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Add up your daily watt-hours, divide by your peak sun hours and multiply by 1.25 for losses: that is your solar array in watts. Multiply daily use by the days of backup you want and divide by the battery’s usable fraction (about 0.85 for LiFePO4, 0.5 for lead-acid) for the battery bank. Size the inverter for 1.25× everything running at once, with surge capacity for the biggest motor. The calculator below does all of it, with each step shown.
Off-Grid Solar and Battery Sizing Calculator
1. What will you run?
We filled in a small cabin as an example. Remove what you don’t need, add your own, and set how many hours a day each one runs.
| Appliance | How many | Hours a day | Running W | Starting W |
|---|
2. Where will it be?
3. What you need
Advanced settings
Show the math
How the Calculator Works
The calculator follows the same method as our how many solar panels do I need guide:
| Component | Formula | Why |
|---|---|---|
| Daily energy | Σ (running W × qty × hours/day) | What the battery has to supply and the panels have to replace each day. |
| Solar array | daily Wh ÷ peak sun hours × 1.25 | Peak sun hours turn daily Wh into panel watts; the 1.25 covers heat, wiring, controller, inverter and dust losses (20–25%). |
| Battery bank | daily Wh × days of backup ÷ usable fraction | LiFePO4 can be cycled to about 85% of its rating day after day; lead-acid lasts far longer kept above 50%. |
| Battery Ah | battery Wh ÷ system voltage | Batteries are sold in amp-hours at a voltage. |
| Inverter (continuous) | 1.25 × running W of everything on at once | Headroom so the inverter isn’t run at its limit. |
| Inverter (surge) | running total + largest (starting W − running W) | Only one motor normally starts at a time, the same rule generator makers use. The suggested inverter is the next standard size that covers the continuous figure and half the surge, since most inverters surge to about twice their rating. |
| Charge controller | array W ÷ battery V × 1.25 | Output amps of an MPPT controller, with a 25% margin. |
| System voltage (auto) | under 2 kWh/day: 12 V; 2–5 kWh: 24 V; over 5 kWh: 48 V | Higher voltage keeps currents, cable sizes and controller counts sensible as the system grows. |
The calculator also checks charge rate: the array should be able to deliver at least 0.2C to a LiFePO4 bank (about one-fifth of the bank’s watt-hours, in watts) so it can refill after a cloudy spell. If it can’t, it tells you.
Worked Example: A Small Off-Grid Cabin
The calculator opens with this example. A cabin in Missouri runs an ENERGY STAR full-size refrigerator (about 1,267 Wh a day), LED lights, a laptop, a Wi-Fi router and a 1/2 hp shallow-well pump for an hour a day. That comes to about 3.3 kWh a day. Designing for Missouri’s December sun (3.2 peak sun hours) with two days of LiFePO4 backup gives a 1.3 kW array (four 400 W panels), a 7.9 kWh battery bank at 24 V, a 2,000 W inverter (it needs about 1,600 W continuous and 2,300 W for a moment when the refrigerator or pump starts), and a 100 A charge controller. Switch the advanced setting to “Year-round average” and the array shrinks by a third — which is why a system sized on annual sun runs short every winter.
Peak Sun Hours by State (Annual and December)
Peak sun hours are the daily solar energy on a panel tilted at the site’s latitude, in kWh/m²/day. These figures are the long-term NASA POWER climatology for one point in each state (the capital, or the largest city where that is more representative). Sun varies inside a state, especially in mountain and coastal states, so treat them as a starting point. Arizona has the most annual sun in this table (6.45); Alaska the least (2.92).
| State | Location | Annual | December |
|---|---|---|---|
| Alabama | Montgomery | 4.91 | 3.58 |
| Alaska | Anchorage | 2.92 | 0.52 |
| Arizona | Phoenix | 6.45 | 4.87 |
| Arkansas | Little Rock | 4.77 | 3.56 |
| California | Sacramento | 5.85 | 3.38 |
| Colorado | Denver | 5.64 | 4.17 |
| Connecticut | Hartford | 4.30 | 2.56 |
| Delaware | Dover | 4.46 | 2.95 |
| District of Columbia | Washington | 4.51 | 3.19 |
| Florida | Orlando | 5.16 | 4.39 |
| Georgia | Atlanta | 4.83 | 3.58 |
| Hawaii | Honolulu | 6.05 | 5.04 |
| Idaho | Boise | 5.14 | 2.51 |
| Illinois | Springfield | 4.47 | 2.72 |
| Indiana | Indianapolis | 4.31 | 2.48 |
| Iowa | Des Moines | 4.59 | 2.96 |
| Kansas | Topeka | 4.81 | 3.43 |
| Kentucky | Frankfort | 4.35 | 2.61 |
| Louisiana | Baton Rouge | 4.93 | 3.68 |
| Maine | Augusta | 4.24 | 2.48 |
| Maryland | Annapolis | 4.65 | 3.19 |
| Massachusetts | Boston | 4.29 | 2.73 |
| Michigan | Lansing | 4.08 | 1.94 |
| Minnesota | Saint Paul | 4.39 | 2.45 |
| Mississippi | Jackson | 4.95 | 3.59 |
| Missouri | Jefferson City | 4.67 | 3.20 |
| Montana | Helena | 4.49 | 2.15 |
| Nebraska | Lincoln | 4.85 | 3.38 |
| Nevada | Carson City | 5.94 | 3.53 |
| New Hampshire | Concord | 4.17 | 2.44 |
| New Jersey | Trenton | 4.32 | 2.75 |
| New Mexico | Santa Fe | 6.26 | 4.98 |
| New York | Albany | 4.13 | 2.22 |
| North Carolina | Raleigh | 4.89 | 3.68 |
| North Dakota | Bismarck | 4.67 | 2.56 |
| Ohio | Columbus | 4.10 | 2.19 |
| Oklahoma | Oklahoma City | 5.23 | 3.89 |
| Oregon | Salem | 4.25 | 1.90 |
| Pennsylvania | Harrisburg | 4.22 | 2.51 |
| Rhode Island | Providence | 4.40 | 2.60 |
| South Carolina | Columbia | 4.94 | 3.77 |
| South Dakota | Pierre | 4.80 | 2.91 |
| Tennessee | Nashville | 4.53 | 3.00 |
| Texas | Austin | 5.11 | 3.99 |
| Utah | Salt Lake City | 5.41 | 3.23 |
| Vermont | Montpelier | 3.79 | 1.81 |
| Virginia | Richmond | 4.68 | 3.40 |
| Washington | Olympia | 3.78 | 1.69 |
| West Virginia | Charleston | 4.21 | 2.49 |
| Wisconsin | Madison | 4.32 | 2.40 |
| Wyoming | Cheyenne | 5.30 | 3.63 |
What the Calculator Doesn’t Do
- Shading and orientation. The sun-hour figures assume an unshaded, south-facing panel tilted at your latitude. A flat RV roof or a shaded site gets less.
- Inverter idle draw. A large inverter can use 20–50 W just being on, 0.5–1.2 kWh a day. Add it as a custom load if yours stays on around the clock.
- Cold-weather battery limits. LiFePO4 cells should not be charged below freezing unless the battery has a low-temperature cutoff or heater; check the battery’s manual.
- Code and permits. Off-grid systems still fall under the electrical code in most states. See our off-grid laws by state for the NEC edition your state enforces.
Sources
- NASA POWER (Prediction Of Worldwide Energy Resources), Climatology API
- U.S. DOE Energy Saver — Estimating Appliance and Home Electronic Energy Use
- Honda Power Equipment — Generator Wattage Estimation Guide
- ENERGY STAR Certified Residential Refrigerators dataset (EPA)
How this calculator was put together
Appliance figures come from our sourced appliance wattage chart. Peak sun hours are NASA POWER long-term climatology (parameter SI_TILTED_AVG_LATITUDE), pulled November 2, 2026. The sizing rules match our solar sizing guide. Everything runs in your browser; nothing you enter is sent anywhere.
Note: This is a planning estimate. Have a licensed electrician or solar installer check the final design, wire sizes and overcurrent protection.
Frequently Asked Questions
How do I size an off-grid solar system?
Add up the watt-hours you use per day, divide by your winter peak sun hours and multiply by 1.25 for losses to get the array size. Size the battery for the days of backup you want divided by the battery's usable fraction, and the inverter for 1.25 times everything that runs at once.
How many solar panels and batteries do I need for an off-grid cabin?
A small cabin using about 3 kWh a day with 4 peak sun hours needs roughly 950 W of panels (three 400 W panels) and about 7 kWh of LiFePO4 storage for two days of backup. Designing for December sun in a northern state can double the panel count.
What size inverter do I need for off-grid?
Add the running watts of everything that can be on at the same time and multiply by 1.25 for the continuous rating. Then make sure the inverter's surge rating covers that running total plus the starting surge of the largest motor, such as a well pump or refrigerator compressor.
Should I use 12V, 24V or 48V for off-grid solar?
As a rule of thumb, 12 V suits systems under about 2 kWh a day (vans, RVs, sheds), 24 V suits 2–5 kWh a day, and 48 V suits anything larger. Higher voltage means lower current, thinner cables and fewer charge controllers for the same power.
Should I size for winter or the annual average?
For a system with no generator backup, size for the worst month you will live there, usually December. Sizing on the annual average leaves you short in winter. Many people size panels for spring and fall and cover the darkest weeks with a generator.
