Off-Grid Solar Battery Calculator: Don’t Buy the Wrong Bank
Off-Grid Solar Battery Calculator: The Ultimate Sizing Guide (2025)
Designing an off-grid power system for a van conversion, RV, or remote cabin is an exercise in precision. If you underestimate your needs, you will wake up to a blackout. If you overestimate, you waste thousands of dollars on lithium that you will never use. This guide and tool by ShockTrail will help you find the perfect balance.
The Calculator
Use the tool below to estimate your required battery bank size in Amp-Hours (Ah) and Watt-Hours (Wh).
Battery Bank Sizing
Smart Tool by ShockTrailWhy “Watts” Don’t Tell the Whole Story
One of the most common mistakes beginners make is confusing Power (Watts) with Energy (Watt-Hours). A 1000W microwave running for 2 minutes consumes very little energy. A 50W fridge running for 24 hours consumes a lot. To size a battery bank correctly, as we teach at ShockTrail, you must calculate the total energy over time.
The 3 Variables That Kill Your Batteries
Our calculator accounts for three hidden factors that manual math often ignores:
- Inverter Efficiency: Converting DC power (Battery) to AC power (Outlet) creates heat. You typically lose 15% of your energy in this process.
- Depth of Discharge (DoD): You cannot drain a battery to 0%. Doing so will destroy it.
- Temperature: Cold weather slows down chemical reactions, effectively shrinking your battery’s capacity temporarily.
Battery Chemistry Comparison: Lithium vs. Lead-Acid
Choosing the right technology is as important as choosing the right size. See the comparison table below:
| Feature | Flooded Lead-Acid | AGM (Absorbed Glass Mat) | LiFePO4 (Lithium) |
|---|---|---|---|
| Usable Capacity (DoD) | 50% | 50-60% | 80-100% |
| Cycle Life | 300-500 cycles | 400-800 cycles | 3000-5000+ cycles |
| Weight | Very Heavy | Heavy | Lightweight (1/3 of Lead) |
| Maintenance | Requires Water Refill | Maintenance Free | Zero Maintenance (BMS) |
Real World Sizing Scenarios
To help you visualize, here are three setups frequently analyzed by the ShockTrail engineering team:
Scenario A: The “Weekender” Minivan
Load: Charging phones, drone batteries, and LED lights.
Requirement: ~300Wh per day.
Solution: A simple “Solar Generator” (portable power station) with 500Wh capacity is cheaper and easier than wiring a custom system.
Scenario B: The Digital Nomad Van
Load: Starlink Internet, Laptop (8 hours), 12V Fridge, Diesel Heater.
Requirement: ~1200Wh per day.
Solution: A 200Ah Lithium Battery (12V) paired with 400W of solar panels. This provides enough buffer for cloudy days.
Scenario C: The Off-Grid Cabin
Load: Well pump, full-size fridge, TV, lighting, kitchen appliances.
Requirement: ~5000Wh per day.
Solution: Move to a 48V system. You will need a server-rack lithium battery (approx 10kWh total capacity) to ensure autonomy.
How to Calculate Manually (The Math)
If you want to double-check the results from our calculator, here is the formula used by professionals:
(Daily Watt-Hours / 0.85 Efficiency) × Days of Autonomy = Total Energy Needed
Then divide Total Energy by the Voltage (12V, 24V, or 48V) to get Amp-Hours.
Maintenance Tips for Longevity
- Never store empty: If you leave a lithium battery at 0% for months, the BMS may lock it permanently.
- Check connections: Loose wires cause resistance, heat, and fire hazards. Retighten your busbars every 6 months.
- Ventilation: Even though Lithium doesn’t off-gas like Lead-Acid, your inverter needs airflow to stay cool.
For more detailed wiring diagrams and safety guides, visit the main hub at ShockTrail.
Frequently Asked Questions (FAQ)
Can I mix different battery brands?
It is highly discouraged. Different brands have different internal resistances and Battery Management Systems (BMS), which can lead to imbalances. Always buy matched sets.
What size inverter do I need?
Add up the wattage of everything you want to run simultaneously. If you run a blender (1000W) and a microwave (1500W) at the same time, you need at least a 3000W inverter.
Is 24V better than 12V?
Yes, for systems over 2000W. Higher voltage means lower current (Amps), which allows you to use thinner, cheaper, and safer wires. ShockTrail recommends 24V for most medium-sized builds.
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