Off-Grid Cabin Solar: How to Size Panels, Batteries and Inverter

Sizing off-grid cabin solar is three jobs: measure daily electricity use, build a battery bank that survives cloudy stretches, and pick panels plus an inverter to refill and deliver that power. Real numbers for weekend and full-time cabins, so you avoid the classic error of oversized panels on an undersized bank.

Step 1: Measure Your Daily Loads

Every off-grid design starts with a load list, not a shopping cart. List everything that draws power, note its watts from the nameplate, and estimate daily run hours. A $25 Kill A Watt meter removes the guesswork on the big loads.

The fridge is the heavyweight: 100 to 150 watts cycling 8 to 10 hours a day, about 1 to 1.2 kWh daily, and it runs all week even when you are away. The water pump is the opposite: 750 to 1,000 watts but only 30 to 60 minutes a day. LEDs sip 5 to 10 watts each, a laptop adds about 60 watts, and remote workers should know Starlink can pull 50 to 100 watts continuously, quietly becoming a top load.

Here is a realistic weekend cabin load list to show how the math works:

This cabin uses about 2.5 kWh per day, and that single number drives everything else. When in doubt, measure for a week. The U.S. Department of Energy has a handy guide to estimating appliance energy use if you want reference figures to sanity-check your list.

Weekend Cabins vs Full-Time Cabins

In a weekend cabin, the fridge hums all week, the bank keeps cycling, and Friday arrival must find a full battery. The upside: you can tolerate imperfection. If Sunday brings clouds and the bank runs low, you drive home and the panels recharge all week, with a small generator as backup.

Full-time cabins typically use double the energy: cooking, laundry, lighting, and sometimes a mini-split every day, with no weekday recharge window. A dead battery means a dead morning. Daily use of 6 to 12 kWh is common, and tolerance for a bad sizing guess is near zero.

Weekend systems can be sized for average conditions with a generator as backup; full-time systems should be sized for the worst solar month, usually December, with 3 days of battery. That is why full-time systems often cost two to three times more for the same cabin. Our guide to RV solar sizing for boondocking covers the same load-first method on wheels.

Step 2: Size the Battery Bank

Size the battery bank first: batteries are the expensive part and they decide how the system feels day to day. The formula is simple:

Rated bank kWh = (daily watt-hours × days of autonomy) ÷ inverter efficiency ÷ depth of discharge

Use 0.9 for inverter efficiency, 80% depth of discharge for LiFePO4 lithium, 50% for old-style lead-acid. Lithium is the default for cabins now: 3,000 to 6,000 cycles, faster charging, and most of its capacity is actually usable.

Weekend cabin example: 2.5 kWh/day × 2 days = 5 kWh; ÷ 0.9 = 5.6 kWh usable; ÷ 0.8 = about 7 kWh rated. Two 48V 100Ah server-rack batteries (5.12 kWh each, 10.2 kWh total) give this cabin a comfortable margin.

Full-time cabin at 8 kWh/day with 3 days of autonomy: 8 × 3 = 24; ÷ 0.9 = 26.7 usable; ÷ 0.8 = about 33 kWh rated, or six to seven 48V 100Ah server-rack batteries. Our full guide to battery bank sizing covers days of autonomy and depth of discharge in more detail.

Step 3: Size the Solar Array

Panels have one job: replace what you used, within the sun hours you actually get. The formula is:

Array watts = daily watt-hours ÷ peak sun hours ÷ 0.75

Peak sun hours are the equivalent hours of full-strength sun, not daylight hours; most of the U.S. gets 3.5 to 5.5. The 0.75 factor covers losses from heat, dust, wiring, and the charge controller.

Weekend cabin: 2,542 Wh ÷ 4.0 ÷ 0.75 ≈ 850 watts; round up to 1 kW (two 550-watt or three 370-watt panels). Full-time cabin at 8 kWh/day: 8,000 ÷ 4.0 ÷ 0.75 ≈ 2.7 kW, so a 3 kW array.

Two judgment calls beat the arithmetic. First, size for your worst month: northern winter sun hours can approach 2, so the array needs to be 30 to 50 percent bigger than summer math suggests. Second, tilt for winter sun and avoid shade; a shaded string produces far less than its rating. Renogy’s learning center has practical explainers on tilt, shading, and wiring if you want to go deeper.

Step 4: Size the Inverter

The inverter must handle your biggest simultaneous load, not your daily total: add up everything that could run at once, plus 25 percent headroom. The trap is motor surge: an 800-watt well pump can pull 2,500 to 4,000 watts for seconds at startup, and the inverter must survive that spike.

Weekend cabin: the pump surging near 3,000 watts plus the fridge kicking on at 600 watts makes a 3 kW inverter the sensible minimum. Full-time with a washer or microwave: a 5 kW split-phase inverter is the common choice. Always buy pure sine wave; modified sine wave can damage motors and electronics.

Two hardware paths: an all-in-one (EG4 and Growatt are popular) combines inverter, charge controller, and charger in one box, simpler and cheaper for cabins. A separates setup, typically a Victron inverter/charger plus standalone MPPT controller, costs more but is easier to repair and expand. Our comparison of hybrid vs string inverters explains the trade-offs.

Bottom line: anything bigger than a basic weekend setup should be 48V. You will spend a little more on the battery up front and save it back on copper and headaches.

Two Real Example Builds

The math as shopping lists (typical DIY budgets):

Weekend cabin (2.5 kWh/day): 1.2 kW of panels, two 48V 100Ah server-rack LiFePO4 batteries (10.2 kWh), a 3 kW 48V all-in-one inverter, plus racking, wiring, and breakers. Typical DIY cost: $4,500 to $6,500 before any generator.

Full-time cabin (8 kWh/day): 3 kW of panels, six 48V 100Ah server-rack batteries (~30 kWh), a 5 kW split-phase all-in-one inverter, a 4 to 5 kW dual-fuel generator, and a monitoring shunt. Typical DIY cost: $12,000 to $16,000.

Tax note for 2026 buyers: the 30% federal residential clean energy credit ended for systems installed after December 31, 2025, so do not subtract it from your budget. For outage backup on a grid-connected cabin, see our guide to sizing a home battery for outages.

Sizing Mistakes That Kill Cabin Systems

Buying panels before measuring loads. Without a load list you are guessing, and guesses are always wrong in the expensive direction.

Forgetting motor surge. A pump that runs fine on paper trips an undersized inverter at startup. Size for surge, then add headroom.

Mounting panels flat for winter use. Flat panels shed snow poorly and catch weak winter sun badly; a steep tilt can add a third more December production.

Mixing old and new batteries. Fresh batteries wired to a two-year-old bank sink to the old level. Buy the whole bank at once.

Skipping the generator. Every system meets a gray November week eventually; a small generator is the cheapest “extra battery” you can buy.

Ignoring phantom loads. An idle inverter draws 30 to 80 watts around the clock, which is 5 to 13 kWh gone over an empty week. Use power-save mode or switch it off between visits.