Off-Grid Solar System Sizing for Cabins and Sheds
A step-by-step sizing walkthrough for a standalone solar system powering a cabin, shed, garden office or tiny home. Covers daily load estimation, panel count, battery bank, inverter selection and system cost. Uses real calculator inputs throughout.
Last updated: May 2026
The scenario
You want to power a small off-grid cabin with solar panels and a battery bank. The cabin is used on weekends and some weekdays. The loads are modest: LED lighting, a laptop, phone charging and occasionally a 12V fridge. There is no mains grid connection. The system must handle at least 2 consecutive cloudy days without fully depleting the battery.
Step 1: Load inventory and daily energy budget
List every load, its rated power in watts, and how many hours per day it runs. Multiply each load's watts by its hours to get the daily Wh contribution, then sum.
| Load | Power (W) | Hours/day | Daily energy (Wh) |
|---|---|---|---|
| LED lighting (4 bulbs) | 40 | 4 | 160 |
| Laptop | 65 | 3 | 195 |
| Phone charging (2 phones) | 20 | 2 | 40 |
| 12V compressor fridge (avg) | 45 | 8 | 360 |
| Small radio / speaker | 10 | 2 | 20 |
| Total | — | — | 775 Wh/day |
Add a system efficiency factor of 15–20% for inverter losses, wiring losses and battery charge/discharge efficiency. Adjusted daily need: 775 × 1.20 = 930 Wh/day.
Step 2: Size the solar panels
Panel size depends on your daily energy need and the average peak sun hours (PSH) for your location. PSH is the number of hours per day the sun delivers full rated irradiance (1000 W/m²). Central Europe averages 3–4 PSH in winter, 5–6 PSH in summer. Use the worst-case (winter) PSH for a system that must work year-round.
Panel power needed = 930 Wh ÷ 3.5 PSH = 266 W → choose 2× 200 W panels (400 W total)
Oversizing the array is intentional: extra panel capacity charges the battery faster, provides buffer for cloudy days, and costs less per watt than extra battery capacity. A 400 W array on a 930 Wh/day load gives comfortable headroom.
| Location example | Winter PSH | Panels needed (for 930 Wh/day) |
|---|---|---|
| Southern UK / Northern France | 2.5 | 372 W → 2× 200 W |
| Central Europe / Netherlands | 3.5 | 266 W → 2× 200 W |
| Southern Spain / Italy | 5.0 | 186 W → 1× 200 W (with margin) |
| US Pacific Northwest | 3.0 | 310 W → 2× 200 W |
| US Southwest | 6.0 | 155 W → 1× 200 W |
Use the calculator: Enter your panel wattage, number of panels and your location's PSH into the Solar Panel Output Calculator to see daily, monthly and yearly kWh estimates. Check PSH for your area in the Peak Sun Hours reference table.
Step 3: Size the battery bank
The battery bank must store enough energy to run the cabin through 2 consecutive days without solar input (the "days of autonomy"). The battery should not be discharged below 50% for lead-acid (AGM/gel) or 20% for LiFePO4 lithium.
Required capacity = (930 Wh × 2 days) ÷ 0.50 = 3720 Wh → at 24V: 155 Ah AGM
At 12V system voltage: 3720 Wh ÷ 12V = 310 Ah — two 150 Ah AGM batteries in parallel. At 24V (better for higher power systems): two 150 Ah AGM batteries in series. Using LiFePO4 at 80% DoD: 3720 ÷ 0.80 = 4650 Wh, 24V: ~194 Ah — one 200 Ah LiFePO4 at 24V handles it easily.
| Chemistry | DoD | 2-day capacity needed | Practical choice |
|---|---|---|---|
| AGM (12V) | 50% | 310 Ah | 2× 150 Ah in parallel |
| AGM (24V) | 50% | 155 Ah | 2× 150 Ah in series |
| LiFePO4 (24V) | 80% | 97 Ah | 1× 100 Ah battery |
Use the calculator: Enter 930 Wh/day load, 2 days autonomy, your system voltage and DoD into the Battery Bank Calculator. Toggle DoD between 50% (AGM) and 80% (lithium) to compare.
Step 4: Size the inverter
The inverter converts 12V or 24V DC battery power to 230V AC for the laptop and other mains devices. Size the inverter for the maximum simultaneous AC load, not the average. For this cabin: laptop (65 W) + lighting (40 W) + radio (10 W) = 115 W continuous. A 300 W or 500 W pure-sine inverter is appropriate.
| Cabin scenario | Peak AC load | Recommended inverter |
|---|---|---|
| Laptop + lights (this example) | 115 W | 300 W pure-sine |
| + Small power tools | 400 W | 600 W pure-sine |
| + Kettle (2 kW, occasional) | 2000 W | 2000–3000 W pure-sine |
| + Water pump, fridge, lights | 500 W | 1000 W pure-sine |
Always choose a pure-sine wave inverter for sensitive electronics (laptops, phone chargers, anything with a switching power supply). Modified-sine inverters are cheaper but cause buzzing in audio equipment, run motors less efficiently and can damage some appliances.
Use the calculator: Enter the connected AC loads into the Inverter Sizing Calculator to see required continuous and surge rating.
Step 5: Estimate cost and payback
For a weekend cabin not connected to mains, the comparison is not against a grid electricity bill but against the cost of running a generator or not having power at all. A simple solar system pays back in avoided generator fuel and maintenance within 2–5 years for typical usage.
| Component | Indicative cost (EUR) |
|---|---|
| 2× 200 W solar panels | 200–300 |
| MPPT charge controller (20–40 A) | 60–150 |
| 2× 150 Ah AGM batteries (24V) | 300–500 |
| 300 W pure-sine inverter | 60–120 |
| Wiring, fuses, connectors | 50–100 |
| Total system | 670–1170 |
Use the calculator: Enter system cost, estimated daily kWh saved and your local electricity rate into the Solar Payback Calculator to see simple and inflation-adjusted payback period.
Component sizing summary
| Component | This example | Notes |
|---|---|---|
| Daily load | 775 Wh (930 Wh adjusted) | Includes 20% system losses |
| Solar panels | 2× 200 W = 400 W | Sized for 3.5 PSH worst-case winter |
| Battery bank (AGM 24V) | 2× 150 Ah in series | 2 days autonomy at 50% DoD |
| Battery bank (LiFePO4 24V) | 1× 100 Ah | 2 days autonomy at 80% DoD |
| MPPT charge controller | 20 A / 48V | Handles 400 W array at 24V |
| Inverter | 300 W pure-sine | Laptop + lights simultaneously |
Common mistakes
- Sizing panels for summer, not winter: a system that works fine in July will run flat batteries by December. Use winter PSH for the panel calculation if the cabin is used year-round.
- Undersizing the battery for autonomy: one day of autonomy feels sufficient until a 3-day grey period arrives. Two days is the practical minimum; three days is comfortable for northern Europe.
- Skipping the charge controller: connecting panels directly to the battery without an MPPT or PWM charge controller will overcharge and destroy the battery within weeks.
- Modified-sine inverter for a laptop: laptop power supplies and many phone chargers work poorly or fail prematurely on modified-sine inverters. Pure-sine costs only slightly more for small inverters.
- Ignoring panel orientation and tilt: panels facing south at 30–45° tilt in Europe or north-facing at the same angle in Australia produce significantly more energy than flat or poorly oriented panels. A 10% improvement in orientation can replace 10% more panel capacity.
Frequently Asked Questions
How many solar panels do I need for a small off-grid cabin?
For a typical light-use cabin (775 Wh/day as in this example) in central Europe using 3.5 winter peak sun hours: at least 266 W of panels, so two 200 W panels (400 W) is the right minimum with comfortable buffer. In sunnier climates (southern Europe, US Southwest at 5–6 PSH), one 200 W panel may be sufficient. In cloud-heavy locations (UK, Pacific Northwest at 2.5–3 PSH), two 200 W panels or one 400 W panel is necessary. Use the Solar Panel Output Calculator with your local PSH from the Peak Sun Hours reference.
Should I use a 12V or 24V battery system?
For systems under 500 W of solar and daily loads under 500 Wh, 12V works fine. For larger systems — 400 W+ of panels, 1 kWh+ daily load, or any inverter over 1000 W — use 24V or 48V. Higher voltage systems run lower currents for the same power, which means thinner wires, less voltage drop and more efficient inverters. This cabin example is borderline: 400 W panels and 775 Wh/day works well at 24V and is awkward at 12V due to the large battery Ah required (310 Ah at 12V vs 155 Ah at 24V).
PWM or MPPT charge controller for an off-grid system?
MPPT for almost all off-grid systems. A PWM controller is only efficient when the panel voltage matches the battery voltage almost exactly — which is rare for modern panels. An MPPT controller extracts 10–30% more energy from the panels at the cost of slightly higher price. For a 400 W panel system charging a 24V battery, an MPPT controller typically produces 15–20% more daily kWh than PWM in the same conditions. The extra cost pays back in a few months.
Can I add a generator as a backup to an off-grid solar system?
Yes, and it is a common hybrid approach. A small petrol or LPG generator sized at 2–3 kW can charge the battery bank through a battery charger when solar production is insufficient during long grey periods. The generator only runs for 2–4 hours to top up the battery, rather than running continuously. This is far more fuel-efficient than a generator-only system. Make sure the battery charger input voltage matches the generator output (230V AC in Europe).
What size cable do I need from the panels to the charge controller?
For a 400 W, 24V system: short-circuit current (Isc) for two 200 W panels in parallel is roughly 2 × 9 A = 18 A. Use 4 mm² (AWG 11) cable for the panel-to-controller run to keep voltage drop under 1% and handle the Isc safely. For runs over 10 m, increase to 6 mm². Always use UV-rated outdoor solar cable (PV cable) between the panels and the controller — standard flex will degrade in sunlight within 2–3 years.