Off-Grid Cabin Solar Sizing: Wattage Calculator and Real-World Examples

Start Here: Most Off-Grid Cabins Need Between 2kW and 6kW of Solar

A basic off-grid cabin with LED lighting, a chest freezer, and phone charging runs fine on a 2kW system. Add a well pump, a mini-split, and a washing machine and you’re looking at 6kW minimum. The gap between those two numbers is where most people get burned — they undersize their array, then wonder why their batteries are dead by 9pm in January.

I’ve sized systems for three off-grid builds in the last five years: a 400-square-foot hunting cabin in the Ozarks, a 900-square-foot year-round homestead in western Montana, and a seasonal retreat in the Texas Hill Country. Same math every time. Here’s how it works.

The Core Formula: Daily Watt-Hours First, Array Size Second

Don’t start with panels. Start with your load. You need to know your daily watt-hour (Wh) consumption before you can size anything else. The formula is dead simple:

Watts × Hours Used Per Day = Daily Watt-Hours

Add up every appliance you plan to run. Divide your total daily Wh by your location’s average peak sun hours. That gives you raw solar wattage. Add 25% for inefficiency losses — inverter conversion, wire resistance, battery charge/discharge losses. That’s your real array size.

The Off-Grid Cabin Load Calculator

Run through this appliance list and fill in your numbers. Wattage draws are from measured loads, not manufacturer spec sheets.

Common Cabin Appliances and Their Real Draws

• LED lighting (8 bulbs, 10W each): 80W × 5 hours = 400 Wh/day
• Chest freezer (7 cu ft, like the Midea MRC070S0AWW): ~85W average draw × 24 hours = 2,040 Wh/day
• Refrigerator (12V compressor, like the BougeRV 30Qt): ~45W average × 24 hours = 1,080 Wh/day
• Well pump (1/2 HP submersible): 750W × 2 hours = 1,500 Wh/day
• Mini-split (9,000 BTU, heating mode): 900W × 6 hours = 5,400 Wh/day
• Laptop + phone charging: 65W × 4 hours = 260 Wh/day
• LED TV (40-inch): 40W × 3 hours = 120 Wh/day
• Washing machine (cold water, front-load): 500W × 1 hour = 500 Wh/day
• Propane range ignition/clock: negligible
• Router/modem (Starlink dish): 50W × 24 hours = 1,200 Wh/day

A cabin running lights, a 12V fridge, a laptop, a TV, and a Starlink dish lands around 3,060 Wh/day. Add a well pump and you’re at 4,560 Wh/day. Throw in a mini-split for winter heating and you’re pushing 10,000 Wh/day — and that’s where propane backup starts making a lot more sense than trying to solar your way through a Montana January.

Peak Sun Hours by Region: This Number Changes Everything

Peak sun hours (PSH) aren’t total daylight hours. They’re the equivalent hours of full 1,000 W/m² irradiance per day. A partly cloudy day in Vermont might give you 2.5 PSH. A clear day in Tucson gives you 6.5. That difference nearly triples your required array size for the same load.

Average Annual Peak Sun Hours by Region

• Southwest (AZ, NM, southern CA, west TX): 5.5–7.0 PSH
• Mountain West (CO, UT, ID, MT): 4.5–6.0 PSH (winter drops to 3.5–4.5)
• Pacific Northwest (OR, WA): 3.0–4.5 PSH (winter: 1.5–2.5)
• Great Plains (KS, NE, OK, ND): 4.5–5.5 PSH
• Southeast (GA, AL, FL, SC): 4.5–5.5 PSH
• Northeast (NY, VT, ME, NH): 3.5–4.5 PSH (winter: 2.0–3.0)
• Midwest (MN, WI, MI, OH): 3.5–4.5 PSH
• Texas Hill Country: 5.0–6.0 PSH
• Ozarks (MO, AR): 4.5–5.0 PSH

Size for your worst-month PSH, not the annual average. If you’re in western Montana, design for 3.5 PSH in December, not the 5.5 you’ll see in July. Your batteries don’t care about summer averages when it’s -10°F in February.

Worked Examples: Three Real Cabin Scenarios

Scenario 1: Seasonal Hunting Cabin (Ozarks, AR) — 2kW System

This is the 400-square-foot cabin I helped a buddy wire up outside of Jasper. Used October through January only. Load: LED lights, a small 12V fridge, phone/radio charging, and a single 40W LED TV. Total daily load: ~1,800 Wh/day. Worst-month PSH in the Ozarks: 4.0.

Raw array needed: 1,800 ÷ 4.0 = 450W. Add 25% for losses: 450 × 1.25 = 562W. We rounded up to a 2kW array for headroom and future expansion. Four 500W panels, a 40A MPPT charge controller, and a 200Ah lithium battery bank. Total component cost came in around $2,800. Three seasons in, no issues.

Scenario 2: Year-Round Homestead (Western Montana) — 5kW System

900 square feet, occupied full-time. Load includes a chest freezer, well pump (run 1.5 hours/day average), LED lighting, laptop, Starlink, and a propane range with electric ignition. No mini-split — they heat with wood. Daily load: ~5,800 Wh/day. Worst-month PSH (December in MT): 3.5.

Raw array: 5,800 ÷ 3.5 = 1,657W. With 25% losses: 2,071W. We sized to 5kW to handle cloudy stretches and give the battery bank time to recover. Ten 500W panels, a Victron SmartSolar MPPT 150/85 charge controller, a Victron MultiPlus-II 5000VA inverter/charger, and 400Ah of LiFePO4. This system handles 4–5 consecutive cloudy days before they need to run their backup generator.

Scenario 3: Texas Hill Country Retreat — 4kW System

Seasonal use, spring through fall. 700 square feet. Load includes a 9,000 BTU mini-split (running 4 hours/day average in shoulder season), a 12V fridge, lights, and Starlink. Daily load: ~6,200 Wh/day. Worst-month PSH (November in central TX): 5.0.

Raw array: 6,200 ÷ 5.0 = 1,240W. With losses: 1,550W. Sized to 4kW for headroom and to handle peak AC startup surge. Eight 500W panels, a 60A MPPT controller, a 3,000W pure sine wave inverter, and 300Ah LiFePO4. The mini-split’s startup surge — around 2,200W — is the spec that drives inverter selection here. Your inverter’s continuous wattage rating needs to exceed your largest motor load’s running watts, and the surge rating needs to cover startup. Don’t skip that math.

Choosing Your Components: What Actually Matters

Solar Panels

For cabin installs, I run monocrystalline panels exclusively. Better efficiency in low-light conditions and better temperature coefficients than polycrystalline — which matters in the Pacific Northwest or during Rocky Mountain winters. The Renogy 400W mono panels are what I spec for most builds. Consistent output, solid frame construction, and they’ve taken hail in Montana without delamination.

For a complete starter kit that includes panels, charge controller, and mounting hardware, the Renogy 400W 12V Solar Panel Starter Kit is a solid entry point for seasonal cabins.

Check Price on Amazon → (paid link)

Charge Controllers

MPPT over PWM, every time. A PWM controller wastes 20–30% of your panel output. On a 4kW array, that’s 800–1,200W gone. The efficiency difference pays for the price gap inside one season. For systems under 3kW, the Victron SmartSolar 100/30 is my go-to — Bluetooth monitoring built in, rock-solid firmware, and Victron’s support actually picks up when something goes sideways.

For larger systems (4kW+), step up to the Victron SmartSolar MPPT 150/85. It handles up to 8,500W of panel input and pairs directly with Victron’s VRM monitoring platform, which I use to track every system I’ve helped build remotely.

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Inverters

Pure sine wave only for cabin use. Modified sine wave inverters will damage variable-speed motors, some LED dimmers, and certain electronics. The Victron MultiPlus-II is the gold standard for serious off-grid builds — inverter, battery charger, and transfer switch in one box. For smaller seasonal systems, the EG4 3000EHV-48 runs around $600 and handles 6,000W surge, which covers most well pump and mini-split startup loads without breaking the budget on a seasonal build.

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Batteries: LiFePO4 or Lead-Acid?

LiFePO4 wins on every metric except upfront cost. You get 80% usable depth of discharge versus 50% for lead-acid, 3,000–5,000 charge cycles versus 500–800, no maintenance, and better cold-weather performance. A 200Ah LiFePO4 bank gives you 160Ah usable. A 200Ah lead-acid bank gives you 100Ah usable. You need twice the lead-acid capacity to match LiFePO4 storage — and lead-acid weighs roughly twice as much per amp-hour. For a cabin where you’re hauling batteries in by hand or ATV, that weight difference is real.

The Battle Born 100Ah LiFePO4 is what I’ve used in two of my three builds. Ten-year warranty, built-in BMS, and I’ve watched them hold capacity at -20°F in Montana where standard lithium chemistry would have dropped 30–40%.

Check Price on Amazon → (paid link)

Seasonal Adjustments and Winter Sizing

Year-round cabin in a northern state? Size for December. In July, a 3kW array in Montana produces 15–18 kWh/day. In December, that same array produces 6–8 kWh/day. Loads also climb in winter — more lighting hours, possible heating loads, and battery capacity drops 15–20% in cold temps.

The practical solution most full-time off-gridders use: size your solar for shoulder seasons, then run a propane or diesel generator for 1–2 hours on the worst winter days to top off the batteries. A 3,500W generator running 1.5 hours puts roughly 4–5 kWh into your batteries. That’s often cheaper than doubling your panel array for 60 days of winter use.

Panel Tilt and Orientation: Free Watts You’re Leaving on the Table

Fixed roof mounts lose 10–25% of potential output if they’re not optimized. For year-round use, tilt your panels at your latitude angle. For winter-optimized systems, add 15 degrees to your latitude. A cabin at 45°N should have panels tilted at 60° in winter. According to NREL’s PVWatts calculator, that adjustment increases December output by 20–30% compared to a flat roof mount at the same latitude.

Azimuth matters too — true south, not magnetic south. In the Mountain West, magnetic declination runs 8–12 degrees east, meaning magnetic south points you significantly off true south. Use a GPS app or NOAA’s declination calculator, not a compass, to orient your array.

Quick Sizing Reference Table

• Weekend/seasonal cabin, minimal loads: 2kW array, 100–200Ah LiFePO4, 2,000W inverter
• Part-time cabin with fridge and well pump: 3–4kW array, 200–300Ah LiFePO4, 3,000W inverter
• Full-time cabin, no HVAC: 4–5kW array, 300–400Ah LiFePO4, 3,000–5,000W inverter
• Full-time cabin with mini-split: 6–8kW array, 400–600Ah LiFePO4, 5,000W+ inverter
• Full-time homestead with heavy loads: 8–12kW array, 600Ah+ LiFePO4, 8,000W+ inverter + generator backup

The Bottom Line

Undersized systems die by 9pm. Oversized systems drain bank accounts because someone skipped the load math and just bought more panels. Do the daily watt-hour calculation. Divide by worst-month PSH. Add 25%. Build from that number.

A 4kW system with 300Ah of LiFePO4 handles the vast majority of three-season cabin use cases in the continental US. Running a mini-split or a well pump year-round in the northern tier? Go to 6kW and plan for generator backup in December and January. That’s not a system failure — that’s December in Montana doing what December in Montana does.

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