Off grid solar system for a small cabin: how I'd size one, step by step

I've lived off grid for 22 years, in a treehouse in a remote part of Hawaii Island. If you're planning power for a small cabin, the first question is usually how big a system it needs. My honest answer is that it depends on what you run and where the cabin is, and that the arithmetic is simple enough to do yourself. This page walks through it the way I'd do it, with the sources, for a weekend cabin and a full-time cabin. If you see a mistake, I'd like to hear about it.

My system, as a reference point

My system is 1,780 W of panels, a 60 A charge controller at 12 V, a 4.02 kWh LiFePO4 bank (four 314 Ah cells in series, 12.8 V), a 3,000 W pure sine inverter and a Honda EU2200i backup generator. It runs two laptops, a monitor, Starlink and networking. Planning on 80% of the bank leaves about 3,215 Wh usable.

It isn't a template. It's one working system, and it's useful as a yardstick for the numbers below. Two honest notes about it. First, PowMr's manual lists the controller's maximum panel input at 12 V as 720 W, well under my 1,780 W array.^[5] How much of the array reaches the battery on a bright day depends on wiring I haven't documented here, so I won't put a number on it. If you're buying for a new cabin, match the controller's listed input to the array on paper first. Second, the generator is a requirement for me, not an extra. You'll see why in the worked examples.

Step 1: what do you run each day, in Wh?

Energy per day is watts times hours. Virginia Cooperative Extension gives it as wattage x hours used per day / 1,000 = daily kWh.^[2] I keep it in Wh, because small cabin loads are small numbers.

• Where the watts come from. VT Extension says the wattage is usually on the bottom, back or nameplate, and that if only amps are listed, amps x volts (120 V for most US appliances) gives watts.^[2] Nameplates tend to list the maximum, so they usually overstate the average.
• Fridges cycle. VT notes a fridge isn't running at full power all the time.^[2] For a new US fridge, the yellow EnergyGuide label's kWh per year divided by 365 is a better daily figure than the nameplate.
• Things that are "off" still draw. VT calls these phantom loads.^[2] In a cabin, the biggest one is often the inverter itself: Victron lists 20 W with nothing plugged in for its 3,000 VA 12 V model, and Renogy lists under 2.5 A for its 3,000 W unit.^[6][7] At 20 W, that's 480 Wh a day just for being on.
• Heat is the budget breaker. VT lists portable heaters at 750–1,500 W, microwaves at 750–1,100 W and coffee makers at 900–1,200 W.^[2] One hour of a 1,500 W heater is 1,500 Wh, more than the whole weekend cabin below uses in a day. On a small solar system, heat usually comes from wood or propane.

Here are the two examples. The quantities and hours are my assumptions for an ordinary small cabin. Swap in your own.

The weekend cabin switches its inverter off overnight, so idle counts 8 hours. Starlink and laptop figures are at the high end on purpose.

The full-time cabin leaves the inverter on, which adds 480 Wh a day, about 13% of its total. The fridge and pump figures are assumptions; read your own labels. If you have a Starlink Standard kit instead of a Mini, Starlink lists 75–100 W average, which would add roughly 0.9–1.6 kWh a day here.^[3]

So the weekend cabin needs about 1,087 Wh a day from the battery and the full-time cabin about 3,679 Wh a day. For comparison, my 3,215 Wh usable would carry the full-time example for a bit under a day with no sun at all.

Step 2: how much sun does your cabin get?

A "peak sun hour" is one hour of sunlight at 1,000 W per square meter, so a site's daily solar energy in kWh/m² is its peak sun hours. NREL/NLR's solar resource data gives this for US locations as monthly averages on a south-facing surface tilted at the site's latitude, from a satellite model averaged over 1998–2009 on cells about 10 km across.^[1] I pulled it for a spread of US cabin country:

Fairbanks, AK returned no data from this dataset, so Alaska isn't in the table. NREL/NLR's PVWatts calculator is a better tool for a specific site, and the 1.8 kW solar guide shows month-by-month PVWatts output for six places.

The number that matters most for a cabin is the lowest month you'll actually be there. Burlington's December, at 2.41 hours, is 43% below its year average. Seattle's is 1.65. These are long-term averages. A cloudy week is worse than an average month, and snow on the panels makes a sunny day worthless until it slides off. That's what the battery and the generator are for.

Step 3: how big a battery, in usable Wh?

Size the battery from what you can use, not the label. Two planning choices go in:

• Days without sun. I use 2 days here. Fewer days means more generator time; more days means more battery. It's a choice, not a rule.
• How much of the bank you plan to use. I plan on 80%, the same assumption as my other guides. The cell maker's limits and your BMS settings decide the real floor. EVE, for example, says to keep its cells above 2.8 V in normal use and calls over-discharge forbidden.^[4]

Cold cabins need a plan for the battery. EVE allows its LiFePO4 cells to be charged only between 0 and 60 °C (32 to 140 °F).^[4] A battery in an unheated mainland cabin can be below freezing on a sunny winter morning, when the panels are ready to charge it. Check your battery or BMS maker's low-temperature charge setting, and think about where the bank lives. The battery runtime guide goes deeper on usable energy and aging.

Step 4: how many panel watts, and what controller?

Panel watts = daily Wh / (peak sun hours x 0.75). The 0.75 is my planning assumption for everything between the panel label and the battery: heat, dirt, wiring, the controller and charging losses. The 1.8 kW guide modeled 14% system losses in PVWatts before the controller and battery take their share, so 0.75 is in that neighborhood, not a measured figure.

Two things jump out. The weekend cabin is a small system almost anywhere: about 421 W in Burlington for its lowest month from May to October. The full-time cabin is where the north hurts: sized for December, Denver needs about 1,057 W, Burlington about 2,035 W and Seattle about 2,973 W. The alternative to buying panels for the darkest month is to size closer to the average and plan generator time for winter. That trade-off is what the backup generator guide works through.

Charge controller: amps at your battery voltage

A charge controller's limit is really a power limit at a given battery voltage. PowMr's manual for its 60 A MPPT controller lists 720 W of panels at 12 V, 1,440 W at 24 V, 2,100 W at 36 V and 2,800 W at 48 V.^[5] That's roughly 60 A times the nominal battery voltage. So a rough controller size is panel watts / battery voltage:

• Weekend cabin, about 421 W at 12 V: 35 A, so a 40 A controller leaves some room.
• Full-time cabin, about 838 W (Denver, sized for the year average): 70 A at 12 V, 35 A at 24 V or 17 A at 48 V. This is the point where a higher battery voltage starts to look sensible, because the same power means fewer amps on the battery side.

Voltage limits matter as much as amps. PowMr says to use the panels' open-circuit voltage (Voc) to make sure the input never exceeds the controller's maximum, 160 V on that model.^[5] REC says to work out the number of panels in a string from Voc at the lowest ambient temperature at the site, because Voc rises in the cold, and that the NEC's Article 690 must be followed for the higher-than-label outputs panels see in real conditions.^[8] A string that's fine in July can be over the limit on a frozen January morning. This is a good calculation to have your controller maker or an electrician check.

Step 5: what size inverter?

The inverter is sized for power at once, not energy per day. Add up everything that could run at the same moment, then check what starts motors.

• Continuous isn't the headline number. Victron's 3,000 VA 12 V inverter is rated 2,400 W continuous at 25 °C and 2,200 W at 40 °C, with a 6,000 W peak.^[6] Renogy lists a 6,000 W peak surge for its 3,000 W model.^[7] Read the continuous watts at a realistic temperature.
• Motors start hard. VT Extension notes a motor draws about three times its running current while starting.^[2] On the full-time example, the assumed 500 W pump could ask for around 1,500 W for a moment. Victron says its inverter is built to start difficult loads such as refrigeration compressors and electric motors.^[6]
• Bigger costs energy. An oversized inverter usually idles higher. If your biggest load is a laptop and a fan, a small inverter, or running some loads on DC, may save more than any extra panel.
• The battery has to keep up. At 12.8 V and 90% efficiency, a 3,000 W inverter at full output pulls about 260 A from the battery. The cells, BMS and fuse all have to be rated for that. The runtime guide shows how my own BMS and cells, not my inverter, set my real limit.

For the examples: the weekend cabin's loads at once are a few hundred watts, so a small pure sine inverter covers it. For the full-time cabin, the pump's assumed 1,500 W start alone, plus the fridge, laptop and lights, puts it near 2,000 W for a moment if they overlap. That points at a 2,000 to 3,000 W class unit. Check the start-up figure on your own pump and fridge.

Wiring, fuses and safety: where I defer

This is the part of a cabin system that can hurt someone or burn the cabin down, and it's the part I'm least willing to summarize. What the manuals say:

• Fuse at the battery. PowMr strongly recommends a fuse connected directly to the battery to protect against a short circuit in the battery wiring, and says wire size must match the expected current.^[5] I use a Class T fuse; Blue Sea's Class T block comes with an insulating cover to meet ABYC/USCG insulation requirements.^[10]
• Follow the code. REC says wiring must follow the National Electrical Code (NEC), and that where the NEC doesn't apply, Isc and Voc are multiplied by 1.25 (or per local rules) when sizing conductors, fuses and controls.^[8] Your county may require a permit and an inspection, even for a cabin, so ask the local building department.
• Connection order. PowMr says the battery must be connected to the controller before the panels, and never the panels without a battery.^[5] Panels are live whenever light hits them.
• Generators make carbon monoxide. Honda says never to run the engine in a closed or even partly closed area, and to keep it at least 3 ft from buildings.^[9] Honda also says standby connections to a building's wiring must be made by a qualified electrician.^[9]

I'm not giving wire gauges or fuse ratings on this page on purpose. They depend on current, run length, temperature, the parts you buy and the code your inspector uses. Your equipment manuals and a licensed electrician are the right sources.

Putting the two examples together

What I don't know

• Your loads. Every quantity and hour in the examples is an assumption, and the loads are where most sizing mistakes start.
• Your site. Shade from one tree, snow, or a north-facing roof can matter more than the region. NREL/NLR's data is a long-term average for a 10 km cell, not your clearing.
• My own daily use. Nothing in my system logs it, so I can't give you a measured number for my treehouse.
• Whether 0.75 is right for your system. It's a round planning factor. If you can measure your own system's output against the sun, that's better.

If you've measured a cabin system like these, I'd like to learn from it.

Keeping the plan in one place

I built GRID COMMAND for this kind of record. You can enter your loads, panels, controller, battery and inverter, see the daily Wh add up, and keep each part's manual next to it, with no signal needed. It's free to try with no account. Saving your system is $5 a month, and the $7 plan adds off-device backup, export and the AI, which answers from your own equipment list and says when something needs an electrician. Open the app.

FAQ

How many solar panels do I need for a small cabin?

Start from your daily loads in watt-hours, not from a panel count. Divide the daily Wh by your peak sun hours and by a loss factor (I use 0.75 as a planning assumption). In the worked example on this page, a weekend cabin using about 1,087 Wh a day in Burlington, VT needs about 421 W of panels for its lowest summer-season month. A full-time cabin using about 3,679 Wh a day in Denver needs about 1,057 W for December. Your own loads and location change the answer.

How big a battery does an off grid cabin need?

Multiply daily Wh by the days you want to ride through without sun, then divide by the share of the battery you plan to use. With 2 days and 80% usable, the weekend example needs about 2,717 Wh of nameplate LiFePO4 (about 212 Ah at 12.8 V) and the full-time example about 9,197 Wh (about 719 Ah at 12.8 V, or 180 Ah at 51.2 V). The 2 days and 80% are planning choices, not rules.

Should a small cabin system be 12 V, 24 V or 48 V?

It's mostly about current. The same power at a higher battery voltage means fewer amps, and so smaller cables and fuses. PowMr's manual for its 60 A controller lists 720 W of panels at 12 V and 2,800 W at 48 V. A weekend cabin often stays at 12 V. A full-time cabin with a fridge and a pump is where 24 V or 48 V starts to make sense. Ask your equipment makers or an electrician before you commit.

Can I size my cabin solar from the yearly average sun?

Only if you're fine running a generator in the dark months. In NREL/NLR's data, Burlington, VT averages 4.26 peak sun hours over the year but 2.41 in December, so a full-time cabin sized on the average comes up short in winter. Size for the lowest month you'll be there, or plan the generator in from the start.

Do I need an electrician for a cabin solar system?

For anything you aren't sure about, yes. REC's panel manual points to the National Electrical Code (Article 690 for PV) and to qualified system designers, and a lithium bank can deliver hundreds of amps into a fault. Wire sizes, fuses, grounding and any connection to cabin wiring belong to the manuals, the code, your local inspector and an electrician, not to a sizing page.

Methodology

• Peak sun hours are NREL/NLR solar resource data (avg_lat_tilt, monthly and annual), pulled from the public API on 2026-10-11 for the coordinates of each city.^[1]
• Daily Wh = watts x quantity x hours. AC loads are divided by an illustrative 90% inverter efficiency (Renogy lists over 90%, Victron 93% maximum), and inverter idle is added at Victron's 20 W zero-load figure.^[7][6]
• Usable battery = daily Wh x 2 days. Nameplate = usable / 0.80. Ah = nameplate Wh / battery voltage (12.8, 25.6 or 51.2 V).
• Panel watts = daily Wh / (peak sun hours x 0.75). Controller amps = panel watts / nominal battery voltage, following how PowMr states its limits.
• The 2 days, 80% and 0.75 are planning assumptions. Nothing on this page is measured.

Sources

1. NREL/NLR, Solar Resource Data API v1 (avg_lat_tilt: monthly and annual average daily solar resource in kWh/m²/day on a fixed surface tilted at latitude; SUNY satellite model, 1998–2009, about 10 km cells; source "Perez-SUNY/NREL, 2012"): https://developer.nlr.gov/docs/solar/solar-resource-v1/ (accessed 2026-10-11)
2. Virginia Cooperative Extension, "ENERGY SERIES: Estimating Appliance and Home Electronic Energy Use" (formula; nameplate wattage; amps x volts; refrigerator cycling; motor starting about three times running current; phantom loads; typical nameplate wattages, including laptop 50 W, ceiling fan 65–175 W, portable heater 750–1,500 W, microwave 750–1,100 W, coffee maker 900–1,200 W, deep well pump 250–1,100 W): https://www.pubs.ext.vt.edu/2901/2901-9014/2901-9014.html (accessed 2026-10-11)
3. Starlink, Mini specification sheet (average power 25–40 W): https://starlink.com/public-files/specification_sheet_mini.pdf, and Standard kit specification sheet (average power 75–100 W): https://api.starlink.com/public-files/specification_sheet_standard.pdf (accessed 2026-10-09)
4. EVE Energy, MB31 314 Ah LiFePO4 cell product specification (keep above 2.8 V in normal use; over-discharge forbidden; charging 0 to 60 °C; copy hosted by a distributor): https://www.solardirekt24.de/media/4b/bf/6c/1762180019/EVE_MB31_314Ah_Datasheet.pdf (accessed 2026-10-07)
5. PowMr, HHJ-60A MPPT charge controller user manual V1.3 (maximum PV input power 720 W at 12 V, 1,440 W at 24 V, 2,100 W at 36 V, 2,800 W at 48 V; input must not exceed 160 V, checked with Voc; fuse directly at the battery; wire size to expected current; battery before panels): https://cdn.shopify.com/s/files/1/0746/0415/1079/files/HHJ-60A_User_Manual_V1.3.pdf (accessed 2026-10-09)
6. Victron Energy, Phoenix Inverter 3 kVA 120 V datasheet (12/3000: 2,400 W continuous at 25 °C, 2,200 W at 40 °C, 6,000 W peak; 93% maximum efficiency; zero-load power 20 W; suited to start refrigeration compressors and electric motors): https://www.victronenergy.com/upload/documents/Datasheet-Inverter-3000VA-120V-EN.pdf (accessed 2026-10-09). Cited as an example; it isn't my inverter.
7. Renogy, 12 V pure sine wave inverter datasheet (3000 W: 6,000 W peak surge; efficiency over 90%; no-load current under 2.5 A): https://www.renogy.com/cdn/shop/files/INVT-P2-Datasheet.pdf (accessed 2026-10-09). Cited as an example; it isn't my inverter.
8. REC Group, REC Alpha Pure series panels installation manual, UL edition (string sizing from Voc at the lowest ambient temperature; wiring per the NEC; NEC Article 690; 1.25 factor on Isc and Voc where the NEC doesn't apply): https://www.recgroup.com/sites/default/files/2025-01/IM_RECAlpha_UL_EN.pdf (accessed 2026-10-09)
9. Honda, EU2200i owner's manual 00X31Z446010 (carbon monoxide warnings; 3 ft clearance; standby connections by a qualified electrician): https://cdn.powerequipment.honda.com/pe/pdf/manuals/00X31Z446010.pdf (accessed 2026-10-07)
10. Blue Sea Systems, Class T Fuse Block with Insulating Cover, 225 to 400 A (insulating cover satisfies ABYC/USCG insulation requirements): https://www.bluesea.com/products/5502100/Class_T_Fuse_Block_with_Insulating_Cover_-_225_to_400A (accessed 2026-10-09)

Cite this page: MiniGridMap, "Off grid solar system for a small cabin: sizing it step by step", https://minigridmap.com/guides/off-grid-solar-system-small-cabin/ (2026). Tables may be reused with a link back.

Related

For the same reference system: How far does a 1.8 kW off-grid solar system go?, How long will my 12 V LiFePO4 battery last? and How big a backup generator does a 12 V LiFePO4 off-grid system need? Once it's built, the off grid solar maintenance checklist covers upkeep, including a cabin that sits empty for months. All guides are on the guides page.

The reference system is the site owner's own. Every figure is arithmetic on cited data and specs, or a labeled assumption. Nothing is measured, and this page is not a system design.