How to Size a Battery Bank for Off-Grid Solar: The Complete Guide
Your battery bank is the most expensive single component in any off-grid solar system, and the one most people size wrong. Too small and you’re in the dark after a cloudy day. Too large and you’ve wasted thousands. Here’s the exact math.
The Two Numbers You Need First
Before calculating battery bank size, you need two inputs:
- Daily energy consumption in watt-hours (Wh) — everything you use in a day
- Days of autonomy — how many days you want to run without any solar input
If you haven’t calculated your daily consumption yet, go through every appliance: watts × hours per day = watt-hours per day. Add them all up, then multiply by 1.25 for inverter losses and wire inefficiency. A comfortable off-grid cabin typically lands between 1,000 and 2,500 Wh/day.
The Core Formula
Required battery capacity (Wh) = Daily Wh × Days of Autonomy ÷ Usable Depth of Discharge
The “usable depth of discharge” is critical:
- LiFePO4 batteries: use 0.80 (you can safely use 80% of rated capacity)
- AGM batteries: use 0.50 (only use 50% — going deeper kills them fast)
Worked example — 1,500 Wh/day cabin, 2 days of autonomy:
- LiFePO4: 1,500 × 2 ÷ 0.80 = 3,750 Wh needed
- AGM: 1,500 × 2 ÷ 0.50 = 6,000 Wh needed
To convert watt-hours to amp-hours at 12V: divide by 12.
- LiFePO4: 3,750 ÷ 12 = 312 Ah → three 100Ah batteries in parallel
- AGM: 6,000 ÷ 12 = 500 Ah → five 100Ah batteries in parallel
This is why LiFePO4 is so attractive: you need far fewer batteries for the same usable capacity.
How Much Autonomy Do You Actually Need?
1–1.5 days: Appropriate for weekend/seasonal cabins in sunny climates where you’re usually not there during bad weather.
2 days: The minimum for a full-time off-grid home in most of the US. Handles normal cloudy stretches.
3 days: Recommended for northern climates with long cloudy periods, or any full-time home where you don’t want to run a generator often.
5+ days: For critical systems or very cloudy climates (Pacific Northwest). Beyond this, you’re better off adding more solar than more batteries.
Choosing Battery Voltage: 12V, 24V, or 48V?
Higher voltage systems move the same power with less current, which means smaller wire and less heat loss. Here’s when to use each:
| System Size | Recommended Voltage |
|---|---|
| Under 1,500W | 12V |
| 1,500W – 4,000W | 24V |
| Over 4,000W | 48V |
For most cabins and small homesteads, 12V is simpler and has more compatible equipment. If you’re planning a serious homestead with a washer, well pump, and multiple large loads, start at 24V or 48V.
🏆 Our Pick: LiTime 100Ah LiFePO4 Battery — ~$219/battery, Grade A cells, 4,000-cycle rated BMS, excellent value
For a 300Ah 12V bank: 3 of these in parallel. For a 200Ah 24V bank: 4 of these in a 2S2P configuration.
As an Amazon Associate we earn from qualifying purchases.
Series vs Parallel Wiring
Parallel wiring (positive to positive, negative to negative) keeps voltage the same and adds amp-hours. Four 12V 100Ah batteries in parallel = 12V 400Ah.
Series wiring (positive to negative) keeps amp-hours the same and raises voltage. Two 12V 100Ah batteries in series = 24V 100Ah.
Series-parallel combines both. Four 12V 100Ah batteries in a 2S2P configuration (two series pairs, then those two pairs in parallel) = 24V 200Ah.
The critical rule for parallel banks: all batteries must be identical — same brand, model, capacity, and ideally age. New and old batteries in parallel will cause the new ones to compensate for the old ones, degrading the whole bank.
Common Battery Sizing Mistakes
Ignoring temperature. LiFePO4 batteries cannot be charged below 32°F (0°C) without risk of plating lithium metal on the anodes — permanent damage. If your battery bank is in an unheated space, you need either self-heating batteries or a small thermostat-controlled heat mat. AGM tolerates charging down to about 20°F.
No battery monitor. LiFePO4 has a nearly flat voltage curve — the voltage stays around 13.2V from 90% SoC down to 20% SoC. You simply cannot tell how full they are by checking voltage. A $30–$80 coulomb-counting battery monitor (like the Victron BMV-700 or Renogy 500A shunt monitor) is non-negotiable.
Buying cheap cells. The LiFePO4 market is full of Grade B and C cells sold as Grade A. Stick to brands like LiTime, Battle Born, Renogy, SOK, or Ampere Time. Saving $50/battery on no-name cells that fail in 2 years is not saving money.
Mixing old and new batteries. If you add new batteries to a bank that’s been cycling for 2+ years, the new batteries will have more capacity than the old ones. In a parallel bank, they’ll effectively be charging and discharging the old batteries to compensate. Add new batteries only to a new bank, or replace all batteries at once.
Quick Reference: Battery Bank Sizing by Use Case
| Use Case | Daily Wh | LiFePO4 Needed (2-day) | Batteries (100Ah) |
|---|---|---|---|
| Small cabin, minimal | 400 Wh | 100 Ah | 1× 100Ah |
| Cabin, lights + laptop | 800 Wh | 200 Ah | 2× 100Ah |
| Cabin + fridge | 1,400 Wh | 350 Ah | 4× 100Ah |
| Small homestead | 2,500 Wh | 625 Ah | 7× 100Ah |
Use Our Free Battery Sizing Calculator
Punch in your daily watt-hours, autonomy days, and battery type at our battery bank sizing calculator and get an exact recommendation.
FAQ
How many batteries do I need for a 1,000W solar system? Solar panel wattage and battery size are calculated independently — a 1,000W panel array might serve a tiny cabin using 600 Wh/day or a larger one using 3,000 Wh/day. Calculate your daily consumption first, then size batteries for 2–3 days of autonomy at your target depth of discharge.
Can I add more batteries to my bank later? Yes, with LiFePO4 you can add identical batteries to a parallel bank. They should be the same brand, model, and ideally the same state of charge (storage charge, ~50% SoB) when connected. Never add new batteries to a bank where the existing batteries are significantly degraded.
What happens if I regularly discharge AGM batteries past 50%? Every discharge past 50% accelerates sulfation on the lead plates, permanently reducing capacity. AGM batteries discharged to 80% regularly may last only 100–200 cycles instead of 300–500. This is why the “usable capacity” of AGM is officially 50% — it’s not arbitrary caution, it’s the real chemistry.
How long do LiFePO4 batteries last off-grid? Quality LiFePO4 cells rated at 4,000 cycles at 80% DoD, cycling once per day, last over 10 years. In real off-grid use where batteries cycle shallowly much of the time, 12–15 years is realistic. Battle Born offers a 10-year warranty; LiTime offers 5 years.
Should I use a BMS? Every LiFePO4 battery should have a BMS (Battery Management System). Quality batteries include an internal BMS that prevents overcharge, over-discharge, short circuits, and thermal issues. For larger banks with multiple batteries in parallel, a separate bank-level BMS or a smart charge controller that monitors individual batteries adds another protection layer.