Off Grid Solar System: Sizing Panels, Batteries and Inverter

An off grid solar system has no safety net. When a grid-tied array comes up short, the utility quietly fills the gap; when an off-grid cabin or home runs out of stored energy on the fourth cloudy day in January, the lights simply go out. That is why off-grid design starts with your loads, not your panels, and why the battery bank and a backup generator matter as much as the array on the roof.

The basic recipe is straightforward. Add up your daily energy use in watt-hours, decide how many sunless days the batteries must carry you, size the battery bank to cover that with a safe depth of discharge, size the array to refill the bank during your worst solar month, then choose a charge controller and inverter/charger that can handle the peak power. Getting each number right is what separates a reliable system from an expensive frustration.

What an Off Grid Solar System Includes

Every standalone solar setup has the same core building blocks:

• Solar array: panels on a roof or ground mount, wired in series and parallel strings.
• Charge controller: regulates panel output to charge the batteries safely. MPPT controllers are the standard for anything larger than a small shed system.
• Battery bank: stores energy for nights and cloudy stretches.
• Inverter/charger: converts battery DC to 120/240-volt AC for household loads and charges the batteries from a generator when needed.
• Generator: covers extended bad weather and heavy seasonal loads.
• Balance of system: fuses, breakers, DC and AC disconnects, grounding, wiring and monitoring.

Many buyers start with an off-grid solar kit that bundles panels, a controller, an inverter and sometimes batteries. Kits simplify component matching, but you still need to confirm the sizes fit your loads and climate. A solar backup power system for a grid-connected house is a different design, since it can lean on the utility most of the time.

Step 1: Do a Load Audit

List every device you plan to run, its wattage and how many hours per day it operates. Multiply watts by hours to get watt-hours per day, then total them. Use nameplate ratings or, better, a plug-in watt meter for a week of real data.

• Efficient refrigerator: roughly 1,000 to 1,500 Wh/day
• LED lighting for a small home: 300 to 800 Wh/day
• Well pump: highly variable, often 500 to 1,500 Wh/day plus a big starting surge
• Laptop, router and phones: 200 to 500 Wh/day
• Washing machine: 300 to 500 Wh per load

A frugal cabin might need 2 to 4 kWh per day; a full-time family home with a well, efficient appliances and no electric heat often lands between 8 and 20 kWh. Electric resistance heat, electric water heating and electric cooking push numbers far higher, which is why most off-grid homes use propane, wood or heat pumps for those jobs. Cutting loads is the cheapest part of solar energy off grid: every kilowatt-hour you remove saves panels, batteries and generator fuel.

Also note peak power. List what might run simultaneously and the starting surge of any motors. A well pump can draw three to five times its running watts for a split second, and the inverter must handle that.

Step 2: Choose Days of Autonomy

Days of autonomy is how long the battery bank should carry your loads with little or no solar input. Sunny, mild climates with a generator often use 1 to 2 days. Cloudy northern climates, or sites without a generator, may need 3 to 5. More autonomy means a bigger, costlier bank, so most designers pair 2 to 3 days of storage with an automatic-start generator for longer storms.

Step 3: Size the Battery Bank

Use this formula:

Battery capacity (Wh) = daily use × days of autonomy ÷ usable depth of discharge ÷ inverter efficiency

Example: 6,000 Wh per day × 2 days ÷ 0.8 (lithium iron phosphate usable fraction) ÷ 0.9 (inverter efficiency) is about 16,700 Wh, or roughly 17 kWh. With flooded lead-acid batteries limited to about 50 percent discharge for decent life, the same home would need around 27 kWh of nameplate capacity.

Lithium iron phosphate (LiFePO4) is now the common choice for new builds: more usable capacity, thousands of cycles, no watering and far less weight. Lead-acid still costs less up front and tolerates very cold charging better, but it needs ventilation for hydrogen gas and regular maintenance. Cold matters for lithium: most LiFePO4 batteries should not be charged below about 32°F unless they have built-in heaters, so plan an insulated, conditioned battery space.

Choose system voltage by size. Small systems run at 12 volts; mid-size cabins at 24 volts; most homes at 48 volts, which keeps currents and wire sizes manageable.

Step 4: Size the Array for Your Worst Month

Size panels to refill the battery during the month with the least sun you plan to rely on, measured in peak sun hours. Look up peak sun hours for your location from a solar resource map; December values in northern states can be 1.5 to 2.5 hours versus 5 to 6 in the summer.

Array size (W) = daily use ÷ peak sun hours ÷ system efficiency (about 0.75)

For 6,000 Wh per day at 3 peak sun hours: 6,000 ÷ 3 ÷ 0.75 is about 2,670 W, so roughly 2.7 to 3 kW of panels. Sizing for the darkest December week can double that, which is why many owners size for spring and fall and let the generator handle the deepest winter. Tilt panels steeper for better winter harvest, and keep them clear of snow and shade.

Step 5: Pick a Charge Controller

MPPT charge controllers harvest 10 to 30 percent more energy than older PWM designs, especially in cold weather and with higher-voltage panel strings. Size by current: array watts divided by battery voltage, plus a safety margin of about 25 percent. A 3,000 W array on a 48-volt bank needs roughly 63 amps, so a 70 to 80 amp controller fits. Also confirm the array’s maximum open-circuit voltage on the coldest morning stays below the controller’s input limit, since panel voltage rises as temperature drops.

Step 6: Choose the Inverter/Charger

An off-grid inverter/charger should be pure sine wave, sized for your continuous peak load with headroom, and able to handle motor surges. For many homes that means 4,000 to 8,000 W continuous at 48 volts, with 120/240-volt split-phase output if you have a well pump or other 240-volt equipment. The built-in charger lets a generator recharge the batteries; check that its charging rate suits your bank size. Idle consumption matters too, since an inverter left on 24 hours a day can quietly burn 1 kWh daily. All-in-one units that combine inverter, charger and MPPT controller simplify wiring for small and mid-size systems.

Step 7: Plan Generator Backup

A generator turns a fragile system into a dependable one. Size it to run the inverter/charger at full charge rate plus any loads running at the same time; many homes use 5 to 10 kW propane or diesel units. Automatic generator start, triggered by battery state of charge, prevents deep discharges when you are away. Keep generators outside, away from windows and doors, and install carbon monoxide alarms in the home. Gas and propane piping to a standby generator is licensed-pro work.

Off Grid Solar System Installation Overview

1. Finalize the load audit and design with a qualified designer or installer.
2. Check permits: many counties require electrical permits even for remote cabins, and some require structural review of roof or ground mounts.
3. Build racking into rafters or footings designed for local wind and snow loads.
4. Mount panels, then run conductors in conduit to a combiner and DC disconnect.
5. Install the battery bank, controller and inverter/charger in a ventilated, temperature-controlled space with required clearances.
6. Install fusing and disconnects on every source, grounding and bonding, and the AC distribution panel.
7. Commission settings for your battery chemistry, test generator auto-start, and arrange inspection.

Panels produce voltage whenever light hits them, so installers cover modules or open DC disconnects before connecting strings. Before any AC work, breakers are turned off and conductors are confirmed dead with a non-contact voltage tester.

Fire and Battery Safety

Battery banks can deliver thousands of amps into a short circuit, so every battery string needs a properly rated DC fuse or breaker close to the terminals. Use listed components and cabling sized for the current with headroom, torque terminals to spec, and never leave tools on top of batteries. Keep batteries protected from extreme cold, away from heaters and fuel storage, and in a room with smoke detection. Lead-acid banks need ventilation to disperse hydrogen. Any lithium battery that swells, smells hot or throws temperature alarms should be isolated per the manual and inspected by a professional.

Troubleshooting and Costs

• Batteries never reach full: array too small for winter, shading, dirty panels or charge settings wrong for the chemistry.
• Inverter trips when the pump starts: surge rating too low; consider a soft starter or larger inverter.
• Generator runs constantly: loads grew or the array is undersized; revisit the audit.

Costs vary with size and remoteness. A small cabin setup might run from a few thousand to around $10,000 in equipment; a full-time home with a 5 to 10 kW array, 20 to 40 kWh of lithium storage, an inverter/charger and a generator commonly lands between $30,000 and $70,000 or more installed. Off grid solar system installation at remote sites costs more for trenching, delivery and labor travel.

When to Call a Licensed Professional

Off-grid does not mean off-code. Battery banks, inverters, generator connections and household wiring carry lethal energy, and most jurisdictions require electrical permits and inspections, plus structural review for roof and ground mounts. A licensed electrician or experienced off-grid solar installer should design and connect the system, and a licensed gas fitter should handle propane or natural gas piping to a generator. If you want to handle part of the work, many installers will let you do site prep, trenching or racking under their supervision. Your most valuable contribution is an accurate load audit and a realistic view of how much generator runtime you will accept.