How to size off-grid solar for a cabin

A cabin far from power lines is a natural place for solar, and an easy place to get the size wrong. This guide walks you through sizing an off-grid system for a US cabin step by step: what you will power, when you will be there, how much sun you get, and what the panels, battery and electronics must handle. No wiring experience needed.

The short version

• Start with a list of what you will run and for how long: watts times hours gives watt-hours per day.
• When you use the cabin changes the size more than anything else. In Montpelier, Vermont, a panel gives about 3.5 times more energy on an average July day than on an average December day.
• Size for the darkest month you will actually be there, not for the yearly average.
• The battery covers nights and cloudy days. Lithium iron phosphate (LiFePO4) batteries must not be charged below freezing.
• Cold raises panel voltage, so the charge controller must handle your panels on the coldest morning.
• The National Electrical Code (NEC) covers the wiring; your county or state decides on permits and inspection. Ask before you buy.
• A year-round cabin in the snowy north can need two and a half times the panels of a summer cabin and still run short. A generator for the darkest weeks is worth pricing.

Step 1: List what you will power

Everything starts with energy per day. The U.S. Department of Energy gives a simple formula: wattage times hours used per day, divided by 1,000, gives kilowatt-hours (kWh) per day (DOE Energy Saver). We skip the division and work in watt-hours (Wh).

Think of water: watts are how fast it leaves the tap, watt-hours how much ends up in the bucket. A 60 W laptop charger running 4 hours uses 240 Wh.

Where to find the watts:

• The label. DOE says the wattage is usually stamped on the bottom, back or nameplate, and is the maximum power drawn. If it shows only amps, multiply by the volts; most US appliances use 120 volts.
• The yellow EnergyGuide label on a refrigerator gives kilowatt-hours per year. Divide by 365.
• A plug-in power meter measures what an appliance really uses over a day: the best number of all.

A refrigerator is "on" all day but its motor runs only part of the time; DOE suggests dividing the time it is plugged in by three to estimate its full-power hours.

Write down two numbers per item: running watts (for the inverter, the box that makes household power) and watt-hours per day (for the panels and battery). Motors such as fridges and pumps also draw a short burst of extra power when they start, called surge.

Step 2: Decide when you will use the cabin

This is the biggest decision of all, because winter days are short and the sun is low.

Here is what 1,000 W (1 kW) of panels, tilted 30 degrees and facing south, gives on an average day in two cabin areas, according to the European Commission's PVGIS solar calculator, using US satellite records from 2005 to 2015 and its standard 14 % allowance for losses in wiring, dirt and ageing (PVGIS Montpelier, PVGIS Flagstaff):

• Montpelier, Vermont: about 4.34 kWh a day in July, 3.58 in September, and only 1.23 in December.
• Flagstaff, Arizona: about 5.93 kWh a day in April, 4.29 in July, and 3.86 in December.

Scaled down, a 100 W panel in Montpelier gives about 434 Wh on an average July day and about 123 Wh in December (our calculation from the PVGIS figures). That 3.5-fold gap is why a summer cabin and a year-round cabin in the same place need very different systems. In Flagstaff the gap is far smaller.

More places: solar in each US region. How watts become energy per day: watts, watt-hours and sun hours.

Step 3: Size the panels for your darkest month

The rough first estimate is: daily energy need divided by what 1 kW of panels gives on an average day in your darkest month of use.

But an average day is not enough: many days are worse than average, and cloudy spells come in runs. Good off-grid design looks at years of real weather, day by day, and asks how often the battery would run empty. PVGIS has a free off-grid mode that does a simple version of this and reports the share of days on which the battery hit its lower limit (PVGIS user manual).

Placement matters too. Steeper south-facing panels catch more of the low winter sun, and trees around a cabin can shade them, so check where shadows fall in your darkest month.

Step 4: Size the battery

The battery carries energy from the sunny hours into the night and the cloudy days that follow. Three ideas decide its size:

• Days without sun. How many gray days in a row should the cabin run?
• Usable capacity. Draining a battery completely shortens its life. The PVGIS manual uses a 40 % cut-off for lead-acid and suggests about 20 % for lithium, so you plan to use about 60 % or 80 % of the rated capacity.
• Losses. Energy is lost in the battery and inverter. PVGIS's off-grid mode assumes the system delivers 67 % of what the panels could make, which its manual says may be conservative.

More battery saves panels only up to a point. See how big a solar battery do you need.

Step 5: The charge controller and the cold-voltage trap

The charge controller sits between the panels and the battery and stops overcharging; the MPPT type also gets more energy from the panels (MPPT vs PWM). It has two limits: the most current it can deliver to the battery and the highest voltage it can take from the panels.

Here is the trap. A panel's voltage goes up when it is cold. Each panel has an "open-circuit voltage" (Voc), its voltage in sunlight with nothing connected, measured at 77 °F. On a freezing, sunny morning it is higher. In many places, the NEC's section 690.7 says the maximum system voltage must be worked out from the panels' Voc corrected for the lowest expected temperature at the site, using the panel maker's temperature figures when they are given (EC&M on NEC 690.7); confirm with your local inspector.

How cold? In the PVGIS hourly records for Montpelier, 2005 to 2015, the air dropped below freezing on about 150 days a year, and the lowest reading was about -26 °F (our count from the PVGIS hourly series; a cold hollow can be colder).

Step 6: The inverter

The battery stores direct current (DC). Household appliances use alternating current (AC), the power from a wall outlet. The inverter translates between them. Size it for the most watts running at the same time, plus room for motor surge. If you only run 12-volt lights, a 12-volt fridge and USB chargers, you may not need one at all.

Step 7: Cold batteries and snowy panels

Batteries. Lithium iron phosphate (LiFePO4) batteries have one firm rule: no charging below freezing. Charging a lithium cell in the cold can plate metallic lithium onto an electrode, and that damage is permanent (Grepow). In the batteries we have checked for our US catalog, the makers' datasheets set the lowest charging temperature at 32 °F for four of them and 41 °F for the other six. In a cabin left cold all week, the battery needs a built-in heater, a spot that stays above that limit, or electronics that refuse to charge in the cold (LiFePO4 batteries in cold weather).

Snow blocks the light until it slides or melts off. A national study by the National Renewable Energy Laboratory (NREL) modeled snow losses for fixed panels at 0 % to 16 % of yearly output across the lower 48 states, with the highest losses in Minnesota and Michigan and up to almost 40 % at one site in northern Alaska. Even Flagstaff came out at 3 % to 5 %, while nearby Phoenix had none. Panels tilted at the latitude angle lost less than panels tilted at 20 degrees (NREL 2017). Those are yearly figures; in a snowy month the share lost is much larger.

Step 8: The NEC and permits, in plain words

The National Electrical Code (NEC, or NFPA 70) is the US rulebook for safe wiring. States, and sometimes cities or counties, adopt an edition and may amend it. As of September 2026, 19 states had adopted the 2023 edition in some form and six had already adopted the 2026 edition, according to a report citing NFPA (EC&M). NFPA publishes maps of which edition each state enforces.

The parts that matter for a cabin:

• Article 690 covers solar panels and their wiring, including the cold-voltage rule above. In many places, section 690.12 requires a "rapid shutdown" function for solar circuits on or in a building, a way to quickly cut the voltage on the roof wiring for firefighters (EC&M); confirm with your local inspector.
• Article 706 covers energy storage systems (batteries), and Article 710 covers stand-alone systems that are not connected to the power grid (IAEI Magazine).

Whether you need a permit, an inspection or a licensed electrician is decided locally, by your county or town building department or state electrical office (the "authority having jurisdiction"). Rules differ, including whether owners may wire their own property. The Department of Energy says a qualified installer will meet local building, fire and electrical codes, and that the industry-standard installer certification comes from NABCEP, the North American Board of Certified Energy Practitioners (DOE).

Our advice, not a legal ruling: call your building department before you buy, say it is an off-grid cabin system with a battery, and ask what they need. Use a licensed electrician for anything you are unsure of, and for anything that connects to cabin wiring, a generator inlet or a breaker panel.

A worked example: a summer cabin near Montpelier, Vermont

Our calculation from the PVGIS figures above; an illustration, not a design for your cabin.

The cabin's day, with wattages read off the labels (our example assumptions):

• Five LED bulbs at 9 W each for 5 hours: 225 Wh.
• Two phones charging, 20 W for 3 hours: 60 Wh.
• A laptop, 60 W for 4 hours: 240 Wh.
• A refrigerator whose EnergyGuide label says 365 kWh a year: 365,000 Wh / 365 days = 1,000 Wh.
• A water pump, 60 W for half an hour: 30 Wh.

Total: 1,555 Wh a day. We round up to 1,600 Wh.

Season: May to September. The darkest month in that season is September, at about 3.58 kWh per kW of panels per average day.

First estimate of panels: 1.6 kWh / 3.58 kWh per kW = 0.45 kW, about 450 W, for an average September day.

Checking against real weather. We put 800 W of panels (tilted 30 degrees, facing south), a 4,000 Wh lithium battery with a 20 % cut-off (3,200 Wh usable, about two days of use) and 1,600 Wh a day into the PVGIS off-grid calculator for Montpelier. Over 2005 to 2015, the battery ran down to its limit on fewer than 1 day in 100 from May to August (PVGIS off-grid run). The jump from 450 W covers losses and cloudy runs.

Controller and inverter. 800 W charging a 12-volt battery means up to about 800 / 12 = 67 amps; at 24 volts, about 33 amps, so cables can be thinner. Running loads add up to about 305 W (lights 45, phones 20, laptop 60, fridge about 120 while running, pump 60), so an inverter of around 1,000 W leaves room for motor surges.

Cold voltage check. Say each panel's datasheet lists a Voc of 22 V at 77 °F and a voltage change of -0.3 % per °C (about -0.17 % per °F). At -26 °F, 103 °F colder, the voltage rises by about 103 x 0.167 % = 17.2 %, to about 25.8 V per panel. Two panels in series give about 52 V, fine for a controller rated at 100 V. Four in series would give about 103 V and could damage it.

What if you want it all year? December gives only 1.23 kWh per kW per day in Montpelier. We tried 2,000 W of panels tilted steeply at 55 degrees with a 6,000 Wh battery: the battery still ran out on about 8 days in 100 in December (PVGIS off-grid run), and PVGIS does not count snow on the panels or cold-charging limits. That is two and a half times the summer panels for a worse result. For many northern cabins, a generator for the darkest weeks is worth pricing against yet more panels.

What to check before you buy

1. Write your load list: running watts and watt-hours per day.
2. Pick your months and find the darkest one; check your sun in that month.
3. Choose how many sunless days the battery should cover.
4. Check the controller's current rating against your panels at your battery voltage.
5. Check the controller's voltage limit against your panels' Voc on the coldest morning.
6. Check the inverter's continuous and surge power against your loads.
7. Check the battery's minimum charging temperature, and whether it has a heater.
8. Plan cables and fuses (solar cable size and fuses).
9. Call your building department about permits and inspection.

What can go wrong

• Sizing for the yearly average. It works in June and is empty by October.
• Cold-voltage damage. Panels fine in summer exceed the controller limit on a frosty, bright morning.
• Charging a frozen lithium battery. Without a heater or low-temperature cut-off, the cells can be damaged for good.
• An undersized controller. It wastes panel power or cannot cope.
• Panel voltage too low for the battery. The panels' working voltage must sit above the battery voltage, or charging stalls.
• Thin cables and missing fuses. Low-voltage systems carry high currents, and thin cable heats up.
• Snow and shade on flat or shaded panels in winter.

Questions people ask

Can I just buy a "cabin kit"? Yes, but check it against the list above, your loads and your darkest month.

Is a power station enough? For light loads it can be: power station or DIY system.

What about a generator? For year-round northern cabins it is worth pricing against more panels. Backup power for outages covers generators and transfer switches.

How our quiz sizes a cabin

Our quiz does these steps for your ZIP code. It runs a day-by-day energy balance over many years of PVGIS satellite weather for your area (a cell about 3 miles across), including panel temperature, losses, battery limits, cold charging and your area's snow records. It then offers three options: Minimum (no shortfall on 90 % of the days in your season), Medium (97 %) and With margin (99.5 %). Every part is checked against the others: controller voltage on your coldest morning, current, battery temperature, inverter surge. When your season's sun cannot do the job, the result says so and suggests a generator or other backup instead of ever more panels.

Size my cabin system. For the full method, see how we size systems, and for any term you met here, the solar glossary.

Our results and guides are estimates and do not replace advice from a qualified electrician.