Cabin solar gets sized backwards most of the time. People start from panel wattage because that is what gets advertised, when the number that actually decides the system is how many watt-hours you consume in a day.
Everything else follows from that figure. Battery capacity carries it overnight, panels replace it during the day, and the controller has to handle whatever the panels produce. Our roundup of solar panels for cabins covers the hardware.
Quick Answer
Add up the watt-hours every device uses in a day, then size the battery to carry a full day with margin and the panels to replace that in the worst month you intend to use the cabin.
What Drives the Number
| Factor | Effect on system size |
|---|---|
| Refrigeration | Usually the single largest load |
| Water pumping | Brief but high draw, needs surge headroom |
| Lighting | Small on LED, was large on incandescent |
| Season of use | Winter needs several times the panel area |
| Days of autonomy | Each cloudy day carried multiplies the battery |
| Heating or cooling | Changes the answer entirely |
Start From Watt-Hours, Not Watts
Watts describe how fast something draws power. Watt-hours describe how much it consumes over time, and time is what a battery has to cover.
A 50 watt device running four hours uses 200 watt-hours. A 500 watt device running twelve minutes uses 100 watt-hours. The second draws ten times as hard and consumes half as much. A vehicle changes the math, which our RV version of this guide works through. Our diesel heater guide covers why heat should not come out of the bank.
That distinction is why a list of device wattages tells you almost nothing on its own. You need wattage multiplied by realistic hours for each thing you actually run.
Adding those up gives a daily total, and that single number is what the whole system gets built around.
Being honest about hours matters more than being precise about watts, since people consistently underestimate how long things actually run. Our note on what appliances you can run off grid covers typical loads. Cabin sizing starts in the same place, so building an off grid load list is the first step worth doing.
Refrigeration Dominates Most Cabins
It runs continuously
A compressor cycles day and night rather than only when someone is present.
Duty cycle matters more than wattage
How often it runs decides consumption, not the number on the plate.
Ambient temperature changes it
A hot cabin makes the same fridge work considerably harder.
Propane is the common alternative
Which removes the largest electrical load entirely. Our roundup of propane appliances covers that route.
Sizing the Battery Bank
The bank has to carry the full daily consumption overnight, plus whatever margin you want for cloudy days.
Usable capacity is not the same as rated capacity. Lead acid gives you roughly half its rating before damage, while lithium iron phosphate gives you most of it.
That means a 200Ah lead acid bank and a 100Ah lithium bank deliver broadly similar usable energy, which is the comparison that matters rather than the headline number.
Days of autonomy is the other decision. One day means a single cloudy stretch drains you; three days is the common recommendation for a cabin where running out is genuinely inconvenient.
Each additional day multiplies the bank, which is where the cost sits in most off-grid builds. Our note on how many batteries you need covers the arithmetic.
Winter Is the Sizing Constraint
Panels are rated at their peak output under ideal conditions, and no real installation achieves that for more than a moment.
What matters is peak sun hours, which is the equivalent number of full-output hours a location receives in a day, and that figure varies enormously by season and latitude.
A location with five peak sun hours in June may have two in December, which means the same array produces less than half as much in the month you most want heat and light.
Sizing on summer figures produces a system that works beautifully for four months and fails in the other eight.
The honest approach is deciding which months you actually use the cabin and sizing for the worst of those, rather than for an annual average that describes no real day. Our note on choosing solar panels covers output ratings.
Panel Sizing From the Daily Total
Panels have to replace the daily consumption during the available sun hours, plus enough extra to cover system losses.
Charge controllers, wiring, and battery charging efficiency all lose a share, and real-world output sits meaningfully below the arithmetic ideal.
Adding a margin above the calculated figure is standard practice rather than over-engineering, because a system that exactly breaks even on a perfect day never catches up after a bad one.
Panels are also the cheapest part of most systems relative to batteries, which makes oversizing the array the more economical way to buy margin.
An oversized array simply throttles when the bank is full, which costs nothing. Our note on oversizing an array covers that.
Heating and Cooling Change Everything
Electric heating is generally out of reach for cabin solar, because resistive heat consumes vastly more than lighting and refrigeration combined.
You can work out how long a system takes to pay back before deciding.
Wood, propane, and diesel handle heat in almost every off-grid cabin for that reason, and it is a practical decision rather than a preference.
Cooling is similarly demanding, though a DC air conditioner or a well-sized fan is more achievable than resistive heating.
A system that would run lights, a fridge, and devices comfortably may need to be several times larger to add climate control.
Deciding that question first prevents sizing a system twice. Our roundup of DC air conditioners covers the cooling side.
Measure Before You Buy
Calculated loads and real consumption differ, usually in the direction of more.
A plug-in watt meter on individual devices, or a clamp meter on a DC system, converts assumptions into figures.
For an existing cabin on a generator, tracking fuel use across a typical stay gives a usable estimate of daily energy.
For a new build, the calculation is all you have, which is an argument for building in more margin than the numbers suggest.
Either way, revisiting the figure after a season of real use is how systems get sized correctly on the second attempt. Our roundup of clamp meters covers measurement.
What to Work Out First
Daily watt-hours, honestly estimated
Wattage times realistic hours for every device.
Which months you actually use it
Winter sizing and summer sizing are different systems.
Whether heat is electric
If yes, the whole calculation changes scale.
How many cloudy days you can tolerate
Each one multiplies the battery bank.
Common Mistakes to Avoid
Starting from panel wattage
Panels are the output of the calculation, not the input.
Sizing on summer sun
Winter peak sun hours can be under half the summer figure.
Using rated battery capacity
Lead acid delivers roughly half its rating before damage.
Forgetting system losses
Controller, wiring, and charging efficiency all take a share.
Recommended Reading
See our note on sizing a charge controller, our note on choosing a solar battery, our roundup of 200Ah LiFePO4 batteries, and a note on LiFePO4 versus lead acid.
Cabin Solar Sizing FAQ
How much solar does a cabin need?
It depends entirely on daily watt-hours. Add up wattage times realistic hours for every device, then size the battery to carry that overnight and the panels to replace it in your worst month of use. To work the numbers for your own load list, our off-grid solar sizing calculator runs the same arithmetic and returns battery and array figures.
Why watt-hours instead of watts?
Watts describe how fast something draws. Watt-hours describe how much it consumes over time, and time is what the battery has to cover.
What uses the most power?
Refrigeration in most cabins, because a compressor cycles day and night whether anyone is there or not. Propane refrigeration removes that load entirely.
Can I run electric heat?
Generally not on cabin solar. Resistive heating consumes vastly more than everything else combined, which is why wood and propane handle heat in almost every off-grid cabin.
How many batteries do I need?
Enough usable capacity for a full day plus your tolerance for cloudy weather. Lead acid gives roughly half its rating, lithium gives most of it.
Should I size for summer or winter?
The worst month you actually intend to use it. Winter peak sun hours can be under half the summer figure at the same location.
Is it better to oversize the panels or the battery?
Panels, usually. They are cheaper per unit of margin, and an oversized array simply throttles when the bank is full.
How do I know my real consumption?
Measure it. A plug-in watt meter on devices or a clamp meter on a DC system converts assumptions into figures, and calculated loads usually understate reality.
