Choosing batteries for off-grid power doesn’t have to be scary. This guide explains what an off-grid battery system is, how a battery bank is built, which chemistries make sense (LFP, AGM, Gel, etc.), how to calculate kWh, and the safety parts (BMS, fuses, low-temp charging) you must include.
What is an off-grid battery and how it works
An off-grid battery stores energy so you can run appliances when there’s no utility connection. It is charged by solar panels (via a charge controller), by a generator, or by wind/micro-hydro if you have them. A small computer called a BMS watches cell voltages, temperature, and current to keep everything safe.
What it powers: lights, fridge, Wi-Fi, well pump, phone charging—up to whole-home loads if you size it correctly.
Off-grid battery bank vs off-grid battery system
- Battery (single unit): one pack or one 12 V/24 V/48 V module.
- Battery bank: several batteries in series (raise voltage) and/or in parallel (raise capacity).
- Battery system: the entire set—battery bank + BMS/monitor + charger/MPPT + inverter + fuses/breakers + wiring and enclosures.
Think of the bank as the Lego bricks, and the system as the finished machine that can safely power your home.
Off-grid battery chemistry: LFP vs AGM vs Gel vs NMC
| Chemistry | Usable DoD (typ.) | Cycle life (typ.) | Safety & temp notes | Maintenance | Best use case |
|---|---|---|---|---|---|
| LiFePO4 (LFP) | 80–100% | 3,000–6,000+ | Very stable; needs low-temp charging protection below 0 °C | None | Most off-grid homes, cabins, RVs |
| AGM Lead-acid | 50–60% | 500–1,000 | Works in cold, heavy and bulky | None | Budget systems, backup rarely cycled |
| Gel Lead-acid | 50–60% | 700–1,200 | Sensitive to high charge rates | None | Low-rate, indoor environments |
| NMC/NCA Li-ion | 80–90% | 2,000–4,000 | Higher energy density; tighter thermal control | None | Space-limited installs with good HVAC |
Why LFP wins for off-grid: longest life per dollar, wide safety margin, deep usable capacity, simple maintenance.
Designing an off-grid battery bank (series, parallel, 12/24/48 V buses)
Choose system voltage first. Higher voltage means lower current for the same power → thinner cables and less heat.
| DC bus | Typical inverter power | Cable & losses | When to pick |
|---|---|---|---|
| 12 V | ≤ 1.5 kW | Highest current, thick cables | Small RVs, tiny cabins |
| 24 V | 1.5–3 kW | Moderate current | Mid-size cabins, light workshops |
| 48 V | ≥ 2–10 kW | Lowest current, neat wiring | Whole-home off-grid or high loads |
Series (S) adds voltage; Parallel (P) adds capacity.
- Four 12 V batteries in series → 48 V.
- Two 48 V strings in parallel → same voltage, double capacity.
Wiring rules for beginners
- Every series string must use identical batteries (age, brand, capacity).
- Balance parallel strings with equal cable lengths and proper bus bars.
- Always include main DC fuse/breaker, disconnect switch, and class-T or DC-rated protection sized for the inverter.
Off-grid battery sizing calculator
Inputs you need
- Daily energy use (kWh/day) – add appliances × hours.
- Days of autonomy – how long you want to last with poor sun.
- Usable DoD – 90% for LFP, ~50% for AGM/Gel.
- System losses factor – 1.10–1.20 (inverter + wiring).
Formula
Required battery (kWh) ≈ Daily kWh × Days ÷ DoD × Losses
Starter assumptions
- Small cabin: 6–8 kWh/day
- Family home: 15–25 kWh/day
- Heavy electric home: 25–40+ kWh/day
Examples (easy to copy)
- Cabin essentials: 6 kWh/day × 2 days ÷ 0.9 × 1.15 ≈ 15.3 kWh → one 48 V LFP bank around 15 kWh
- Family home: 20 kWh/day × 1.5 days ÷ 0.9 × 1.15 ≈ 38.3 kWh → 3× ~13 kWh LFP units in parallel
- Workshop with well pump: 18 kWh/day × 2 days ÷ 0.9 × 1.15 ≈ 46 kWh, plus inverter sized for pump surge
Tip: if your inverter is 6 kW and you plan long loads, prefer 48 V to keep current manageable (6 kW ÷ 48 V ≈ 125 A DC).
Charge and protection: BMS, low-temp charging, fuses and breakers
- Battery Management System (BMS): monitors cells, balances them, and cuts charge/discharge if voltage or temperature is unsafe.
- Low-temperature charging: most LFP packs should not charge below ~0 °C. Use heaters or a BMS with low-temp cut-off.
- Over-current protection: place a DC fuse or breaker close to the battery positive. In many installs, a Class-T fuse protects high-current inverters.
- Disconnects & isolation: include a lockable DC disconnect, proper ground/earth, and surge protection on AC and PV where code requires.
- Cables & lugs: size for <2–3% voltage drop at max continuous current; use tinned copper lugs and torque to spec.
Off-grid battery life, warranty and maintenance tips
- Cycle life vs DoD: deeper daily cycles shorten life; LFP tolerates 80–100% DoD well, lead-acid prefers ≤50% DoD.
- Temperature effect: every 10 °C above room temps can reduce life; avoid hot enclosures.
- Throughput warranty: many packs warrant MWh of energy as well as years—read both.
- Routine checks: keep vents and clearances open, log monthly state-of-health (SOH), and update firmware where applicable.
Off-grid battery bank wiring for beginners
- 12 V RV bank: 2× 12 V AGM in parallel → 12 V, ~200 Ah total; 1–2 kW inverter.
- 24 V cabin bank: 2× 12 V LFP in series → 24 V; add a second series string in parallel when you expand.
- 48 V home bank: 4× 12 V LFP in series → 48 V; or single 48 V LFP module; parallel more modules for extra kWh.
Label strings S1/S2, use bus bars, and land the main positive from one end and the main negative from the opposite end for even sharing.
Frequently asked questions on off-grid batteries
What is the best battery for off-grid living?
In most cases LiFePO4 (LFP) gives the best mix of safety, deep usable capacity, and long cycle life.
How big of a battery bank do I need for a house?
Multiply your daily kWh by the days of autonomy, divide by usable DoD, then add ~10–20% for losses. Many homes land between 20–50 kWh.
How much does an off-grid battery bank cost?
Very rough: LFP ≈ $300–$600 per usable kWh for modules (installed systems higher). Lead-acid is cheaper upfront but costs more over its shorter life.
Can I run AC on an off-grid system?
Yes, if the inverter kW and battery surge can start the compressor. Many homes need 48 V and several LFP modules for whole-home AC.
What is the 80/20 battery rule?
Keep 20% in reserve (don’t empty every cycle) to extend life. With LFP you can use 80–100% in emergencies, but daily use at ~80–90% DoD is gentle.
Is it possible to be 100% off-grid year-round?
Yes, but plan for winter solar: more PV, a backup generator, or extra battery days of autonomy.
Do I need a special charger for LFP?
Use a charger/MPPT with an LFP profile and correct voltage limits; include low-temp charging protection for cold climates.
What appliances cannot be used with solar power?
You can use nearly any, but high-surge tools, welders and big resistive heaters demand a large inverter and heavy DC wiring.
Quick selection guide for off-grid battery choices
- Budget small system: AGM 12/24 V, shallow daily cycles, upgrade later.
- Main home off-grid: 48 V LFP modules, scalable 20–60 kWh, hybrid inverter.
- Cold climate cabin: LFP with battery heaters or insulated cabinet.
- Occasional outage backup: AGM or LFP with generator assist.