Key takeaways
- Usable, not nameplate. Half of an AGM bank is off-limits before you damage it. Plan from the smaller number, always.
- The bank is the bottleneck. Solar refills it, but the usable capacity is the thing that carries you through the night.
- Chemistry drives everything. Lithium versus AGM changes your usable depth, your weight, your charge speed, and your lifetime cost.
- Cold is two separate problems. Capacity drops as it gets cold, and lithium can't charge below freezing without a heater.
- Size for the cloudy stretch. One day of usable margin is the floor. Two is comfort.
The four numbers that size a battery bank
Your bank is the tank. Solar, the alternator, and a generator are just hoses that fill it, and the whole game of boondocking power is keeping that tank from hitting empty before the next good charging window. Four numbers decide how big the tank needs to be, and they only work in this order.
Usable capacity is the number that matters
Here's the part the spec sticker skips. A 100Ah battery is not 100 amp-hours of energy you get to use. Two numbers stand between the label and the power you can actually pull: how deep you can safely discharge it, and the losses on the way out.
Depth of discharge is the big one, and chemistry sets it. AGM and flooded lead-acid should only be drawn down about 50% before damage starts, so a 100Ah AGM is really about 50Ah usable, good for roughly 300 to 600 cycles. LiFePO4 gives you 80 to 100% usable and 3,000 to 6,000 cycles, per the chemistry comparisons at MANLY Battery and Battery Tender. That single fact, usable versus nameplate, is behind most "my battery died way too fast" stories.
| Chemistry | Usable depth | Cycle life |
|---|---|---|
| LiFePO4 (lithium) | 80 to 100% | 3,000 to 6,000 |
| AGM | ~50% | 300 to 600 |
| Flooded lead-acid | ~50% | 300 to 500 |
Then the smaller cut: inverters, wiring, and charge-discharge inefficiency skim roughly 10% off the top in a typical 12V system, and cold weather skims more. So when you size, you start from usable capacity and work backward to the nameplate you actually have to buy.
How big does your bank need to be?
Size the bank to carry your daily draw times the number of cloudy days you want to survive without the sun refilling it. One day of margin is the floor. Two is comfort. The math is simple: usable amp-hours needed equals your daily draw times your days of autonomy, then convert to the nameplate you have to buy by dividing by your usable depth and a small efficiency factor.
Run realistic first-timer numbers. Say you pull 100 amp-hours on a normal day and you want a two-day cushion. That's 200 usable amp-hours to keep on hand. In LiFePO4 at 80% usable, that's about a 300Ah nameplate bank. In AGM at 50% usable, the same 200 usable amp-hours needs roughly 450Ah of nameplate, and AGM weighs about twice as much per amp-hour, so you've added a few hundred pounds to carry the same usable power. That comparison is the whole argument for lithium. The calculator below does this against your real rig, loads, and climate, and hands the number straight to the chemistry decision.
Lithium vs. AGM vs. flooded
Chemistry isn't a detail you bolt on at the end. It sets your usable capacity, your weight, your charging hardware, and your cost over the life of the rig. Lithium iron phosphate (LiFePO4) is the default for serious boondocking, and it isn't close: 80% usable depth instead of 50%, roughly a third of the weight per usable amp-hour, a flat voltage curve that keeps your 12V gear happy down to near-empty, and several thousand cycles. It takes a charge fast, too. The catch is upfront cost, and one real limit: most cells can't be charged below freezing without a heater.
AGM is a sealed, maintenance-free lead-acid battery. Cheaper to buy and tolerant of imperfect charging, genuinely fine for light or occasional use, but you live with 50% usable depth, real weight, shorter cycle life, and slow charge acceptance that wastes solar and generator runtime. Flooded lead-acid is cheapest to buy and the most work to own: watering, venting, and real sensitivity to being left part-charged. Lithium looks expensive until you divide by usable cycles. Over the life of the rig it's usually the cheaper battery, because you buy one bank instead of replacing lead-acid two or three times.
Cold weather, charge acceptance, and the tradeoffs nobody mentions
Charge acceptance is the hidden bottleneck. Lead-acid only takes a charge so fast and tapers hard as it fills, so the last 20% can take hours, and a big array or a generator spends much of its time throttled by what the battery will accept. Lithium takes a high, steady charge almost to full, so the same panels and generator put far more back in the same window. Cold is two problems. Capacity drops for every chemistry as it gets cold, and charging LiFePO4 below about 32°F damages the cells, so you want a battery with a built-in heater, a warmed bay, or the discipline to wait until it warms above freezing.
Your charger has to match your chemistry. Owners who've made the jump back this up. In one Airstream Forums upgrade thread, the repeated advice on a lithium swap is to replace the converter for a lithium profile and add a DC-DC charger for the tow vehicle, since a truck's roughly 13-volt charging won't fully fill a lithium bank. The owner who started it had already ruined a set of AGMs through mismatched charging, and the group's math was blunt: a couple hundred dollars for the right charger is cheap insurance on a two-thousand-dollar bank.
One owner with a lithium bank and five solar panels ran the furnace two nights running, sure the capacity made the draw irrelevant. On the third morning the readout was dead, the bank had sagged to 10.47 volts, and there wasn't enough left to spark the stove for coffee. They ended up heating water on the outdoor grill.
The fix they landed on is the one this page is built around: learn what each load actually pulls before you commit to a site, and treat the furnace fan as a real overnight cost. A line that runs through that whole thread is worth keeping: a battery is reserve, not a power source. It stores energy, it doesn't make it.
Run your numbers
Size your bank in about 60 seconds
Tell us your rig, your daily amp-hour target, and how cold the bay gets. We'll pick the chemistry and the nameplate size your real setup needs, break out how much you can actually use, and point you at the generator math.
The mistakes that leave you rationing a dead bank
Sizing by the label instead of the usable number. "I have 200 amp-hours" means 100 usable in AGM, and the bank quits halfway through the night. Buying lead-acid to save money. Lower sticker, higher lifetime cost once you count replacements. Skipping the cold plan. Lithium taken into the mountains in October won't charge in the morning. Ignoring the furnace and inverter draw. The blower and an always-on inverter pull far more overnight than the lights people plan around. Oversizing solar to rescue an undersized bank. Panels can't store energy overnight, only the battery can. If the nights are the problem, more panel isn't the fix.
RV house batteries, frequently asked
How many batteries do I need to boondock?
What's the difference between usable and nameplate capacity?
Lithium or AGM for boondocking?
How long will a 100Ah battery last off-grid?
Can I charge a lithium battery in freezing weather?
How many amp-hours do I use in a day?
Do I really need a battery monitor?
Will adding more solar fix my dead-battery problem?
Next: what size generator do you need?
You'll know your bank size and chemistry. The Generator calculator sizes the generator your rig actually needs, set by the startup surge, usually the AC, and tells you whether you even need one. About 90 seconds.
Start the Generator calculator →Sources: battery chemistry, depth of discharge, and cycle life from MANLY Battery and Battery Tender. Lithium-conversion and charger-matching patterns from the Airstream Forums AGM-to-lithium thread and boondocking-on-lithium thread.