Fully Off-Grid Solar System: Step-by-Step Sizing and Wiring Guide

By Mason Hollis

Updated November 2026

A fully off-grid solar system produces all of your electricity from sunlight and stores enough energy in batteries to run your home through nights, cloudy stretches and winter months without any utility connection. In this guide you will learn how to calculate your daily energy use, size the panels, batteries, charge controller and inverter, and wire everything into a safe, working system. If you are still deciding between approaches, the overview at What Is Meant by Off Grid Solar System? A Plain-English Answer explains the basics.

What Makes a System Fully Off-Grid

A fully off-grid setup has no utility tie-in and no backup generator carrying the load on a regular basis. Every watt-hour your home uses must come from panels or from the battery bank that the panels charge. That single requirement changes the sizing math: you cannot rely on the grid to cover a bad week of weather, so you design for your worst realistic stretch of sun, not your average day.

Shopping for one? See our guide: 7 Best DIY Solar Power System Kit Options for Off-Grid Beginners (2026).

Compare this with a grid-tied or hybrid design, covered in On-Grid vs Off-Grid Solar System: Which Should You Choose? and Off-Grid, Backup and Hybrid Solar Systems: How They Differ. Those systems can export surplus or pull from the utility when batteries run low. A fully off-grid system cannot, so the margins need to be deliberately larger.

Step 1: Calculate Your Daily Energy Load

Start with an inventory of every appliance and device you plan to run, including the ones you use only occasionally. For each item, record its power rating in watts, how many hours per day it runs, and the resulting watt-hours. Watt-hours per day equal watts multiplied by hours of use.

A simplified example for a small off-grid home:

Manufacturers’ nameplate ratings often overstate real use, but for a first pass they are safer than guessing. Motors and compressors draw a surge at startup, so note those separately for inverter sizing later.

Add a 20 percent margin to cover losses, aging batteries and appliances you forgot. In the example, 2,005 Wh becomes roughly 2,400 Wh per day.

Step 2: Determine Your Design Sun Hours

Panels are rated under standard test conditions, which assume peak sun. Your site gets fewer effective hours. Use peak sun hours for your location and season, which you can find in utility-scale solar resource maps or from your installer’s data. Design for the worst month, typically winter, because a system that works in July may fail in December.

Peak sun hours in a northern winter can be a third or less of summer values. This is the factor most DIYers underestimate, and it drives the whole array size.

Step 3: Size the Solar Array

Use this formula:

1. Required panel output (W) = daily load (Wh) ÷ design peak sun hours
2. Apply a derate factor of about 0.75 for wiring, temperature, dust and controller losses
3. Panel count = required output ÷ panel wattage, rounded up

Continuing the example: 2,400 Wh ÷ 2.5 winter sun hours = 960 W, then 960 ÷ 0.75 = 1,280 W. Four 400 W panels give 1,600 W, which leaves room for cloudy days and panel aging. Mount panels with tilt adjusted for winter if you can, since low-sun months dominate the design.

Step 4: Size the Battery Bank

The battery bank must cover your nightly load plus several days of reduced sun. Use this sequence:

1. Choose days of autonomy. Fully off-grid designs commonly use two to three days.
2. Battery energy needed (Wh) = daily load × days of autonomy
3. Divide by usable depth of discharge (DoD). Lithium iron phosphate typically allows around 80 to 90 percent; lead-acid is usually limited to about 50 percent.
4. Divide by system voltage to get amp-hours.

Example with 2,400 Wh per day, two days of autonomy, 90 percent DoD on a 48 V lithium bank: 4,800 Wh ÷ 0.9 = 5,333 Wh; 5,333 ÷ 48 V ≈ 111 Ah. A 100 Ah 48 V battery would be slightly undersized, so a 150 Ah or two 100 Ah units in parallel is a safer target. Follow the manufacturer’s maximum parallel configuration.

For a deeper look at battery packaging and matching, see Off Grid Solar System Packages with Batteries: How to Choose and Size One.

Step 5: Choose the Charge Controller

The charge controller regulates current from the panels into the batteries. Use an MPPT controller for a system of this size, since MPPT handles higher array voltages and recovers more energy than PWM units, especially in cold weather.

Size the controller on both current and voltage:

• Controller amperage = total array watts ÷ battery voltage × 1.25 safety margin
• Maximum PV open-circuit voltage must stay below the controller’s rated input, checked at the coldest expected temperature because panel voltage rises in cold.

In the example, 1,600 W ÷ 48 V × 1.25 ≈ 42 A, so a 50 A MPPT controller fits.

Step 6: Select the Inverter

The inverter converts battery DC to household AC. Two numbers matter: continuous wattage and surge capacity. Continuous rating must exceed the sum of simultaneous loads. Surge rating must handle motor startup, such as a well pump or refrigerator compressor, which can draw two to three times running power for a moment.

In the example, simultaneous loads of about 1,200 W with a 750 W pump starting suggests a 3,000 W continuous inverter with generous surge capacity. Most off-grid inverters sold for this purpose are pure sine wave units, which are safer for electronics and motors. Further selection detail is in Off Grid Solar System Inverter: How to Choose and Size One (Step-by-Step).

Step 7: Wire the System

Wiring follows a set order, and the sequence matters for safety:

1. Mount panels and run PV cable to the controller, keeping runs short to limit voltage drop.
2. Connect the battery bank to the controller first, with a fused disconnect.
3. Connect the inverter to the battery bank through a properly sized DC breaker or fuse.
4. Connect the AC output of the inverter to your distribution panel.
5. Ground the system according to your local electrical code.

Use conductor sizes that keep voltage drop under about 3 percent on DC runs. Fuse every positive battery conductor close to the battery terminal. Never connect or disconnect batteries while the inverter or controller is powered.

Recommended gear

• Victron Energy SmartSolar MPPT controller: an MPPT controller with Bluetooth monitoring that many off-grid builders use to track charge state and array output. View on Amazon
• Renogy Rover MPPT controller: a widely sold MPPT option for small to mid-size off-grid arrays, with multiple battery voltage settings. View on Amazon
• Battery monitor shunt: a shunt-based monitor shows true state of charge in amp-hours, which voltage alone cannot reliably give. View on Amazon
• Pure sine wave inverter: a continuous-rated unit with surge capacity sized to your largest motor load. View on Amazon

Common mistakes

• Sizing the array on summer sun hours instead of winter values.
• Forgetting the derate factor, which leaves the system short on real cloudy days.
• Ignoring motor startup surge when picking the inverter.
• Using lead-acid batteries and assuming 100 percent of rated capacity is usable.
• Exceeding the controller’s maximum PV voltage on cold mornings.
• Skipping fuses or breakers on DC lines, which creates a serious fire risk.

Tips for a Reliable Build

• Log actual daily consumption for a month before expanding the system.
• Build in a generator input for emergencies; a fully off-grid system still needs a plan for extended storms.
• Track system performance with a battery monitor and check connections at least twice a year.
• Read the battery and controller manuals for temperature limits, especially for lithium chemistry in freezing conditions.

For a comparison of how a full system fits a home versus a cabin, see Off Grid Solar System for Small Cabin: 5 Best Picks Compared (2026) and Renogy Off-Grid Solar System: How to Size and Wire One (Step-by-Step).

FAQ

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

It depends on daily consumption and winter sun hours. A home using about 2,400 Wh per day in an area with 2.5 winter peak sun hours typically needs roughly 1,500 W of panels after derating.

How many days of battery backup should I plan for?

Most fully off-grid designs use two to three days of autonomy. Cold, cloudy regions often justify the higher end.

Can I run a fully off-grid system without batteries?

Not in a practical sense for a home. Without storage, power is available only while the sun is shining, and most households need electricity at night.

Is lithium or lead-acid better for off-grid storage?

Lithium iron phosphate offers deeper usable capacity, longer cycle life and lighter weight. Lead-acid costs less up front but needs more capacity to deliver the same usable energy. Choose based on your budget priorities and maintenance tolerance.

Do I need a backup generator?

Many builders keep one for extended bad weather or battery service. A generator still counts as off-grid if you own and run it yourself rather than connecting to a utility.

Can I DIY a fully off-grid system safely?

Many people do, but DC wiring, battery systems and grounding carry real hazards. Have a licensed electrician or qualified solar installer review your design and inspection requirements before energizing the system.

How long do off-grid batteries last?

Lifespan depends on chemistry, depth of discharge and temperature. Manufacturers specify cycle counts at stated depths of discharge; check those figures against your planned daily use.

Conclusion

A fully off-grid solar system is built around your worst-case week, not your best day. Calculate your daily load, size the array on winter sun, add enough battery capacity for several days of autonomy, and match the controller and inverter to real current and surge demands. Verify your design against local electrical code and, where needed, a licensed professional before you go live.

About Mason Hollis

Mason puts together our diy solar power systems guides, digging through specifications, manufacturer details and owner reviews so you don't have to.