Ethan BrooksBy Ethan Brooks

Updated November 2026

To make an off grid solar system, you calculate your daily energy use, size the panels, battery bank, charge controller and inverter to match that load, then wire the components in the correct order and test each stage before you connect the loads. This guide walks through each step in plain terms, with the math shown so you can check your own numbers before buying anything.

What an Off Grid Solar System Actually Does

An off grid system generates its own electricity from sunlight and stores the surplus in batteries, so it runs without a utility connection. Unlike a grid-tied array, nothing feeds back to the utility, which means the battery bank and the sizing math carry the whole load. If you are still deciding between approaches, the comparison in On-Grid vs Off-Grid Solar System: Which Should You Choose? covers the tradeoffs.

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

Step 1: Calculate Your Daily Energy Use

Start with a load list. Write down every device you plan to run, its wattage, and how many hours per day it operates. Multiply watts by hours to get watt-hours (Wh) per day, then add everything up.

Device Watts Hours/Day Watt-hours/Day
LED lights (5 fixtures) 50 5 250
Refrigerator (efficient) 60 average 24 1,440
Laptop 60 4 240
Well pump (intermittent) 750 0.5 375
Total 2,305 Wh/day

Refrigerators and pumps have a startup surge far above their running wattage, so note the surge rating from the nameplate. Your inverter must handle that surge, not just the steady load.

Step 2: Size the Battery Bank

Battery capacity depends on your daily Wh, how many days of autonomy you want (cloudy days with no sun), the allowable depth of discharge (DoD), and system voltage.

The formula is:

Battery Wh needed = Daily Wh × Days of autonomy ÷ DoD

Using the example above with two days of autonomy and lithium iron phosphate (LiFePO4) batteries rated for about 80% DoD:

2,305 × 2 ÷ 0.8 = 5,763 Wh, or about 5.8 kWh of usable capacity.

Convert to amp-hours at your system voltage. At 24 V: 5,763 ÷ 24 = 240 Ah. At 48 V it is 120 Ah. Higher voltage means lower current, thinner wire and smaller breakers, so 48 V is common for systems above roughly 3 kWh per day. Lead-acid batteries typically need a lower usable DoD (about 50%), which roughly doubles the bank size.

For a closer look at how packaged battery sets are built, see Off Grid Solar System Packages with Batteries: How to Choose and Size One.

Step 3: Size the Solar Array

Panel sizing uses peak sun hours, the average daily solar energy for your location (check NREL’s PVWatts tool or a local solar resource map). Account for system losses, which typically run 20% to 25% from wiring, temperature, dust and conversion.

Array watts = Daily Wh ÷ (Peak sun hours × 0.75)

With 2,305 Wh per day and 4.5 peak sun hours: 2,305 ÷ (4.5 × 0.75) = 683 W. Round up to about 800 W to cover winter months and degradation. Four 200 W panels or two 400 W panels would meet this. Check that your panel open-circuit voltage stays within the charge controller’s maximum input rating, especially in cold weather when voltage rises.

Step 4: Choose the Charge Controller

The charge controller regulates battery charging from the panels. Two types are common:

  • PWM (pulse width modulation): simpler and lower cost, but the array voltage must closely match battery voltage.
  • MPPT (maximum power point tracking): extracts more power from higher-voltage strings and is generally the better choice for arrays above a few hundred watts.

Size the controller current as array watts ÷ battery voltage, then add a 25% margin. For 800 W on a 48 V bank: 800 ÷ 48 = 16.7 A, so a 20 A or larger controller is appropriate.

Step 5: Size the Inverter

The inverter converts battery DC into household AC. Choose continuous output rating above the sum of simultaneous loads, and surge capacity above the largest motor start. For the example load list, a 2,000 W pure sine wave inverter with a surge rating of at least 4,000 W for a few seconds handles the pump and refrigerator comfortably. Pure sine wave output is strongly preferred for motors and electronics. A deeper guide to matching inverter size appears in Off Grid Solar System Inverter: How to Choose and Size One (Step-by-Step).

Step 6: Plan the Wiring and Protection

Wire in this order: panels to charge controller, charge controller to battery bank, battery bank to inverter, and inverter to the AC distribution panel. Every connection from the battery bank needs overcurrent protection within a few inches of the battery terminal, sized to the cable ampacity.

  1. Mount panels and run DC cable from the array to the controller, using wire sized for less than 2% voltage drop.
  2. Connect the controller to the battery bank. Do not connect panels to the battery until the controller is wired and powered.
  3. Install a DC disconnect and fuse or breaker between the battery and inverter.
  4. Connect the inverter AC output to a subpanel with its own breakers and a ground fault protection device where code requires it.
  5. Bond and ground the system per local electrical code, including the array frame and inverter chassis.

Use cable rated for DC and outdoor exposure where needed. Torque all lugs to the battery and terminal manufacturer’s specification.

Step 7: Test Before Connecting Loads

Energize the system in sequence. Connect the battery first, verify polarity with a meter, then turn on the controller and confirm charging. Switch on the inverter with no load and check the AC output voltage. Add loads one at a time and watch for overheating connections and voltage sag during startup surges.

  • Victron SmartSolar MPPT controller: a widely used MPPT controller with Bluetooth monitoring, useful for tracking charge state through the build. View on Amazon
  • Multimeter with DC voltage and clamp current: needed to verify polarity, measure string voltage and check for voltage drop during testing. View on Amazon
  • Battery monitor shunt: reports state of charge and net amp-hours, which is far more accurate than voltage alone for LiFePO4 batteries. View on Amazon
  • Crimping tool and lug kit for battery cable: proper crimps on heavy cable prevent the hot spots that cause most DIY wiring failures. View on Amazon

Common mistakes

  • Sizing only for average load and ignoring inverter surge demand, which trips the inverter the first time the pump starts.
  • Forgetting to account for temperature: panel voltage rises in cold, and batteries lose usable capacity below freezing.
  • Using undersized wire for long runs, causing voltage drop and heat.
  • Skipping fusing on the battery side. A short across unfused battery terminals can release enough energy to cause a fire.
  • Mixing battery chemistries or ages in one bank, which leads to uneven charging and early failure.

Tips for a Reliable Build

  • Oversize the array by 20% to 30% beyond the minimum to absorb winter output and aging.
  • Use a battery monitor from day one so you learn your real consumption.
  • Document wire gauges, breaker ratings and battery serials in one place for future troubleshooting.

Electrical work at battery and high-DC-voltage levels carries real shock and fire risk. If you are unsure about any step, consult a licensed electrician or solar installer, especially for grounding and any connection to a home’s existing wiring.

FAQ

How much does it take to start an off grid solar system?

The starting point is your daily energy use in watt-hours, calculated from your load list. Every other component size follows from that number, so the calculation matters more than any single part.

Can I run a refrigerator on an off grid system?

Yes. An efficient refrigerator averages around 1 to 2 kWh per day in most homes, so it usually needs the largest share of battery and array capacity. Check the surge rating and make sure the inverter can start it.

What is the difference between PWM and MPPT controllers?

PWM controllers connect the array closely to battery voltage and waste power when panel voltage is higher. MPPT controllers convert excess voltage into additional charge current, making them more efficient, especially in cooler weather.

How many days of autonomy should I plan for?

Two to three days is a common target for most homes. Remote sites with long cloudy seasons may need more, which increases battery cost and size considerably.

Is lithium or lead-acid better for DIY off grid systems?

LiFePO4 offers higher usable depth of discharge, longer cycle life and lighter weight, so fewer amp-hours are needed for the same usable energy. Lead-acid has a lower upfront complexity but needs more capacity for the same usable energy.

Do I need a permit for an off grid solar system?

Many jurisdictions require permits and inspections for electrical and structural work, even for off-grid systems. Check with your local building department before installing.

Can I add more panels later?

Yes, provided the charge controller has spare input capacity and the battery bank can absorb the additional charge. Plan the initial wiring with room for expansion where possible.

Conclusion

To make an off grid solar system, begin with an accurate daily energy number and size every component from it: battery bank, array, controller and inverter. Wire in the correct order with proper fusing and grounding, then test each stage before connecting loads. Careful sizing and conservative wiring are what separate a reliable system from a frustrating one, and a licensed professional should review any step involving grounding or utility interconnection.

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.