Ethan BrooksBy Ethan Brooks

Updated November 2026

Planning an off-grid solar system comes down to one core task: matching the energy you use each day with the energy your panels and batteries can reliably supply, including during cloudy weeks and winter. This guide walks through that process in order, from listing every load to sizing the array, battery bank, charge controller and inverter, and wiring the pieces together. By the end, you will have a rough design you can refine with a proper calculator or a licensed installer.

Step 1: Define What the System Must Power

Start by deciding whether the system will run an entire house, a cabin, a workshop or a shed. The answer changes everything downstream. A cabin that only runs lights, a small fridge and a laptop needs a very different design than a full home with a well pump, electric range and air conditioning. If you are still deciding on the general layout, the overview in What Is Meant by Off Grid Solar System? A Plain-English Answer explains the basic components before you commit to a design.

Shopping for one? See our guide: 6 Best DIY Solar Power System Kit Options for Off-Grid Homes (2026).

Step 2: Build an Energy Audit (Watt-Hours per Day)

Every appliance needs to be converted into daily energy use, measured in watt-hours (Wh). The formula is simple: A related read: Water System Off Grid: A Step-by-Step Guide to Planning Yours.

Daily Wh = Power (W) × Hours of use per day

For a 60 W LED lamp used for 5 hours, that is 300 Wh per day. For a refrigerator, check the nameplate or the energy guide label, which often lists kilowatt-hours per year. Divide that by 365 to get an average daily figure. Add a margin for appliances you have not yet planned for. Most planners add 20 to 30 percent to the total so the system is not run to its limit on day one.

Watch Out for Surge Loads

Motors and compressors draw several times their running wattage at startup. A refrigerator compressor, well pump or table saw can require a surge rating two to three times its running watts. The inverter must handle that surge, so note it alongside the daily energy figure.

Step 3: Choose a System Voltage

Most small and mid-sized off-grid systems run at 12 V, 24 V or 48 V DC. Higher voltage reduces current for the same power, which means thinner wire and lower losses over distance. A rough guide:

System Size (Daily Energy) Typical Battery Voltage Notes
Under about 1.5 kWh/day 12 V Simple wiring, widely available parts
About 1.5 to 5 kWh/day 24 V Halves current compared with 12 V
Above about 5 kWh/day 48 V Standard for larger homes and workshops

These ranges are planning guidelines, not hard rules. Stick with one voltage for the entire system, and make sure your inverter and charge controller support it.

Step 4: Size the Battery Bank

The battery bank stores energy for nights and cloudy periods. Use this sequence:

  1. Take your total daily Wh from Step 2.
  2. Decide how many days of autonomy you want (often 2 to 3 days in sunny climates, more in regions with long winter overcast).
  3. Divide by the usable depth of discharge. For lithium iron phosphate (LiFePO4) batteries, 80 to 90 percent is common. For lead-acid, 50 percent is the usual limit.
  4. Divide again by system voltage to get amp-hours (Ah).

Example: A cabin uses 2,000 Wh per day, wants 2 days of autonomy, uses LiFePO4 at 90 percent depth, and runs at 24 V.

(2,000 × 2) ÷ 0.90 = 4,444 Wh. Divide by 24 V = about 185 Ah. Round up to the next available battery size.

Also account for inverter and wiring losses, which typically add another 10 to 15 percent to the energy you must store.

Step 5: Size the Solar Array

Next, determine how much solar power you need to recharge the batteries and run daytime loads. Use your daily energy figure and your site’s peak sun hours, which you can find in NREL’s solar resource maps or similar regional data.

Array watts = Daily Wh ÷ (peak sun hours × system efficiency)

A typical efficiency factor is 0.7 to 0.8 to cover temperature, dust, wiring and charge losses. For 2,000 Wh per day, 4 peak sun hours, and 0.75 efficiency: 2,000 ÷ (4 × 0.75) = about 667 W. Round up to 700 W or more. Winter sun hours can be far lower than summer, so size for your worst month, not the annual average, if the system must run year-round.

Mounting and Orientation

Tilt panels roughly equal to your latitude for year-round performance, or steeper for winter-heavy use. Face them south in the Northern Hemisphere and north in the Southern Hemisphere. Shading from trees or nearby structures can cut output sharply, so check the site at different times of day.

Step 6: Select the Charge Controller

The charge controller regulates current from the panels into the batteries. Two main types exist:

  • PWM (pulse-width modulation): Simpler and lower cost, suitable for smaller arrays where panel voltage is close to battery voltage.
  • MPPT (maximum power point tracking): Converts higher panel voltage down to battery voltage and typically harvests more energy, especially in cold weather or when panels are wired in series.

Size the controller for the array’s short-circuit current or wattage at battery voltage, with a safety margin. Check the maximum PV input voltage rating against your open-circuit panel voltage in the coldest expected temperature, since panel voltage rises when it is cold.

Step 7: Choose the Inverter

An inverter converts DC battery power into the 120 V or 240 V AC most household appliances need. Select a continuous wattage rating at least 25 percent above your highest expected simultaneous load, and confirm the surge rating covers your largest motor startup from Step 2. Pure sine wave inverters are the safer choice for electronics, motors and modern appliances.

Step 8: Plan Wiring, Fusing and Protection

Wiring is where many DIY systems fail. Follow these rules:

  • Size DC cable for the current it carries and the distance involved. Longer runs and lower voltages need thicker wire to limit voltage drop, commonly kept under 3 percent.
  • Place a fuse or breaker within 7 inches of each battery positive terminal, as most inverter and battery manufacturers specify.
  • Include a DC disconnect between the array and controller, and between the battery and inverter.
  • Ground the system according to the National Electrical Code (NEC) and local requirements.

Because off-grid wiring involves high-current DC and sometimes AC backup connections, consult a licensed electrician or qualified solar installer before finalizing the design, especially for any connection to a home’s main panel.

Tips for a Reliable Plan

  • Write the design down as a single-line diagram before buying anything.
  • Plan for growth: leave space on the controller and battery bank for future expansion.
  • Run a worst-case check using the lowest sun month and your highest load month.
  • Keep a log of actual usage for the first month, then adjust.

Common Mistakes

  • Sizing only for average sun. Winter output can fall dramatically, leaving the batteries empty for weeks.
  • Ignoring surge loads. Inverters shut down when a motor starts if the surge rating is too low.
  • Undersizing the battery bank. Lead-acid batteries discharged deeply every night age quickly.
  • Forgetting phantom loads. Standby power from chargers, routers and clocks adds up over a day.
  • Mixing battery types or sizes. Unmatched batteries in one bank lead to uneven charging.
  • Skipping voltage drop calculations. Thin wire over long runs wastes power and can overheat.
  • Kill A Watt P3 Electricity Usage Monitor: Plugs between an appliance and the wall to measure real watts and daily watt-hours, which makes the energy audit far more accurate than nameplate ratings. View on Amazon
  • Victron Energy SmartShunt: Tracks battery state of charge, voltage and current so you can verify how much usable energy your bank actually delivers. View on Amazon
  • Fluke 117 Multimeter: Measures DC voltage, current and resistance, useful for checking voltage drop across cables and confirming panel output. View on Amazon

FAQ

How many solar panels do I need for an off-grid home?

It depends on daily energy use and local sun hours. Divide your daily watt-hours by peak sun hours and an efficiency factor of about 0.75. A home using 10 kWh per day in an area with 4 peak sun hours would need roughly 3.3 kW of panels.

How long should I plan for battery backup?

Two to three days is common for sunny climates. Regions with long overcast winters often plan for more, or add a generator as backup.

Can I use lead-acid batteries instead of lithium?

Yes, but lead-acid usually allows only about 50 percent depth of discharge, so the bank must be roughly twice the size of a LiFePO4 bank for the same usable energy. Lead-acid is heavier and needs more maintenance.

Do I need a PWM or MPPT controller?

MPPT is generally the better choice for arrays above a few hundred watts or when panel voltage is much higher than battery voltage. PWM suits small, simple systems.

What is the difference between a pure sine wave and modified sine wave inverter?

Pure sine wave output matches grid power and works with all appliances, including motors and sensitive electronics. Modified sine wave is cheaper but can cause heating and noise in some devices.

Should I have a backup generator?

A generator is useful insurance for long cloudy stretches or heavy seasonal loads. Many off-grid systems include an AC or DC generator input for this purpose.

Do I need permits for an off-grid solar system?

Requirements vary by jurisdiction. Many areas require electrical permits even for standalone systems. Check with your local building department and consult a licensed installer for final wiring.

Conclusion

Planning an off-grid solar system is a sequence of measurable steps: audit your loads, pick a voltage, size the battery bank and array, then match a controller and inverter. Work with your worst-case month, leave room to expand, and verify wiring against code. Use the calculations here as a starting design, and have a professional review the final layout before installation.

Ready to choose one?

Ethan Brooks

About Ethan Brooks

Ethan Brooks is part of our editorial team and reviews and edits our DIY solar power systems guides, checking product details against manufacturer specifications and owner feedback.