How to Size Your 12V Battery Bank

One of the most common mistakes RV and off-grid beginners make is guessing their battery bank size. Too small and you’re scrambling for hookups after one day. Too large and you’ve spent money on capacity you’ll never use. Getting it right takes about 20 minutes of math — and this guide walks you through every step.

Step 1: List Every Electrical Load You Plan to Run

Start by making a complete inventory of every 12V device (or AC device powered through an inverter) you plan to use in your rig. For each one, note the wattage and how many hours per day you expect to use it.

Here’s a typical RV electrical load example:

Be honest about your usage habits. If you’re not sure, err on the high side — it’s cheaper to size correctly now than to add batteries later.

Step 2: Convert Watt-Hours to Amp-Hours

Battery capacity is rated in amp-hours (Ah), not watt-hours (Wh). The conversion is simple:

Amp-Hours = Watt-Hours ÷ System Voltage

For a 12V system: 2,380 Wh ÷ 12V = 198 Ah per day

That’s your daily amp-hour consumption. This is the foundation of all the remaining calculations.

Step 3: Account for Depth of Discharge

You can’t use 100% of your battery’s rated capacity without damaging it — every battery chemistry has a maximum recommended depth of discharge (DoD).

• LiFePO4 lithium: 80–90% DoD (use 80% to be conservative)
• AGM sealed lead-acid: 50% DoD maximum
• Flooded lead-acid: 50% DoD maximum

To find the minimum battery bank size needed, divide your daily consumption by the allowable DoD:

• LiFePO4: 198 Ah ÷ 0.80 = 248 Ah minimum
• AGM: 198 Ah ÷ 0.50 = 396 Ah minimum

This is why lithium batteries effectively give you twice the usable capacity for the same rated size — and why two 100Ah AGM batteries barely match one 100Ah LiFePO4 in practice.

Step 4: Add a 20–25% Safety Buffer

Real-world efficiency losses, wiring resistance, inverter inefficiency, and colder-than-expected temperatures all reduce your effective capacity. Add a 20–25% buffer to the minimum bank size you calculated above.

• LiFePO4 example: 248 Ah × 1.25 = 310 Ah
• AGM example: 396 Ah × 1.25 = 495 Ah

This buffer also gives you room for higher-than-expected consumption days, unexpected overnight guests, or the fact that you’ll probably use more power than you thought.

Step 5: Factor In Days of Autonomy

How many days do you want to go without charging — either from shore power or solar? If you boondock for 3 days at a stretch between generator runs or sunny days, multiply your daily requirement by your target autonomy.

Example: 2 days of autonomy with LiFePO4

310 Ah (buffered daily need) × 2 days = 620 Ah bank

That might sound like a lot, but for serious boondockers, a 400–600Ah LiFePO4 bank paired with 400–600W of solar is a very common and practical setup.

Real-World Example: Full Build Calculation

Let’s walk through our example setup from start to finish:

• Daily consumption: 2,380 Wh = 198 Ah at 12V
• LiFePO4 DoD (80%): 198 ÷ 0.80 = 248 Ah minimum usable
• 25% safety buffer: 248 × 1.25 = 310 Ah rated capacity
• 2 days autonomy: 310 × 2 = 620 Ah bank

Recommended battery bank: Four 100Ah LiFePO4 batteries wired in parallel (2P2S for 24V, or 4P for 12V) — or two 200Ah batteries. A 400W solar array with an MPPT charge controller would keep this bank topped up during a normal sunny day.

If this household primarily runs at night or in cloudy conditions frequently, three days of autonomy (930 Ah) would be worth considering.

Tips for Getting This Right

Use a Battery Monitor from Day One

Voltage alone is a notoriously unreliable way to gauge battery state of charge, especially with LiFePO4’s flat discharge curve. A shunt-based battery monitor like the Victron SmartShunt measures actual current flow in and out of the battery, tracking state of charge with coulomb counting. It’s one of the best investments in your electrical system.

Measure, Don’t Guess

If you already have an existing setup, put a clamp meter on your main load wire for a day and log actual consumption. Real data beats estimates every time.

Account for Seasonal Differences

Your power consumption in summer (running fans or an AC via inverter) versus winter (heating pads, less solar input) can differ by 30–50%. If you camp year-round, size to your highest-consumption season.

Battery Banks Grow Over Time

Plan your wiring and battery location for the bank you eventually want, even if you start smaller. Running new cables later is expensive and annoying. Put in 4/0 wire and a bus bar now, and adding batteries later becomes a simple parallel connection.

Sizing your battery bank correctly is the single most important step in building an electrical system you’ll actually enjoy using. Take the time to do the math — your future self will thank you every time you wake up to a full battery in the middle of nowhere.