Article: Off-Grid Battery Bank Sizing: Cabin, Homestead, and Tiny House
Off-Grid Battery Bank Sizing: Cabin, Homestead, and Tiny House
The battery bank is the center of any off-grid power system. Everything else — solar panels, charge controllers, inverters — either fills it or draws from it. Get the size wrong and you're either constantly managing a depleted bank or you've spent thousands of dollars hauling capacity you'll never use.
The right size is a specific number, and it's not that hard to get to once you know the method. Here's how to do it.
The Core Formula
Battery bank size (Wh) = Daily energy use (Wh) × Days of autonomy ÷ Usable capacity factor
Then convert to amp-hours: Wh ÷ System voltage = Ah
Input 1: Daily Energy Use
Build a load list. Write down every electrical device you plan to run, its wattage, and how many hours per day it operates.
| Load | Wattage | Hours/Day | Daily Wh |
|---|---|---|---|
| LED lighting (full cabin) | 60W | 5 | 300 |
| 12V compressor fridge | 50W avg | 24 | 1,200 |
| Chest freezer (efficient) | 30W avg | 24 | 720 |
| Laptop | 65W | 4 | 260 |
| Phone/tablet charging | 40W | 2 | 80 |
| Satellite internet (Starlink) | 50–75W | 10 | 625 |
| Water pump (pressure tank) | 500W | 0.3 | 150 |
| Ceiling fans | 30W each | 8 | 240 |
| TV / streaming | 80W | 3 | 240 |
Typical daily totals by application:
| Application | Daily Energy Use |
|---|---|
| Minimal cabin, seasonal use | 600–1,000 Wh |
| Weekend cabin, regular use | 1,000–1,800 Wh |
| Full-time remote cabin, no A/C | 1,800–3,000 Wh |
| Small homestead, light production loads | 2,500–4,500 Wh |
| Homestead with well pump, shop tools | 4,000–8,000+ Wh |
| Tiny house, full-time living | 1,500–3,500 Wh |
Input 2: Days of Autonomy
Days of autonomy is how many consecutive days you want to run on stored energy without any recharging — no solar, no generator. This is the most consequential decision in battery bank sizing.
- 1 day: Bare minimum — fine only with a reliable generator backup.
- 2 days: The most common baseline. Covers one fully cloudy day without stress.
- 3 days: The sweet spot for most serious off-grid installations. Covers a multi-day cloudy stretch without triggering generator use.
- 4–5 days: For very remote locations, no generator, or extended deep-winter use.
Most cabin and homestead systems land at 2–3 days of autonomy.
Input 3: Usable Capacity by Chemistry
LiFePO4: 95–100% of rated capacity is safely usable. A 200Ah LiFePO4 bank delivers ~190–200Ah.
AGM: Safe depth of discharge is 50%. A 200Ah AGM bank gives you only ~100Ah of usable energy. Regularly discharging deeper accelerates capacity fade and cuts service life to 2–3 years.
Running the Math
LiFePO4 example — weekend cabin (1,200 Wh/day, 3-day autonomy):
1,200 × 3 ÷ 0.95 = 3,789 Wh
At 24V: 3,789 ÷ 24 = 158Ah → round up to a 200Ah 24V bank
AGM equivalent — same scenario:
1,200 × 3 ÷ 0.50 = 7,200 Wh
At 24V: 7,200 ÷ 24 = 300Ah of AGM — nearly twice the size and weight, shorter service life.
Quick Reference: Bank Sizes by Load Level
LiFePO4, 3-day autonomy. Round up to the next available configuration.
| Daily Energy Use | 24V Bank | 48V Bank | Application |
|---|---|---|---|
| 800 Wh | 100Ah | 50Ah | Minimal seasonal cabin |
| 1,200 Wh | 150–200Ah | 75–100Ah | Weekend cabin |
| 2,000 Wh | 250–300Ah | 125–150Ah | Full-time remote cabin |
| 3,000 Wh | 400Ah | 200Ah | Cabin with freezer + Starlink |
| 4,500 Wh | 600Ah | 300Ah | Small homestead |
| 6,000 Wh | 800Ah | 400Ah | Homestead with well + tools |
System Voltage: 24V or 48V?
24V: Most common for cabin and small homestead builds. Good compatibility with available components; manageable wire sizing for most runs.
48V: Better efficiency at high loads and long cable runs — significantly reduces current, which means smaller wire and less heat. Preferred for homestead-scale systems (4+ kWh/day). Much easier to design in from the start than migrate to later.
Don't Skip the Battery Monitor
A shunt-based battery monitor — like the Victron SmartShunt — measures actual current in and out and gives you a real state-of-charge reading at all times. LiFePO4 batteries hold a nearly flat voltage curve from 20% to 90% state of charge, which makes voltage-based gauges nearly useless. Without a shunt, you'll think you have capacity and suddenly hit the BMS cutoff.
Install it from day one.
Browse the full lithium battery collection at Blue Marine, or schedule a free consultation with our team. We spec off-grid builds regularly and can work through your load profile, autonomy requirements, and system voltage to land on a bank size that's right-sized — not guessed at.
Related reading:
How to Design an Off-Grid Solar System from Scratch
LiFePO4 vs AGM Marine Battery: Which Should You Buy?
Generator vs Solar for Off-Grid: When Does Each Make Sense?
