Off-Grid Solar System: How to Size Panels, Batteries and Backup
An off grid solar system powers a home or cabin with no utility connection at all. That independence is appealing, but it comes with responsibility. Because there is no grid, nothing fills the gaps at night, during storms or through a dark winter. Every watt-hour must come from your panels, your batteries or your backup generator. This guide shows how to size off grid solar correctly, so it works in January as well as June.
Last reviewed and updated: October 2, 2026. Written by Jeremy Jarvis.
Key takeaways
- An off grid solar system must cover every day of the year on its own, so size it for the darkest month, not summer.
- Start with daily watt-hours, choose days of autonomy, then size usable battery capacity and panel wattage.
- A backup generator turns a fragile system into a dependable one for long cloudy stretches.
- Cut big electric loads first; propane, wood or efficient appliances shrink the system you need.
We will walk through daily load, days of autonomy, battery sizing, panel sizing for the worst month, inverters, charge controllers and backup power. You can follow along with our free Solar Panel Calculator and Home Battery Backup Calculator.
Off grid vs. grid-tied solar
Most home solar systems are grid tied. They send extra power to the utility and draw from it when the panels fall short. The grid acts as an unlimited battery. By contrast, an off grid solar system stands alone. Batteries store energy for night and cloudy days, and a backup generator often covers long stretches without sun.
Because of that, off-grid systems are designed differently. As a result, they need larger batteries, careful load management and planning for the worst weather of the year. They also need a different kind of inverter, because it must create power on its own rather than follow the grid.
Step 1: Find your daily energy use
First, everything starts with daily watt-hours. List every device, its watts and the hours it runs each day. Multiply watts by hours for each, then add them up. Include the loads people forget: a refrigerator that cycles all day, a well pump, internet equipment, phone chargers, fans and the standby draw of electronics.
Above all, be realistic and be honest. Off-grid systems punish optimism. A plug-in energy meter helps measure real use of appliances that plug in. For hardwired loads, a professional can measure current. If you are moving off grid from a grid-connected home, your bills show yearly use, but your off-grid lifestyle may change it a lot.
Step 2: Choose days of autonomy
Days of autonomy means how many days your batteries can carry the load without any solar input. For example, one or two days may suit a summer cabin with a generator nearby. Full-time homes in cloudy climates often plan for more. The more autonomy, the bigger and more expensive the battery bank.
In addition, your backup plan shapes this choice. If you have a dependable generator, you can plan for fewer days of autonomy and let the generator cover longer cloudy spells. Without a generator, you need more battery and more panels to stay safe.
Step 3: Size the off grid solar system battery bank
Multiply daily watt-hours by days of autonomy. Then account for usable capacity and inverter losses. Lithium iron phosphate batteries allow you to use most of their rated capacity. Lead-acid batteries last longer when you avoid deep discharges, so you can use less of their rated capacity day to day. Divide your energy need by usable capacity per battery to find how many you need.
Here is an example. A small off-grid home uses about 5 kWh a day and wants two days of autonomy, or about 10 kWh. With a 90 percent efficient inverter, the batteries must deliver about 11 kWh. If each lithium battery offers 5 kWh usable, that means three batteries, with a little margin. The Home Battery Backup Calculator runs this math with your own figures.
Step 4: Plan for the worst month
However, this is where many off-grid systems fall short. Sun hours vary greatly by season. Summer may bring long, bright days, while winter may bring short days, a low sun and frequent clouds. If you size panels from an annual average, the system can run short exactly when you need it most.
Instead, find the peak sun hours for your worst month. Many off-grid designers use December or January for northern locations. Size the panels so they can refill your daily use in those conditions. Then decide whether a generator will cover the rare stretches when even that is not enough.
Step 5: Size the off grid solar system array
Divide your daily watt-hours by your worst-month sun hours. Then allow for losses from heat, dust, wiring, charge controller efficiency and battery charging. A loss allowance of around 20 to 30 percent is common for off-grid planning. The result is the array wattage you need.
Continuing the example, 5 kWh a day with 3 winter sun hours and 25 percent losses needs about 2.2 kW of panels. In summer, by contrast, the same array will produce far more than needed. That surplus is normal in off-grid design. Some owners use it for extra loads in summer, such as a water pump or a freezer.
| Setup | Typical daily use | Notes |
|---|---|---|
| Weekend cabin | Lights, phone charging, small fridge | Small battery, small array, generator optional |
| Full-time small home | Fridge, well pump, lights, internet, laptop | Moderate battery and array, generator recommended |
| Full-time family home | Larger fridge, laundry, more electronics | Large battery and array, generator strongly recommended |
| All-electric home | Electric heat, cooking and water heating | Very large and costly; usually redesigned around propane or wood |
Off grid solar system controllers and inverters
First, a charge controller sits between the panels and batteries. It regulates charging so batteries are not overcharged. Maximum power point tracking (MPPT) controllers extract more energy from panels, especially in cold weather and with higher-voltage arrays. They are standard for most off-grid systems.
The inverter turns battery DC power into household AC. Off-grid inverters must form the grid on their own, and they must handle your largest loads and motor starts. Check continuous and surge ratings, and choose a pure sine wave model for modern appliances. Many off-grid systems now use combined inverter-chargers that also charge batteries from a generator.
System voltage
In general, off-grid battery banks run at 12, 24 or 48 volts. Small systems often use 12 volts. Larger systems usually use 48 volts, because higher voltage means lower current for the same power, which allows smaller wires and less loss. As a rule, the bigger the system, the more sense 48 volts makes. Choose components that match the system voltage.
Backup generators for an off grid solar system
A generator turns an off-grid system from fragile to dependable. It recharges the batteries during long cloudy periods and covers unexpected loads. Without one, you would need a much larger battery bank and array to handle rare worst-case weather. Many off-grid owners run a generator only a handful of times each year.
Inverter-chargers can start some generators automatically when batteries fall to a set level. Size the generator to charge the batteries efficiently and run essential loads at the same time. Our guide to choosing a generator for home use covers types and safety. Run generators outdoors, away from windows and doors.
Loads to rethink
Meanwhile, some loads make off-grid solar impractical. Electric resistance heat, electric water heaters, electric ranges and clothes dryers use large amounts of energy. Central air conditioning is also demanding. Most off-grid homes use propane, wood or other fuels for heat, cooking and hot water. They choose efficient refrigerators, LED lighting and efficient well pumps.
After all, every watt-hour you avoid saves panels and batteries. Therefore, efficiency is the cheapest part of any off-grid system. Measure, then cut waste, then size.
Off grid solar system permits, codes and safety
Even so, off-grid does not mean exempt from codes. Many jurisdictions still require electrical permits and inspections for systems connected to building wiring. The National Electrical Code includes requirements for solar and battery systems. A qualified installer or electrician should design and inspect anything beyond a simple portable setup.
Also, batteries store a lot of energy. Install them according to the manufacturer’s instructions, with proper fusing, disconnects and ventilation where required. Keep them within their temperature range. Lithium batteries often have built-in management systems, while lead-acid batteries can release gas when charging and need ventilation.
Living with an off grid solar system
Daily habits matter more off grid than on. On sunny days, run the big loads: laundry, the well pump for filling a storage tank, power tools and vacuuming. On cloudy days, conserve. Many off-grid households check their battery state of charge each evening, the way others check the weather. That simple habit prevents most surprises.
Fortunately, monitoring makes this easy. Most modern charge controllers and inverters report production, consumption and battery state in an app or on a display. Watch the patterns for a few weeks. You will learn which days the generator is likely to be needed, and which loads drain the batteries fastest.
Maintenance and lifespan
An off grid solar system needs regular, simple care. Keep panels clear of snow, leaves and dust. Check wiring connections and fuses yearly. For lead-acid batteries, follow the maker’s maintenance steps, which may include equalization charges and, for flooded types, checking water levels. Lithium batteries need far less hands-on care but still benefit from staying within their temperature range.
Over time, components wear at different rates. Panels often carry long performance warranties. Inverters and charge controllers may need replacement sooner. Batteries wear with cycles and age, so budget for eventual replacement. Keeping a log of installation dates and warranty terms helps you plan.
What an off grid solar system costs to own
Up-front, the battery bank is usually the largest single cost, followed by panels, inverter-charger, controller, wiring and labor. Over time, battery replacement, generator fuel and maintenance add up. In remote locations, those costs can still be far lower than bringing in utility lines. In other places, a grid connection with solar and a battery may cost less overall.
Therefore, compare options honestly. Get a quote from the utility for a line extension if one is possible. Then compare it with the full lifetime cost of going off grid, including the generator. Some owners choose off-grid for independence even when it costs more, and that can be a fine choice when made with open eyes.
Starting small
If a full off-grid system feels like a leap, start small. A portable power station with folding panels can power a cabin’s lights and phones, or keep essentials running during outages at home. You will learn how sun, weather and loads interact before investing in a permanent system. Our solar generator guide explains that path.
Off grid solar system checklist
- List every load with watts and hours to find daily watt-hours.
- Choose days of autonomy based on climate and backup.
- Size usable battery capacity with losses included.
- Find worst-month sun hours for your location.
- Size the array for winter, with 20 to 30 percent losses.
- Choose an MPPT controller and a pure sine wave inverter.
- Pick a system voltage that suits the size.
- Plan a backup generator and safe outdoor placement.
- Check permits and hire a qualified installer.
Where to compare off-grid gear
Once you have numbers, compare current off-grid solar kits, 48-volt LiFePO4 batteries and off-grid inverter-chargers. Confirm specifications and compatibility with each manufacturer, and have a qualified installer review the design. For more, visit our Home Solar hub.
Frequently asked questions about off-grid solar systems
How big an off grid solar system do I need?
Add up your daily watt-hours, multiply by the days of autonomy you want for battery size, and size panels to refill that daily use in your worst month’s sun hours with allowance for losses. A cabin with lights and a fridge needs far less than a full-time home with electric heat.
How many batteries do I need for off grid solar?
Divide your daily energy use times days of autonomy by the usable capacity of each battery. Lithium iron phosphate batteries allow deeper use of their rated capacity than lead-acid batteries, so fewer may be needed for the same usable energy.
Can an off grid solar system run a whole house?
It can, but whole-house loads such as electric heat, central air, electric water heating and cooking make systems very large and expensive. Most off-grid homes use propane or wood for heat and cooking and efficient appliances to keep the solar system practical.
Do I need a generator with off grid solar?
Most off-grid systems benefit from a backup generator for long cloudy periods and winter. It recharges the batteries when solar cannot keep up, which avoids oversizing the solar and battery bank for rare worst-case weather.
What is the difference between off grid and grid tied solar?
Grid-tied solar connects to the utility, which supplies power when panels do not. Off-grid solar has no utility connection, so batteries and backup generation must cover every hour, including nights and cloudy days.
Sources and how we researched this guide
This guide is research-based. We did not build the systems described; we explain standard off-grid sizing methods using Department of Energy guidance and the math in our free Solar Panel and Home Battery Backup calculators. A qualified installer should design any system connected to building wiring. Found something out of date? Tell us and we will check it against the source.
- U.S. Department of Energy: Homeowner’s guide to going solar
- U.S. Department of Energy: Solar integration and distributed energy
- NFPA 70: National Electrical Code
- Ready.gov: Power outages
Keep planning
- Solar Panel Maintenance: What Panels Need and What They Do Not
- Net Metering Explained: How Solar Export Credits Work and Why They Matter
- Solar Tax Credit in 2026: What Ended, What Remains and What to Do Now
- Are Solar Panels Worth It? How to Decide With Your Own Numbers
Go deeper: Home Solar hub · Solar Panel Calculator · Solar Payback Calculator
