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

A typical weekend cabin using 5 kWh/day needs roughly 1,500 W of panels (six 250W or four 400W modules) in a 4 peak-sun-hour climate. Divide your daily watt-hours by worst-month peak sun hours, then divide again by 0.70 to account for real-world off-grid losses.

How many days of battery autonomy should an off-grid system have?

Plan for 3-5 days of autonomy for a full-time off-grid system with no generator backup, or 2-3 days if you have a reliable propane or gas generator. A weekend cabin can get away with 1-2 days because nobody is drawing power between visits.

What size inverter do I need for an off-grid cabin?

Add up the wattage of everything that could run simultaneously, then add 25% headroom. Most small cabins land at a 2,000-3,000 W inverter; full-time cabins with a well pump or larger fridge need 4,000-6,000 W. Size for surge as well as continuous — well pumps and compressors spike to 3-5x their running wattage at startup.

Do I need a generator for an off-grid solar system?

Not strictly, but I recommend one. A small propane or inverter generator lets you size the PV array and battery bank for average conditions instead of worst-case weeks. Without a generator, you have to oversize both by 40-60%, which often costs more than the generator itself.

Why use worst-month sun hours instead of annual average?

An off-grid system has to work in December, not on paper. Annual averages hide the fact that northern latitudes can drop to 2.0-2.5 peak sun hours in winter while averaging 4.5 hours for the year. Sizing on annual numbers guarantees a dead battery bank every January.

Can I use LiFePO4 batteries in an unheated cabin?

Yes, but the BMS must disable charging below 0°C (32°F). Most quality LiFePO4 batteries do this automatically. For winter-only cabins in cold climates, either insulate the battery compartment, use self-heating batteries, or accept that solar charging will pause during freezes.

Off-Grid Solar System Sizing: Complete Cabin & Homestead Guide

Key Takeaways

Size off-grid systems against worst-month peak sun hours, not annual averages. The difference is 30-50% more panels in northern climates.

Use a 0.70 derate factor for off-grid builds (vs. 0.86 for grid-tied PVWatts default). Battery round-trip losses, charge controller losses, and winter-temperature losses stack up.

LiFePO4 at 80% depth-of-discharge gives you the most usable watt-hours per dollar in 2026. Plan for 3-5 days of autonomy, or 2-3 with a generator.

Inverter sizing is about surge, not just continuous load. A 3,000 W inverter can easily fail to start a 1 HP well pump even though the pump only “runs” at 1,200 W.

A small cabin (5 kWh/day) needs roughly 1.5 kW PV, 10 kWh LiFePO4, and a 3 kW inverter. Scale from there.

Every off-grid build should include a generator input on the inverter. Skipping it is the #1 regret I hear from friends with cabins.

Short Answer: Quick Sizing Heuristics

If you just want ballpark numbers to price a system, here’s the rule of thumb I use before I ever open a spreadsheet:

Weekend cabin, 2 kWh/day: ~600 W PV, 4 kWh LiFePO4, 1,500 W inverter
Small full-time cabin, 5 kWh/day: ~1,500 W PV, 10 kWh LiFePO4, 3,000 W inverter
Full-time small cabin with well pump, 8 kWh/day: ~2,400 W PV, 15 kWh LiFePO4, 4,000 W inverter
Homestead, 20 kWh/day: ~6,000 W PV, 40 kWh LiFePO4, 8,000-12,000 W inverter + generator

These numbers assume 4 peak sun hours in the worst month and 3 days of autonomy. If you live somewhere with worse winters (Maine, Montana, the Pacific Northwest) multiply panel wattage by 1.3-1.5. If you have a generator, you can shrink both panels and batteries by 25-30%.

Want to tune these for your specific site? Plug your loads into the Solar System Sizer and Battery Bank Calculator and the math below will do itself.

Why Off-Grid Sizing Is Different from Grid-Tied

Grid-tied sizing is forgiving. If you undersize, you just buy the difference from the utility. Nothing breaks, nothing freezes, nothing goes dark. The grid is an infinite battery that never asks questions.

Off-grid systems don’t get that luxury. Every watt-hour you consume has to come from panels you already paid for, through a controller you already sized, into a battery bank that has a finite depth of discharge. Undersize any link in the chain and you learn about it at 2 AM in January when the lights flicker out.

I run a grid-tied hybrid system on my house in Rhode Island, but I also built a small off-grid system on my shed — two 200W panels, a 100Ah LiFePO4 battery, and a 1,000W inverter. It’s a toy compared to a real cabin, but it taught me the same lesson every cabin owner learns: off-grid systems have to be sized for the worst week of the year, not the average week. Since then I’ve helped two friends plan cabin systems (one in Vermont, one in upstate New York), and the sizing methodology below is what I walk them through every time.

If you’re new to the four core components and the general DIY process, start with our getting started with DIY solar guide — this article assumes you already know what a charge controller does and are ready to run actual numbers.

Step 1: Do a Real Load Audit

The entire design flows from one number: your daily watt-hour consumption. Get this wrong and nothing downstream will be right.

The right way to do a load audit is to list every appliance, its running wattage, and its real hours of use per day. Not nameplate wattage — real running wattage. A “1,500W” microwave pulls 1,500W for maybe 3 minutes a day, which is 75 Wh, not 1,500.

Here’s a template for a small full-time cabin:

Appliance	Running Watts	Hours/Day	Wh/Day
12V DC compressor fridge	55 W	10 h (duty cycled)	550
LED lights (8 fixtures)	80 W	5 h	400
Laptop + monitor	90 W	4 h	360
Internet (Starlink)	60 W	24 h	1,440
Water pump (on-demand)	400 W	0.3 h	120
Phone/tool charging	30 W	3 h	90
Vent fan	20 W	6 h	120
Coffee maker	900 W	0.2 h	180
Microwave	1,500 W	0.1 h	150
Propane furnace blower	100 W	4 h (winter)	400
Total			~3,810 Wh

Call it 4 kWh/day in summer. Now here’s the trap: this number changes seasonally. In winter, the furnace blower runs longer, you use more lights (dark by 4:30 PM in New England), and your fridge actually uses less because the cabin is cold. In summer, if you have a vent fan or a small window AC, that dominates. Audit both seasons and size on whichever is worse combined with worst-month sun.

For the cabin in the table above, winter load climbs to roughly 4.8 kWh/day. I’d size this system on 5 kWh/day to leave margin.

Step 2: Find Your Worst-Month Peak Sun Hours

Peak sun hours (PSH) is the number of hours per day your location receives the equivalent of 1,000 W/m² of sunlight. It’s the single most important number after load.

Go to pvwatts.nlr.gov (the former pvwatts.nrel.gov — it now redirects to the National Laboratory of the Rockies after the 2026 rename). Enter your address, accept the defaults, and look at the monthly “Solar Radiation” column on the results page. That’s your peak sun hours by month.

For an off-grid system, use the worst month, not the annual average. In my Rhode Island zip code, annual average PSH is about 4.3 but December drops to 2.4. That’s a 44% difference. If I size on 4.3, I’ll be running on fumes for all of December and January.

A few representative worst-month values I see in client planning:

Location	Annual Avg PSH	Worst Month PSH
Phoenix, AZ	6.5	4.8 (Dec)
Denver, CO	5.5	3.8 (Dec)
Providence, RI	4.0	2.3 (Dec)
Burlington, VT	4.0	1.9 (Dec)
Seattle, WA	3.7	1.1 (Dec)

PVWatts also lets you set panel tilt. For off-grid, tilt the array at latitude + 15° to favor winter production. You give up a little summer output (which you’d waste anyway — batteries full by 11 AM) in exchange for meaningful winter gains. This alone can bump worst-month production 15-25% in northern climates.

DOE’s Homeowner’s Guide to Going Solar is a good general primer on solar resource assessment, though it’s written for grid-tied. For off-grid the winter bias is mandatory.

Step 3: How Big Should the Panel Array Be?

Here’s the formula I use:

PV watts = Daily Wh / (Worst-month PSH × 0.70)

The 0.70 factor is the off-grid system derate — it’s tighter than PVWatts’ default 14.08% loss assumption (which gives 0.86). Why? Because off-grid has losses grid-tied systems don’t:

Battery round-trip efficiency: ~95% for LiFePO4 (5% loss)
Charge controller losses: 2-4%
Wiring and connector losses: 2-3%
Panel temperature derate (summer): 8-12%
Soiling, mismatch, LID: 5-6%
Charge profile tapering in absorption stage: 3-5% (the sun is there but the battery doesn’t want it all)

Stack those and you land around 0.65-0.72. I use 0.70 as my planning number for a well-designed system. For a compromised site (heavy soiling, some shading, long wire runs) drop to 0.60.

Example: 5 kWh/day cabin in Rhode Island with worst-month PSH of 2.4:

PV = 5,000 / (2.4 × 0.70) = 5,000 / 1.68 = 2,976 W

Round up: 3,000 W of panels, or eight 400W modules. That might feel like a lot for a 5 kWh/day load, but that’s what December in New England demands. Move the same cabin to Colorado (3.8 PSH worst month) and it drops to 1,880 W — about half.

This is the single biggest misunderstanding new off-gridders have. Winter sun is brutal, and the array has to be sized for it.

Step 4: Sizing the Battery Bank

Batteries have two separate sizing drivers: days of autonomy and daily discharge depth. Both matter.

Days of autonomy is how many consecutive no-sun days the bank can carry. Typical guidance:

1-2 days: Weekend cabin (nobody there mid-week anyway)
2-3 days: Full-time cabin with reliable generator backup
3-5 days: Full-time cabin with no generator
5+ days: Critical medical loads, or locations with week-long overcast stretches

Daily discharge depth for LiFePO4 should be 80% maximum. This comes straight from the LiFePO4 chemistry reference: per-cell safe floor is 2.80 V (LVD), and at 80% DoD you see 3,000-6,000 cycles to 80% remaining capacity, versus 2,000-3,000 at 100% DoD. The extra cycle life dwarfs the cost of slightly oversizing.

The formula:

Battery kWh = (Daily kWh × Autonomy days) / 0.80

Example: 5 kWh/day with 3 days of autonomy:

Battery = (5 × 3) / 0.80 = 18.75 kWh

At 48V nominal, that’s 18,750 / 51.2 = 366 Ah at 48V. Four 48V 100Ah server-rack batteries, or two 48V 200Ah units. For a curated list of what’s worth buying in 2026, see the best LiFePO4 batteries roundup. If you’re building your own from prismatic cells, the DIY LiFePO4 battery bank guide walks through top-balancing, BMS selection, and the 16S assembly process.

Why 48V for Anything Non-Trivial?

LiFePO4 16S packs land at 51.2 V nominal, 58.4 V full charge, 44.8 V LVD. This is the de facto standard for home storage. At 48V, moving 5,000 W through your inverter is ~104 A. At 12V, the same power is 417 A — more than 4x the current, which means 4x the wire cost and voltage drop. The solar wire gauge chart and our wire sizing reference show exactly how this cascades. For any off-grid system above about 3,000 W continuous, 48V is the only practical choice. Even systems in the 1,500-3,000 W range benefit from 24V or 48V over 12V.

Step 5: How Do I Size the Inverter?

Two numbers matter: continuous wattage and surge capacity.

Continuous: Add up everything that could plausibly run at the same time. Fridge (100W running) + lights (80W) + laptop (90W) + well pump (1,200W running) + microwave (1,500W) = 2,970 W. Add 25% headroom → 3,700 W. Round up to a 4,000 W or 5,000 W inverter.

Surge: This is where cabins get wrecked. Induction motors — well pumps, compressors, power tools — draw 3-5x their running wattage for the first 1-3 seconds of startup. A 1 HP well pump at 1,200 W running will surge to 4,000-6,000 W. A 3,000 W inverter with a 6,000 W “2-second surge” spec can often start it; a 3,000 W inverter with only a 4,500 W surge cannot.

Read the surge spec carefully. Some manufacturers list “peak” in milliseconds (useless for motor starts) versus a 1-second or 5-second surge (what matters).

For a cabin with a well pump and a decent fridge, I recommend 4,000-6,000 W continuous minimum with surge ratings over 9,000 W. The EG4 6000XP is a popular option at this tier; see the getting started guide for context on why 48V hybrid inverters dominate 2026 DIY builds.

Step 6: Charge Controller Sizing (NEC 690.8)

The charge controller has to handle the array’s current without tripping and without melting wire. NEC 690.8 is crystal clear on this:

Source circuit current = Isc × 1.25           (irradiance factor)
Conductor ampacity     = Isc × 1.25 × 1.25    (continuous duty factor)

The double 1.25 is cumulative. A panel with 13.5 A Isc needs conductors and an OCPD sized for 13.5 × 1.5625 = 21.1 A minimum, which typically means 10 AWG PV wire and a 15-20 A fuse. For strings in parallel, add Isc of each string before applying the factors.

Controller amp rating: The controller’s output amp rating is what the manufacturer lists (e.g., 60 A, 80 A, 100 A). You want your array’s maximum output current to stay at or below that, where:

Array max current = Array STC watts / Battery nominal voltage

Example: 3,000 W array at 48V battery = 62.5 A → needs an 80 A controller (with headroom). Most brands allow you to slightly oversize the array beyond rated watts because MPPT clips on sunny days, but you have to stay under the max PV input voltage at the coldest ambient temperature — this is the other critical constraint.

Voltage ceiling (NEC 690.7):

Voc(cold) = Voc(STC) × [1 + β × (T_min − 25°C)]

Where β is the Voc temperature coefficient (negative, typically -0.28%/°C for mono silicon) and T_min is your ASHRAE 2% extreme minimum temperature. For a string of four panels with Voc 48.0 V at -20°C design low:

Voc(cold) = 48.0 × [1 + (-0.0028)(-20 - 25)] = 48.0 × 1.126 = 54.0 V
4S string = 216 V

That fits a 250 V max controller but would destroy a 150 V controller. Exceeding max PV voltage kills the MPPT instantly — no warranty coverage, no second chance. Get this right the first time.

The full treatment including hot-day minimum voltage checks is in the MPPT charge controller sizing guide. For tested controller recommendations, see the best MPPT charge controllers for 2026 roundup.

Step 7: Where Does the Generator Fit In?

Here’s what most guides skip: a generator is almost always cheaper than the solar it replaces. A $900 3kW inverter generator burning 20 gallons of gas per winter displaces 30-40% of the PV and battery you’d otherwise need. That’s often $3,000-$5,000 in hardware.

I tell every friend building a cabin: buy the generator. You probably won’t run it much, but knowing it’s there lets you size PV and battery for the average bad week instead of the worst bad week. You’ll sleep better, and on the week it rains for 8 days straight you’ll have a warm cabin and a charged bank.

Your inverter should have:

AC input (generator input): Passes generator power through to loads and charges the battery bank simultaneously.
Auto-start relay (optional): Drops the generator on when battery SOC hits a threshold. Most hybrid inverters support this via a dry contact.
Charge current limit: Match to your generator’s capacity. A 3 kW generator at 240 V can deliver ~12 A AC, which converts to roughly 2,000-2,500 W of DC charging at 48V. Set the charger limit accordingly or you’ll trip the generator’s breaker.

Propane generators store fuel indefinitely (no stale gas problem), which is why most cabin owners I know run dual-fuel units.

Ground-mounted solar array tilted at a steep angle next to a New England cabin surrounded by fall foliage

Worked Example 1: Weekend Cabin (2 kWh/day)

A weekend getaway in upstate New York. Fridge runs on propane. Used Friday evening through Sunday afternoon, roughly 40 weekends a year.

Loads (weekend only):

LED lights 5h × 60W = 300 Wh
Phone/laptop charging = 200 Wh
Water pump = 80 Wh
Small TV/entertainment 3h × 80W = 240 Wh
Vent fan 6h × 15W = 90 Wh
Coffee maker = 150 Wh
Misc = 140 Wh
Total: ~1,200 Wh used days, amortized ~2 kWh/day planning

Worst-month PSH: 2.0 (upstate NY, December)

PV: 2,000 / (2.0 × 0.70) = 1,428 W → 1,600 W (four 400W panels)

Battery: Only 1 day autonomy needed (cabin is empty mid-week, bank recharges); (2 × 1.5) / 0.80 = 3.75 kWh → 4 kWh (one 48V 100Ah server-rack unit is ideal, or a single 24V 200Ah)

Inverter: Small loads, no motors → 1,500 W continuous with 3,000 W surge

Controller: 1,600 W / 48 V = 33 A → 40-50 A MPPT

Cost ballpark (2026): $600 panels + $700 battery + $300 inverter + $250 controller + $400 BOS/wire/mounting = ~$2,250

No generator needed for a cabin this small. If nobody’s there Monday-Thursday, the bank is at 100% by Tuesday regardless of weather.

Worked Example 2: Full-Time Small Cabin (8 kWh/day)

Single-occupant homesteader in Vermont. No grid available. Has a shallow well with 1/2 HP pump. Wood heat, propane cooking.

Loads:

12V compressor fridge: 800 Wh
Starlink 24/7: 1,440 Wh
LED lights + evening use: 500 Wh
Well pump (~15 min/day at 800W): 200 Wh
Laptop + monitor 6h: 540 Wh
Chest freezer (outdoor in winter, in cellar in summer): 1,200 Wh
Washing machine (3x/week amortized): 400 Wh
Water heater element (supplemental, propane main): 1,500 Wh
Power tools + misc: 400 Wh
Vent fan + ceiling fan: 500 Wh
Total: ~7,500 Wh/day → plan for 8 kWh

Worst-month PSH: 1.9 (Burlington, VT, December)

PV: 8,000 / (1.9 × 0.70) = 6,015 W → 6,000 W (fifteen 400W panels). This is a big array for 8 kWh/day, but Vermont in December is brutal.

Battery: (8 × 3) / 0.80 = 30 kWh → 30 kWh (six 48V 100Ah server-rack batteries)

Inverter: Fridge + freezer + lights + well pump starting (surge to 3,200 W) + Starlink + laptop = peak ~4,500 W momentarily → 6,000 W continuous, 12,000 W surge

Controller: 6,000 W / 48 V = 125 A → 150 A (or two 80 A units in parallel). Consider splitting into two arrays on two controllers to reduce single points of failure.

Generator: 5 kW dual-fuel inverter generator, auto-start on SOC < 30%. Used maybe 80 hours/year.

Cost ballpark: $2,400 PV + $4,800 battery + $1,600 inverter + $900 controllers + $1,200 mounting + $900 generator + $1,200 BOS = ~$13,000

Worked Example 3: Homestead (20 kWh/day)

Family of four on a rural homestead in central Colorado. Full-size fridge, freezer, chest freezer, well pump (1 HP), electric water heater as backup to solar thermal, workshop with occasional power tools, satellite internet, LED lighting throughout.

Loads: 20 kWh/day average, up to 25 kWh/day in winter with heating circulators running.

Worst-month PSH: 3.8 (Colorado, December — tilted at latitude + 15°)

PV: 25,000 / (3.8 × 0.70) = 9,398 W → 10,000 W (twenty-five 400W panels or fewer higher-wattage modules). Tilted ground mount.

Battery: (20 × 3) / 0.80 = 75 kWh. With a generator present and 3 days autonomy this is reasonable. Go to (20 × 5) / 0.80 = 125 kWh for no-generator → 80 kWh with generator, 125 kWh without

Inverter: 1 HP well pump surge + fridge + freezer + dryer possibility → 12,000 W continuous, 24,000 W surge. Often a split-phase pair of 6 kW units or a single Sol-Ark 12K / EG4 18kPV class unit.

Controller(s): 10 kW / 48 V = 208 A → two or three high-current MPPT units, or a hybrid inverter with built-in MPPTs (Sol-Ark, EG4 18kPV).

Generator: 8-12 kW propane standby with auto-transfer.

Cost ballpark: $4,000 PV + $12,000 battery + $5,000 inverter + $1,500 controllers + $2,500 mounting + $3,500 generator + $2,000 BOS = ~$30,500 before tax credits. After the 30% federal ITC on eligible components, net is around $22,000.

How Do I Know If I’ve Oversized or Undersized?

The honest answer: you don’t, until you live with the system through a full year. But here are the warning signs I watch for in the first winter:

Undersized PV: Batteries never reach 100% SOC on sunny December days. If the bank tops out at 85% by noon on your best day, the array is too small.

Undersized battery: SOC drops below 30% on normal overnight loads. You wake up to a BMS that disconnected overnight.

Oversized PV: Array hits absorption voltage by 10 AM every summer day and the controller clips for 6 hours. This isn’t a failure — just money sitting idle. Consider diverting to a resistive water heater (opportunity load).

Undersized inverter: Lights dim when the well pump starts. Microwave + coffee maker trips the inverter. Add surge capacity.

Undersized controller: Controller hits its amp limit and clips on sunny days, or triggers over-temperature faults in summer.

Monitor everything. A Victron Cerbo GX or equivalent logging solution is worth every dollar for the first year — you’ll see patterns that let you right-size the next round of components.

Common Off-Grid Sizing Mistakes

1. Using annual average PSH. The single most common mistake. A system sized on 4.5 PSH annual average when the worst month is 2.0 will run out of energy every December.

2. Forgetting the Starlink tax. Modern cabins run Starlink, and Starlink pulls 50-75W continuously — 1.2-1.8 kWh/day. That’s half the entire load budget of a weekend cabin, silently running while nobody’s there.

3. Ignoring parasitic draws. Inverter idle draw (20-40 W), charge controller standby (2-5 W), monitoring gear, propane detectors — 60 W continuous is 1.4 kWh/day of loads you didn’t put on the spreadsheet.

4. Undersizing wire. A 3% voltage drop budget is industry best practice for PV circuits, 2% for DC feeders. Use the Wire Gauge Calculator and reference the solar wire gauge chart. Undersized wire is silent money burning into heat.

5. Skipping the generator input. Even if you never plan to use one, wire the inverter for generator input during installation. Adding it later means tearing into commissioned wiring.

6. Cold-charging LiFePO4. Quality batteries have low-temp charge cutoff in their BMS. Verify it. Winter-only cabins at -10°F need heated battery boxes or self-heating units.

7. Treating batteries as a buffer instead of the limit. Your battery bank is the hard cap on consecutive bad days. No amount of panels saves you if the bank is too small.

8. Mounting panels flat for “aesthetic reasons.” Flat panels lose 20-30% winter production and don’t self-clean with rain. Always tilt.

Final Checklist Before You Buy

Before placing a single order, I run through this checklist:

Run the numbers twice. I cannot count the number of times I’ve caught an arithmetic error on a second pass that would have cost hundreds of dollars in wrong parts.

Next Steps

You now have the methodology. Here’s how to finish planning your build:

Plug your loads into the Solar System Sizer — it factors your location and worst-month sun hours automatically.
Design your bank with the Battery Bank Calculator for exact Ah and series/parallel configuration.
Pick your battery chemistry and brand using the best LiFePO4 batteries for 2026 roundup, or learn to build from cells via the DIY LiFePO4 battery bank guide.
Size and select your charge controller using the MPPT charge controller sizing guide and cross-reference tested units.
Plan your wiring with series vs. parallel wiring and the wire gauge chart.
Review the DOE homeowner guide at energy.gov for general context (written for grid-tied, but useful background).

The cabin systems I’ve helped friends build all started as spreadsheets. Spend three hours on the math now and you’ll save three months of frustration later. Off-grid is less forgiving than grid-tied, but when it’s sized right it’s quieter, more reliable, and more satisfying than any grid connection.