in Tennessee (the middle state for sunshine). A 200 W panel takes about 1.5 days in June and 2.5 days in December.
How long does a 200 W solar panel take to charge a 1,200 Wh battery?
Days from empty = 1,200 Wh ÷ (200 W × the day’s solar output per kW × 0.85). In Tennessee that’s 1.5 days in June and 2.5 days in December; lying flat, 3.9 days in December. Your state is in the table.
Does a bigger panel always charge faster?
Only up to the solar input limit of the power station or charge controller (in watts and volts, on its label or manual). Panels beyond that limit add nothing, and exceeding the voltage limit can damage it; check the manual before connecting.
Does it matter if it’s lithium or lead-acid?
Lead-acid batteries shouldn’t be run below about half, so refilling from 50% takes half as long as the figures here; lithium batteries usually charge more efficiently than the 85% we assume. A 100 Ah × 12 V = 1,200 Wh rating is the full capacity.
Sources and assumptions
- Solar output (kWh per kW of panels, by month): Global Solar Atlas 2.0, long-term averages to 2025 (data version 2.2.68, updated 2026-04-01), for crystalline-silicon panels facing south at the best fixed tilt, after typical system losses. Data obtained from the “Global Solar Atlas 2.0, a free, web-based application is developed and operated by the company Solargis s.r.o. on behalf of the World Bank Group, utilizing Solargis data, with funding provided by the Energy Sector Management Assistance Program (ESMAP). For additional information: https://globalsolaratlas.info” Licensed CC BY 4.0; we rounded and combined it as described here. Not affiliated with or endorsed by Solargis or the World Bank Group.
- State figure: population-weighted average of the state’s three largest cities (U.S. Census Bureau Vintage 2024 population estimates; coordinates from the 2020 Census Gazetteer). Hawaii and DC use one city.
- Cross-check: NASA POWER sunshine climatology (2001–2025), tilted with a standard isotropic-sky model × 0.80. States more than 12% apart were reviewed by hand; the gaps are explained on our methodology page.
- Home use: EIA Table 5A, 2024 average monthly residential kWh by state. Rates: EIA Electric Power Monthly, August 2026.
- Tax credit: IRS FAQ on the One, Big, Beautiful Bill (FS-2025-05, Aug. 21, 2025).
- Assumptions: 85% charging efficiency into a battery, 85% from battery to appliance, panel counts rounded up, off-grid sizing for the worst month. Days to full assume an empty battery and no power drawn while charging.