How to Size a Solar System for a Remote Cabin

Sizing a solar system for a remote cabin is more demanding than for a van or a garden office, because you cannot easily top up from the mains if your estimates are wrong. This guide walks through a systematic approach that ensures your system reliably covers your energy needs, season by season.

Step 1: Build Your Load Profile

List every electrical load in the cabin and how long it runs each day. Be honest about seasonal variation. A cabin used primarily in summer has very different demands from one used through winter.

Common cabin loads and typical power draws:

• LED lighting: 5 to 10W per bulb, typically 30 to 80W total for 4 to 6 hours
• Fridge (12V compressor type): 30 to 50W average draw, running 24 hours
• Laptop: 45 to 65W for 4 to 8 hours
• Water pump (12V): 60 to 120W, running intermittently (estimate 30 minutes total per day)
• Phone charging: 10 to 20W for 2 hours
• Small TV or entertainment: 30 to 80W for 2 to 4 hours
• Diesel heater controller/fan: 15 to 30W when running

Add all watt-hours together. A basic cabin with lighting, a fridge and device charging might total 600 to 900Wh per day. A more equipped cabin with a washing machine or electric cooking could reach 3 to 5kWh per day. Avoid electric space heating with solar unless you have a very large system.

Step 2: Account for Seasonal Sun Hours

The number of usable peak sun hours per day varies significantly by location and season. In the UK and Ireland:

• Summer (June to August): 3.5 to 5 peak sun hours per day
• Spring and autumn: 2 to 3.5 peak sun hours per day
• Winter (November to February): 0.5 to 1.5 peak sun hours per day

If the cabin is used year-round, you must size for winter conditions. If it is a summer-only cabin, size for the lowest figure in your occupancy window. Using a winter figure of 1.5 peak sun hours is the prudent approach for year-round designs in the UK.

Step 3: Calculate Required Panel Wattage

The formula is: Required panel wattage = (Daily Wh x 1.25) / peak sun hours per day.

For a cabin consuming 900Wh per day sized for 1.5 winter peak sun hours: (900 x 1.25) / 1.5 = 750W of panels.

For a summer-only cabin at 3.5 hours: (900 x 1.25) / 3.5 = 321W. Round to 400W.

Step 4: Size the Battery Bank

Off-grid cabins typically need 3 to 5 days of battery autonomy to cover extended cloudy periods. For a 900Wh daily load with 3 days autonomy: 900 x 3 = 2700Wh. Using LFP batteries (90% usable depth of discharge), you need around 3kWh of rated capacity.

For year-round use in the UK, extend to 5 to 7 days autonomy to cover the long grey spells common in winter: 900 x 6 = 5400Wh rated capacity. This is a significant battery bank and may require a 24V or 48V system to keep cable sizes manageable.

Step 5: Choose Your System Voltage

• 12V: suitable for small systems up to around 1kWh battery and 400W panels
• 24V: better for medium cabins (1 to 5kWh battery, 400W to 2kW panels)
• 48V: best for larger systems where high current on 12V would require impractically thick cables

Higher system voltage means lower current for the same power, allowing smaller cable cross-sections and reducing resistive losses in the wiring runs that are often long in a cabin installation.

Step 6: Select an Inverter

The inverter must handle your peak simultaneous AC load. If you run a laptop (65W), lights (50W) and a water pump (120W) at the same time, that is 235W peak. Size the inverter to at least 1.5x your expected peak load for headroom. For most cabins, a 1000W to 3000W pure sine wave inverter is appropriate.

Generator Backup

For year-round remote cabins, a small petrol or diesel generator as a backup charger is highly recommended. During extended winter periods with minimal solar, a generator running for 2 to 3 hours can top up the battery bank and prevent a complete blackout. Size the generator to the battery charger's input capacity, not the full load.

Panel Mounting at Remote Locations

Ground-mounted panels on a tilt frame are often easier to manage at a remote cabin than roof mounting, particularly for future access and cleaning. Position panels away from shadows cast by trees or outbuildings. In snowy locations, a steeper tilt angle (up to 60 degrees) helps snow slide off naturally and also improves winter performance by angling panels more directly at the low winter sun.