☀ Off-Grid Power Systems

Off-Grid Solar System Size Calculator:
Complete 4-Component System Design

Size your solar panels, battery bank, charge controller, and inverter in one tool — with US AWG wire sizing, system voltage selection, and full cost breakdown. No guesswork, no missing components.

☀ Design Your System

Step 1 — Daily Energy Load
Enter your total daily electricity use. Add up all appliance watt-hours or use your monthly kWh bill divided by 30. Most off-grid cabins use 2-8 kWh/day; full homes 10-30 kWh/day.
kWh/day
W
Step 2 — System Voltage
24V: Sweet spot for 2-5kW systems — half the amps of 12V.
Step 3 — Location and Sun
hrs/day
Peak sun hours by region: Phoenix AZ 6.5 / Los Angeles 5.6 / Dallas TX 5.2 / Denver CO 5.1 / Atlanta GA 4.7 / Boston MA 4.0 / Seattle WA 3.8. Use NREL PVWatts for your exact ZIP code.
Step 4 — Components
Step 5 — Generator Backup (Optional)

☀ Your Off-Grid System Design

☀

Enter your daily load, choose your system voltage and battery type, then hit Design My System to get your complete off-grid solar BOM.

What Goes Into an Off-Grid Solar System — The Four Components You Cannot Skip

The internet is full of solar calculators that size one thing. You type in your kWh per day and get back “you need a 4 kW array.” That is exactly where most DIYers go wrong. A solar panel array is only one quarter of an off-grid system. Without the right battery bank, charge controller, and inverter — all properly sized to each other — the system either underperforms, destroys your batteries, or trips breakers at the worst possible moment. This calculator sizes all four components together, in one pass, so your system actually works as designed.

Here is a plain-English breakdown of what each component does:

  • Solar panels convert sunlight into DC electricity. The array size determines how much energy you can harvest per day.
  • Battery bank stores that energy for use at night or on cloudy days. The bank size determines how many days you can run without any sun.
  • Charge controller sits between the panels and the battery, regulating the incoming current so you do not overcharge and destroy your batteries. MPPT controllers squeeze 15-30% more power out of your panels than PWM.
  • Inverter converts the battery DC voltage (12V, 24V, or 48V) into the standard 120V AC electricity that your appliances actually use.

System Voltage: Why 12V vs 24V vs 48V Matters More Than You Think

Most beginners default to 12V because car batteries are 12V and all the cheap solar gear is labeled 12V. This is the single most common and expensive off-grid mistake. Here is why it matters. If you are running a 3,000-watt inverter on a 12V system, the battery has to deliver 250 amps continuously. That requires 4/0 AWG welding cable the thickness of your thumb, fuses rated at 300+ amps, and every connection point becomes a fire hazard if not perfectly torqued. The same 3,000W at 48V only requires 62.5 amps — thin 6 AWG wire, standard 80A breakers, and dramatically lower voltage drop across the system. Our calculator automatically adjusts your wire gauge recommendations based on your system voltage. This one input alone can save hundreds of dollars in wiring and eliminate significant safety risks.

How the Off-Grid Solar System Calculator Works

Our calculator uses a standard engineering sizing methodology used by professional off-grid installers across the US. Here is the exact math behind each output:

Solar Panel Array Sizing

The raw panel size needed equals your daily load divided by peak sun hours. But raw panels do not deliver 100% of their rated watts to usable electricity. You lose about 3% through the MPPT controller, 3% through battery charge-discharge inefficiency, 2% in wiring and connectors, and 1% in miscellaneous diode and fuse losses. Our calculator chains these losses together to find your true gross panel requirement, then adds a 20% reserve factor for panel degradation, seasonal variation, and dust accumulation. This is why a 5 kWh/day load requires roughly 2.2 kW of panels rather than the naive 5 / 4.5 = 1.1 kW that the simplified calculators show. Undersized panels mean your battery never fully charges, which destroys lead-acid batteries within 18 months and accelerates LFP degradation.

Battery Bank Sizing and the DoD Factor

Battery sizing depends critically on your chemistry choice. Lead-acid batteries (AGM, Gel, and Flooded) should only be discharged to 50% of their rated capacity. Going deeper accelerates sulfation and cuts your battery lifespan in half. LFP (lithium iron phosphate) batteries can safely discharge to 90% or lower, meaning you need about 40% fewer cells for the same usable storage. This is why our calculator asks for battery type before sizing the bank. A 10 kWh usable storage requirement needs 20 kWh of AGM nameplate capacity, but only 11 kWh of LFP nameplate capacity. At $150/kWh for AGM versus $600/kWh for LFP, the actual cost difference is smaller than it appears once you factor in the DoD adjustment and the LFP’s 10+ year lifespan versus AGM’s 3-5 years.

Three Real Off-Grid System Examples

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Off-Grid Cabin in the Tennessee Mountains

Polk County, TN | 4.8 peak sun hours | 24V LFP system

The Wilson family built a 600 sq ft weekend cabin in the Hiwassee River valley. No grid access within 2 miles — utility connection quote was $28,000. They chose to go off-grid with a 24V LFP system for a 3-day autonomy target.

ApplianceWattsHours/DaykWh/Day
LED lights (8 bulbs)64W5h0.32
Refrigerator150W24h3.60
Laptop + phone charging80W4h0.32
Water pump (12V DC)60W1h0.06
Fans (2x)100W6h0.60
Total daily load4.9 kWh
System output: 4 x 400W panels (1.6 kW array) | 24V 400Ah LFP bank (9.6 kWh) | 40A MPPT controller | 2,000W inverter. Wire sizing: 10 AWG panel-to-controller, 8 AWG controller-to-battery, 4 AWG battery-to-inverter. Estimated cost: $6,800-$9,200. The grid connection would have cost $28,000 installed with $1,200/year minimum utility charges. The off-grid system pays off in under 3 years.
🏡

Full-Time Off-Grid Homestead in Central Texas

Mason County, TX | 5.8 peak sun hours | 48V LFP system

Rancher Jim and his wife Rita run a 1,400 sq ft home completely off-grid in the Texas Hill Country. They have a well pump, chest freezer, window AC in the bedroom, and a full kitchen. The decision to use 48V instead of 24V cut their battery-to-inverter wire from 2/0 AWG to 6 AWG, saving over $400 in cable alone.

ApplianceWattsHours/DaykWh/Day
Refrigerator/freezer200W24h4.80
Window AC (bedroom)1,000W6h6.00
Well pump (1 HP)745W1.5h1.12
LED lights + electronics200W6h1.20
Washer (cold only)500W1h0.50
Total daily load13.6 kWh
System output at 48V/3-day autonomy: 10 x 400W panels (4.0 kW) | 48V 400Ah LFP bank (19.2 kWh) | 100A MPPT controller | 5,000W inverter. Wire: 8 AWG panels-to-controller, 4 AWG controller-to-battery, 8 AWG battery-to-inverter (only 62A at 48V vs 312A at 12V for same load). Total system cost: $18,000-$24,000 before any federal or Texas incentives.
🚗

Vanlife Build in the Pacific Northwest

Cascades Region, OR/WA | 3.8 peak sun hours | 12V LFP system

Outdoor photographer Keiko built a 200-watt system in her 144-inch wheelbase Transit van. The Pacific Northwest averages only 3.8 peak sun hours year-round, requiring an oversized panel array relative to the storage. She added a 12V compressor fridge and charges her camera and drone batteries daily.

ApplianceWattsHours/DaykWh/Day
12V compressor fridge45W24h1.08
Laptop + camera charging120W3h0.36
Drone battery charging80W2h0.16
LED lighting + fans40W4h0.16
Total daily load1.76 kWh
System output: 2 x 200W flexible roof panels (0.4 kW) | 12V 200Ah LFP (2.4 kWh) | 20A MPPT controller | 1,000W pure sine inverter. Wire: 12 AWG panels-to-controller, 8 AWG controller-to-battery, 4 AWG battery-to-inverter. Total: $2,200-$3,100. Keiko uses a 30A shore power hookup at campgrounds to top up during extended cloudy stretches.

Expert Tips for Sizing and Building Your Off-Grid Solar System

1

Always Oversize Panels, Undersize Battery Slightly

Panels degrade 0.5% per year and performance drops in heat (up to 25% at high temps). Size your array 20-25% larger than the math suggests. For batteries, start with 3 days of autonomy; you can add parallel strings later. Undersizing panels destroys batteries. Undersizing storage just means earlier generator runs.

2

Go MPPT, Not PWM — Always

A 40A MPPT controller costs $80-$150 more than a 40A PWM. MPPT recovers 15-30% more energy from your panels by tracking their maximum power point. On a 3 kW array getting 4.5 hours of sun, that is 0.2-0.4 kWh of extra energy per day — enough to run a refrigerator for 1-2 extra hours, every day, for 10+ years. MPPT also lets you use any voltage panels with any battery bank voltage.

3

Never Undersize Your Wires

NEC requires wire ampacity to be at least 125% of the calculated load. For DC systems especially, undersized wires cause voltage drop that robs performance and creates heat that causes fires. Our AWG sizing guide adds 25% to all calculated currents. Use fine-stranded welding cable (not romex) for the high-current DC runs from battery to inverter — it is more flexible and makes better contact with lugs at battery terminals.

16 Frequently Asked Questions About Off-Grid Solar Systems

How much solar do I need to power a house off-grid?+
A typical 1,500-2,000 sq ft American home uses 30-50 kWh per day if you run central HVAC, an electric range, and a water heater. Off-grid at that scale requires 12-20 kW of solar and 50-100 kWh of battery storage — a $50,000-$120,000 system. Most off-grid homeowners dramatically reduce their consumption first: switching to propane cooking, mini-split HVAC instead of central, and on-demand water heaters. With conservation, a comfortable off-grid home runs on 8-15 kWh/day, bringing the system cost down to $20,000-$40,000.
What is the difference between MPPT and PWM charge controllers?+
PWM (Pulse Width Modulation) controllers work by directly connecting the panels to the battery and chopping the current. They are simple and cheap but the panel voltage must closely match the battery voltage, and they operate at 70-80% efficiency. MPPT (Maximum Power Point Tracking) controllers use a DC-DC converter to transform the panel output to the battery voltage at the highest available current. MPPT operates at 93-97% efficiency, works with any panel voltage, and recovers 15-30% more energy from the same panels. In most systems with more than 400 watts of panels, the MPPT pays for itself within 1-2 years through the extra energy harvested.
How many days of autonomy should I design for?+
This depends on your location and how often you experience multiple consecutive cloudy days. In the American Southwest (AZ, NM, NV, Southern CA), 2-3 days of autonomy is adequate because cloudy streaks rarely last longer. In the Pacific Northwest, Upper Midwest, and New England, 4-5 days provides much better reliability, as winter storm systems can block the sun for 5-7 consecutive days. If you have a backup generator, you can design for 2 days and simply run the generator for a few hours on the 3rd dark day, dramatically reducing your battery bank cost.
What is LFP battery chemistry and why is it recommended?+
LFP stands for Lithium Iron Phosphate — a specific type of lithium battery chemistry that is far more stable and safe than the lithium cobalt oxide used in phones and laptops. LFP batteries can be safely discharged to 90-100% of their capacity (vs 50% for lead-acid), have virtually no fire risk, require zero maintenance, work well in cold weather (though capacity drops), and last 3,000-6,000 full cycles — about 10-15 years in daily off-grid use. The higher upfront cost compared to AGM is almost always recovered in the first battery replacement cycle that lead-acid requires at 3-5 years.
Can I use grid-tied solar panels in an off-grid system?+
Yes — standard 72-cell 400-450W panels designed for grid-tied systems work perfectly in off-grid applications. The difference is not the panels themselves but the inverter/charge controller. Grid-tied microinverters (like Enphase IQ7) cannot be used off-grid because they require grid voltage to synchronize. For off-grid, you use a standalone MPPT charge controller with the panels, paired with a separate battery inverter or a combined inverter-charger unit like the Victron Multiplus or Schneider XW series. These are completely separate products from grid-tied inverters.
How do I wire my batteries for 24V or 48V systems?+
Standard 12V batteries are wired in series to increase voltage. Two 12V batteries connected positive-to-negative gives you 24V at the same Ah capacity. Four 12V batteries in series gives 48V. To increase capacity (Ah) without changing voltage, wire strings of batteries in parallel — positive-to-positive and negative-to-negative. A 24V 400Ah bank typically uses four 12V 200Ah batteries: two pairs in series (for 24V each), then those two pairs in parallel (for 400Ah total). Our calculator shows this configuration as 2S x 2P = 4 batteries.
Do I need a permit for an off-grid solar system?+
This varies significantly by state and county. Many rural counties have minimal permit requirements for off-grid systems that are not connected to the utility grid. However, the National Electric Code (NEC) Article 690 governs solar PV systems in the US, and local building codes often adopt NEC by reference. Systems over a certain size (often 1,000 watts) typically require a permit and inspection in most jurisdictions. Energy storage systems follow NEC Article 706 and may have additional fire separation requirements. Always check with your local building department before installation. Unpermitted systems can create problems with homeowner insurance and property sales.
How do peak sun hours affect my system size?+
Peak sun hours (PSH) measure equivalent full-intensity sunlight hours per day — not total daylight. Phoenix AZ averages 6.5 PSH annually; Seattle WA averages 3.8. A 1 kW array in Phoenix produces 6.5 kWh/day; the same array in Seattle produces 3.8 kWh/day. This means a Seattle homeowner needs 71% more panel capacity than a Phoenix homeowner to produce the same daily energy. Always use your specific location PSH from NREL’s PVWatts tool at pvwatts.nrel.gov — generic “US average” figures will significantly under- or over-size your system.
What size inverter do I need?+
Your inverter must handle two different power demands: continuous running load and surge (starting) load. For continuous power, size the inverter to cover all appliances you might run simultaneously plus a 25% safety margin. For surge, motors like well pumps, air conditioners, and refrigerators draw 3-6 times their running wattage for 1-2 seconds at startup. A refrigerator that runs at 150 watts may surge to 800 watts. A 1,500W inverter might not handle a 1,200W window AC that surges to 4,000W on startup. Always check appliance startup (LRA) specifications or use a clamp meter to measure actual startup current, then add 25% per NEC.
Should I use a pure sine wave or modified sine wave inverter?+
For any off-grid system powering a home with appliances, always use a pure sine wave inverter. Modified sine wave inverters cost 30-50% less but cause problems with motor loads (pumps, HVAC, power tools draw more current and run hotter), CPAP machines (most specifically require pure sine wave), LED dimmer switches, some battery chargers, and audio/video equipment. The cost difference for a 3,000W unit is typically $200-$400 — not worth the long-term appliance damage and compatibility headaches for any permanent installation.
How long will my off-grid batteries last?+
LFP (LiFePO4) batteries are typically warranted for 3,000-6,000 full charge/discharge cycles at 80% depth of discharge, translating to 8-15 years of daily cycling in a well-designed system. AGM batteries last 400-700 cycles at 50% DoD — roughly 2-4 years in daily off-grid use. Gel batteries are similar to AGM. Flooded lead-acid batteries last 500-1,000 cycles when properly maintained (monthly equalization, watering every 2-3 months, clean terminals). In all cases, keeping batteries at partial state of charge for extended periods and deep discharging beyond their rated DoD dramatically shortens lifespan.
What AWG wire size do I need for my battery-to-inverter connection?+
The battery-to-inverter connection carries the highest current in your entire system — especially at 12V or 24V. For a 3,000W inverter at 24V: 3,000 / 24 = 125A running, x 1.25 NEC = 156A required wire ampacity. That requires 1/0 AWG copper at a maximum run of 3 feet (one-way). Most battery manufacturers specify maximum wire lengths for each wire gauge; keep battery-to-inverter runs as short as possible (under 6 feet round trip is best). Use fine-stranded 105C rated copper cable with ring terminals properly crimped with a hydraulic die crimper — not the squeeze handle type from hardware stores. NFPA 70 (NEC) Table 310.12 provides the authoritative ampacity ratings for wiring.
Can I add more solar panels or batteries later?+
Yes, but plan ahead. For batteries: you can add parallel strings to increase Ah capacity, but best practice is to only parallel identical batteries of the same age, brand, and capacity. Mixing old and new batteries causes the new batteries to charge the old ones and reduces system performance. For panels: additional panels can be added to most MPPT controllers up to the controller’s maximum input wattage and voltage. If your charge controller is maxed out, you add a second controller in parallel. Always check your charge controller’s maximum PV input voltage — in cold weather, open-circuit panel voltage can exceed rated limits and damage the controller.
What is the 30% federal tax credit for off-grid solar?+
The Residential Clean Energy Credit under IRA 2022 provides a 30% federal income tax credit for solar panel systems and standalone battery storage. Critically, this credit applies to off-grid solar systems that power a dwelling — not commercial/agricultural buildings. The credit applies to panels, batteries, inverters, charge controllers, wiring, and installation labor. The credit is non-refundable (reduces taxes owed, not a check) but unused amounts can carry forward to future tax years. See IRS guidance on the Residential Clean Energy Credit. Always consult a tax professional for your specific situation.
What fuses do I need and where do I put them?+
NEC Article 690 requires overcurrent protection (fuses or circuit breakers) on any conductor that is not short-circuit protected by the panel or battery itself. Required fuse points: (1) at each positive battery terminal within 18 inches for any conductors leaving the battery — this protects against catastrophic shorts; (2) between each string of panels and the combiner or charge controller; (3) between the charge controller and battery bank if the distance is over 18 inches; (4) on the AC output of the inverter. Use Class T fuses (Bussmann JJN/JJS) for DC battery protection — they interrupt DC faults much faster than standard automotive fuses. Fuse sizing should match the wire ampacity, not the load ampacity.
How does a generator integrate with an off-grid solar system?+
Most off-grid inverter-charger units (Victron Multiplus, Magnum Energy, Schneider XW) include a built-in transfer switch and battery charger that allows seamless generator integration. When batteries drop below a set state of charge (typically 20-30%), the system automatically starts the generator and charges the batteries at 50-80A while continuing to power loads from the generator. Once batteries reach 80-90%, the generator stops. For manual integration without a combined unit, you need a separate automatic transfer switch (ATS) and battery charger. Size your generator to match the inverter-charger’s maximum AC input — a 5,000W inverter-charger needs a 6,000W+ generator to charge at full rate.

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Legal Disclaimer and Editorial Transparency

The Off-Grid Solar System Calculator on USCalculators.com provides estimates for educational and planning purposes only. All outputs are based on standard engineering sizing methodologies and the inputs you provide. Actual system performance depends on your specific location, site conditions, equipment specifications, installation quality, local climate, and actual energy consumption patterns.

Cost estimates reflect typical US wholesale and retail prices as of mid-2024 for the specified components and do not include installation labor (typically $2,000-$8,000 for professionally installed systems), permit fees, trenching, or mounting hardware. The 30% federal tax credit calculation is based on current IRA 2022 law — always verify current eligibility with a qualified tax professional and see IRS.gov for current guidance.

All electrical installations must comply with the National Electric Code (NEC), specifically Articles 690 (Solar PV Systems), 706 (Energy Storage Systems), and applicable local building codes. Hire licensed electrical contractors for all installations and obtain required permits. Peak sun hours data sourced from NREL’s National Solar Radiation Database. Wire sizing follows NEC Table 310.12 and 690.8.

Editorial policy: USCalculators.com is an independent educational resource. We do not accept payment for brand recommendations and have no affiliate relationships with equipment manufacturers or solar installers.