Solar Power for an Off-Grid Cabin

A cabin looks like an easy version of a tiny house: similar loads, less of the time. In practice it is the harder sizing problem, for one reason that has nothing to do with how much power the appliances use.

Intermittent use breaks the usual method

The standard method takes daily consumption, multiplies by days of autonomy, and sizes a bank. That assumes a load that is roughly the same every day.

A weekend cabin is not that. It draws nothing for five days, then two days of real use, then nothing again. Average it across the week and you get a figure that describes no actual day. Size the bank on that average and you will run out on Saturday evening, every time.

Size the bank on the days you are there. The load list above is a day of occupancy, not a weekly average, and the autonomy figure covers a weekend of poor weather.

The array is the opposite case, and it works in your favor: it has five quiet days to refill the bank. A cabin can carry a smaller array relative to its bank than a full-time house can, because the recharge window is longer than the discharge window.

The fridge decides the system

The single biggest question for a cabin is whether the refrigerator runs between visits.

Left running, it becomes a continuous load the array must cover 365 days a year, including the darkest fortnight in December when nobody is there to appreciate it. On the load list above the fridge is roughly half the daily total, so this is not a detail.

Switched off between visits, the array only has to cover the days you are present, and the system can be substantially smaller.

The cost is arriving to a warm cabin and waiting several hours, and having to manage what you leave in it. Plenty of cabin owners accept that trade; plenty do not. It is worth deciding deliberately rather than by default, because it is the largest single lever on the cost of the build.

A generator makes more sense here than anywhere else

For a full-time home, running a generator is an intrusion. For a cabin, arriving after a dark fortnight and running one for an hour while you unload the car is barely an inconvenience.

That changes the economics considerably. The last two days of storage are the most expensive part of a battery bank and the least used. A cabin can reasonably carry two days rather than four, and cover the rare gap with fuel.

The winter problem, in both directions

Cabin use often peaks exactly when solar production is worst: hunting season, snow weekends, holidays.

Size on the sun hours for the worst month you actually intend to use the place. If it is genuinely a summer cabin, say so and size accordingly, which will save a great deal of money. If you use it year round, December is the design case.

The opposite risk applies to the bank when nobody is there. A lead-acid bank left partly discharged through a cold spell can freeze and split its cases, which is how people return in spring to a destroyed bank and an acid spill. LiFePO4 will not freeze that way, but most cells must not be charged below freezing without a heater, which matters because a cabin battery box is rarely heated. See cold climates.

The water pump is the surge to watch

Look at the inverter surge figure above against the running watts. The pump is a fraction of the daily energy and most of the inverter requirement.

Deep well pumps are worse still, and are the usual reason a cabin needs a far larger inverter than its energy consumption suggests. If your water comes from a deep well rather than a shallow one or a tank, get the pump’s actual starting current before sizing anything.

Adjusting this

Every figure in the specification is computed from the load list, so change the list and the system changes. Put your own loads into the appliance power budget, then the daily total into system sizing.

The two inputs that move the answer most are whether the fridge stays on and the sun hours in your worst month of use.