I get some version of this question almost every week: “I’m putting in a 300Ah lithium bank because that’s what everyone recommends, does that sound right?” The honest answer is usually no, not because 300Ah is wrong, but because nobody asked what they’re actually running. Amp hour sizing isn’t a guess or a popular number you copy from a forum thread. It’s arithmetic, and it takes about fifteen minutes if you do it in order.
The quick version
- List every device you’ll run, its watts or amps, and how many hours a day you’ll use it.
- Convert everything to amp hours at your system voltage (usually 12V) and add them up for a daily total.
- Add 10 to 15 percent for inverter and wiring losses, then divide by how many days of autonomy you want without recharging.
- Because LiFePO4 gives you close to 100 percent usable capacity, you rarely need to double your number the way you did with lead acid.
- Match your charging sources (solar, DC-DC, shore power) to actually refill that bank, or the size of the battery barely matters.
Start With a Real Load List, Not a Guess
Grab a notebook or a spreadsheet and write down every 12V and 120V device you’ll actually use on a typical day. Not the day you’re camped at a full hookup site with unlimited power. The day you’re boondocking in the desert for four nights.
For each item, note its wattage (check the label or spec sheet) and roughly how many hours it runs per day. A 12V compressor fridge like a Dometic CFX pulls around 40 to 50 watts but only runs maybe 8 to 10 hours a day in cycles, not continuously. A laptop charger draws 60 watts for 3 hours.
This is the step people skip, and it’s the one mistake I see constantly. Someone buys a 400Ah bank because their buddy has one, then finds out they’re only using 60Ah a day. Or the opposite happens, and they underestimate their induction cooktop’s draw and run out of power by day two.
Convert Watts to Amp Hours
Here’s the formula you’ll use over and over: amp hours = (watts x hours) / voltage. If a device runs on 120V AC through an inverter, use 12V as your battery-side voltage since that’s what’s actually being drawn from the bank.
Example: a 700-watt microwave used for 10 minutes (0.167 hours) a day is 700 x 0.167 = 117 watt hours, divided by 12V, which is about 9.7Ah. Do this for every item on your list and you’ll build a real daily total instead of a guess.
Tip: Keep a running spreadsheet with columns for device, watts, hours per day, and resulting amp hours. It takes ten minutes to build and you’ll reuse it every time you add or swap a device down the road.
A Realistic Worked Example
Let’s size a bank for a couple living in a van part-time, boondocking most nights. Here’s what a typical day looks like on their list.
| Device | Watts | Hours/day | Amp hours (at 12V) |
|---|---|---|---|
| Compressor fridge | 45 | 8 (cycling) | 30 |
| LED lighting | 20 | 4 | 6.7 |
| Laptop x2 | 120 | 4 | 40 |
| Phone charging | 15 | 3 | 3.75 |
| Water pump | 60 | 0.5 | 2.5 |
| Induction cooktop | 1200 | 0.5 | 50 |
| Fan / vent | 15 | 6 | 7.5 |
That totals roughly 140Ah of raw DC-equivalent draw per day. Add 12 percent for inverter and wire losses and you’re at about 157Ah a day. If they want three days of autonomy without any recharging at all, that’s 470Ah, which is unrealistic for most vans on cost and space. In practice, most people don’t need full autonomy because solar or driving refills the bank daily, so this couple would likely be well served by a 200 to 280Ah bank paired with decent solar input.
Why Lithium Changes the Sizing Math
With lead acid or AGM, you were taught to size the bank at double your daily need because you could only safely use about 50 percent of rated capacity before damaging the battery. LiFePO4 flips that. You can comfortably use 80 to 100 percent of rated capacity without shortening its life in any meaningful way, according to Battle Born’s own FAQ documentation and most other lithium manufacturers.
That means if your daily load calculation says you need 150Ah of usable power, a 150 to 170Ah lithium bank actually covers you. A 300Ah lead acid bank would have been needed to deliver that same usable 150Ah. This is the real reason lithium banks look smaller on paper but perform bigger in the field, and it’s worth reading more on why lithium gives you nearly double the usable power of an equivalent lead acid setup.
Autonomy: How Many Days Without Sun or Driving?
Autonomy is the number of days you want to run entirely off the battery bank with zero recharging. This is a personal choice, not a fixed rule, and it depends heavily on your travel style.
- Weekend camper with occasional shore power: 1 to 1.5 days of autonomy is usually plenty.
- Full-time boondocker with solar: 2 to 3 days covers most cloudy stretches.
- Overlander or sailor crossing remote stretches: 3 to 5 days if recharging sources are unreliable.
- Anyone relying mainly on driving to recharge via DC-DC charger: size closer to your longest expected stationary stretch.
A mistake worth flagging here: autonomy and charging capacity are two separate problems. A bigger battery doesn’t help if your solar array or DC-DC charger can’t refill it in a reasonable time. Match your charge rate to your bank size, generally aiming to replace a full day’s usage within 4 to 6 hours of good sun or driving.
Common Bank Sizes and Who They Fit
| Bank size | Typical fit | Notes |
|---|---|---|
| 100Ah | Weekend trips, minimal loads | Fine without an inverter or with light inverter use only |
| 200Ah | Couples, moderate boondocking | Sweet spot for most van conversions |
| 300 to 400Ah | Full-timers, families, induction cooking | Needs matching solar or alternator charging capacity |
| 500Ah+ | Heavy power users, workshops on wheels | Often built from multiple 100Ah or 200Ah units in parallel |
Brands like Battle Born, Li Time and Ampere Time all sell in these common increments (100Ah, 200Ah, 230Ah, 300Ah), which makes it easy to combine units in parallel to hit an odd target number like 350 or 460Ah if your worksheet calls for it.
Don’t Forget the Inverter’s Peak Draw
Your amp hour total tells you how big the battery needs to be for energy storage. It does not tell you whether your battery, cables and fuse can handle the peak current an inverter demands the instant you switch on a microwave or hair dryer.
Warning: A 2000-watt inverter running at full load pulls around 165 amps continuously from a 12V bank, and surge loads on motor-driven appliances can spike well above that for a second or two. Check your battery’s continuous and peak discharge rating against your inverter size before wiring anything up.
Most 100Ah drop-in lithium batteries are rated for 100A continuous discharge, which caps you around a 1000 to 1200 watt inverter per battery unless you’re paralleling units.
Building In a Buffer
Once you’ve got your calculated number, add a 20 to 30 percent buffer before finalizing your order. Real life adds loads you didn’t plan for (a visiting friend’s CPAP, a heat wave that runs the fan longer, a string of cloudy days), and lithium is forgiving enough that oversizing slightly doesn’t hurt cycle life the way it once mattered with lead acid.
The first van I wired for a client, we calculated 165Ah of daily need and installed a single 200Ah battery. It felt tight on paper but has worked fine for three seasons, mostly because their solar keeps up and they rarely hit true zero-autonomy days.
Putting It All Together
Sizing a lithium bank isn’t about matching a number you saw online. It’s your load list, converted to amp hours, adjusted for losses, multiplied by your desired autonomy, and padded with a sensible buffer. If you’re still deciding whether lithium is the right move before you get this deep into sizing, it’s worth reading our full comparison of LiFePO4 and lead acid batteries first, and if you’re unsure whether to buy a finished unit or build your own bank from cells, our piece on drop-in lithium versus DIY builds covers the tradeoffs. Once you know roughly what a lithium battery is and how it behaves, check our plain-language guide to LiFePO4 batteries for the fundamentals first.
Do the worksheet before you shop. It takes less time than reading five conflicting forum threads, and it’s the difference between a battery bank that quietly does its job for a decade and one you’re already regretting by the second trip.
