Off-grid cabin solar and battery sizing

An off-grid cabin has no utility to fall back on. That changes how you size everything. Here is the method, step by step, with free tools.

An off-grid system has four main parts: solar panels to make power, a charge controller to charge the battery safely, a battery bank to store power, and an inverter to turn battery DC power into the AC power household appliances use. Many cabins also keep a generator for long cloudy spells.

Step 1: List your loads, season by season

Write down every device: watts and hours per day. Do it separately for your busiest season and your darkest season. A cabin used only in summer is a very different system from one used at Christmas. Our house battery calculator works for cabins too; it adds up daily watt-hours as you list devices.

Big loads change everything. Electric heat, electric water heaters, electric ranges and air conditioning use far more energy than lights and a fridge. Some off-grid cabins use propane or wood for heat, hot water and cooking to keep the electrical system manageable.

Step 2: Size the solar for your worst month

A grid-tied home can average across the year. An off-grid cabin cannot. Size for the month when you have the least sun relative to your use.

Open the free NLR (formerly NREL) PVWatts calculator and enter the cabin's location.
Model a 1 kW array at your planned tilt and direction. PVWatts gives estimated output for each month.
Find the month you will use the cabin with the lowest output. Divide by the days in that month to get kWh per day per kW of panels.
Divide your daily load (in kWh) by that number. That is roughly the panel size in kW, before extra margin for snow, dirt and battery charging losses.

Example only. Purely to show the arithmetic: if PVWatts estimated 60 kWh for a 1 kW array in your worst month of 30 days, that is 2 kWh per day per kW. A cabin using 4 kWh a day would need about 4 ÷ 2 = 2 kW of panels, plus margin. Your PVWatts numbers will differ.

Tilt changes winter output. Try several tilt angles in PVWatts and compare the months you care about before you build the mount.

Step 3: Size the battery for days without sun

Pick how many days the cabin must run with little or no solar: your "days of autonomy." Then:

Battery size (kWh) = daily use x days of autonomy ÷ usable depth of discharge

Depth of discharge depends on chemistry. Battle Born, a lithium battery maker, says AGM lead-acid generally should not go much below about 50% regularly, while lithium can use 80 to 100% of rated capacity. The calculator does this math and converts to amp-hours at 12, 24 or 48 V.

If you keep a generator, you can choose fewer days of autonomy and run the generator in long storms. If you do not, you need more battery, more solar, or both.

Step 4: Choose the system voltage

Higher voltage carries the same power with less current, which allows thinner cables and smaller fuses. Victron's free Wiring Unlimited guide explains why. Small cabins may use 12 or 24 V; larger systems often use 48 V. Choose before buying components, because batteries, controllers and inverters must all match.

Step 5: Size the inverter

The inverter must handle everything that can run at once, plus the surge when motors start (well pumps, fridges, power tools). Add up the watts of loads that may run together and compare with the inverter's continuous rating, then check its surge rating against your largest motor. Our home backup calculator shows a total-watts figure that helps here.

Step 6: Plan for cold

Lithium cannot be charged below freezing without help. Battery University's BU-410 explains why. Look for batteries with low-temperature charge protection or built-in heating, or keep them in a space that stays above freezing.
Lead-acid delivers less in the cold and should be kept charged to avoid freezing. See winter battery storage.
Snow covering panels blocks the sunlight they need. Plan how you will safely clear it.

Step 7: Leave room to grow

Many people add loads over time. Choose a charge controller and inverter with headroom, and leave space and cabling for more panels or batteries if your equipment supports expansion.

What we do not cover here

We did not find a cost range for off-grid cabin systems from a source we consider reliable enough to quote, so we do not give one. For battery cost context, see home battery cost in 2026. Get quotes from local off-grid installers, and ask your county or local building office which permits and inspections apply, even without a utility connection.

All-in-one alternatives

For a small, occasional-use cabin, a large portable power station with portable solar panels can avoid a custom build. Compare options on power station vs installed battery and see prices on power station and solar bundle deals.

A sizing method, not a design. Have a qualified installer check any permanent system.