Battery Bank Calculator: Size a 12V House Battery from a Load List

A battery bank is sized from a load list, never from a budget and never from what somebody else built. The arithmetic is four steps: add up the watt-hours your gear consumes in a day, convert that to amp-hours at 12V, multiply by the number of days you want to survive with no charging, then divide by the fraction of the battery you are actually allowed to use. Skip the last step and you will buy a bank that is half the size you think it is.

How do you turn a pile of gear into a number?

Daily energy consumption is the sum of every load's power multiplied by the hours it actually runs. That is the whole calculation, and the only hard part is being honest about the hours.

Two things trip people up here. The first is duty cycle. A compressor fridge is switched on for 24 hours a day but its compressor only runs for a fraction of that, and the fraction depends on the ambient temperature, the insulation quality and how often the lid opens. A 45 to 55 quart fridge drawing 55W runs roughly 35 to 45 percent of the time at a comfortable 75F, which is around 9 hours of compressor time, and closer to 60 percent at 100F. That is why the load list above asks for run hours rather than hours switched on, and why the fridge draw by ambient temperature chart exists as a separate page.

The second is the inverter. Anything plugged into a 120V outlet is paid for twice: once for the energy the appliance uses and again for the conversion loss on the way there. A good inverter is 88 to 92 percent efficient at a decent load and considerably worse at a trivial one, so a 100Wh laptop charge costs about 114Wh at the battery. Run that same laptop from a 12V USB-C PD socket and the round trip disappears. Over a year of full-time travel that is a meaningful fraction of a solar array.

Convert watt-hours to amp-hours by dividing by the nominal pack voltage. This site uses 12.8V for LiFePO4 rather than 12.0V, because that is where a lithium pack actually sits through most of its discharge. A 1,035Wh day is 81 amp-hours, not 86, and the difference compounds across the rest of the sizing.

What is usable capacity, and why is it not the number on the label?

Usable capacity is the fraction of a battery's nameplate rating you can take out repeatedly without shortening its life. It is the single most misunderstood figure in 12V builds, and it is where nameplate comparisons between chemistries fall apart.

Read the last column first. To get 80 usable amp-hours you buy a 95Ah lithium battery or a 160Ah lead acid one, and the lead acid version weighs somewhere north of 100 lb. That weight comes straight out of the number on the door jamb sticker, which is why the payload calculator is not an afterthought on this site. The battery chemistry comparison chart works through the rest of the trade, and lithium versus AGM argues it out properly.

Two notes on the 85 percent figure for LiFePO4. It is a working convention rather than a hard limit: the chemistry tolerates full discharge, but cycle life improves markedly if you leave something in the tank, and the battery management system will disconnect before you get there anyway. And it is a figure you can only act on if you know the state of charge, which on lithium means fitting a shunt. Terminal voltage on a LiFePO4 pack moves about 0.6V across the entire useful range, so a voltmeter cannot tell you anything. A Victron SmartShunt 500A Battery Monitor is the component that turns this calculator from a plan into a gauge you can read, and the battery monitor roundup covers the options.

How many days of autonomy should you plan for?

Autonomy is how long the bank alone can carry the load with nothing coming in. It is the number people reach for first because it feels like the point of a battery, and it is the number that most often gets oversized.

The figures above are nameplate amp-hours, already divided by the usable fraction. Notice how quickly the AGM column becomes absurd. At three days of autonomy an AGM bank is 486Ah, which is five Group 31 batteries and roughly 350 lb of payload before you have added a single panel. That is the practical reason lead acid disappeared from serious vehicle builds even though its purchase price is lower.

The honest counter-argument to buying autonomy is that most builds do not need much of it. If you drive two hours most days, a Renogy Smart 50A DC-DC MPPT Battery Charger 12V (Dual Input) puts roughly 600W into the bank for every one of those hours, which refills a 100Ah pack in about two hours of driving. That changes the question from "how much can I store" to "how fast can I refill", and the second question usually has a cheaper answer. The solar versus alternator charging comparison lays out the arithmetic, and the DC-DC charging guide covers the install.

What does cold weather do to the number?

Two separate things, and they are often confused with each other.

The first is a temporary capacity loss. A LiFePO4 cell delivers roughly 10 percent less usable capacity near freezing and around 20 percent less well below it, because the internal resistance rises and the cell reaches its cutoff voltage sooner under the same load. The energy is not gone: the same pack warmed back up gives its full capacity again. It just is not available on the cold morning when you want it, which is exactly when a diesel heater and a longer night of lighting are pushing the load up.

The second is a hard limit on charging, and it is the one that ruins trips. LiFePO4 must not be charged below freezing. Doing so plates lithium metal onto the anode, which is permanent damage rather than a temporary loss. Every reputable pack has a battery management system that simply refuses charge below about 32F, which means a winter trip with an unheated pack gives you a battery that discharges perfectly and never refills. A self-heating pack such as the Renogy 12V 100Ah Self-Heating LiFePO4 Battery (DuoHeat, Mini) draws heater power from the incoming charge source rather than from the battery, so it costs you charging time in the cold rather than stored capacity. The winter power guide covers this in full.

AGM behaves differently and not necessarily better. It will accept charge below freezing, which is its one genuine advantage here, but its capacity falls off faster with temperature and its charge acceptance rate at low temperature is poor, so a cold AGM bank takes far longer to refill than the specification sheet suggests.

Where does the bank size stop being the right answer?

Three places, and it is worth knowing them before you spend.

When charging cannot keep up. A bank is a buffer, not a source. If your daily load is 81 amp-hours and your array delivers 50 on an average day with no driving, the bank drains at 31 amp-hours a day regardless of its size, and a bigger bank only changes how long the decline takes. Run the solar array calculator against the same load list before you buy capacity, because more panel is usually cheaper per delivered amp-hour than more battery.

When payload runs out. A 400Ah lithium bank is roughly 105 lb, which sounds manageable until you add 30 gallons of water at 250 lb, a rooftop tent at 130 lb, cabinetry, recovery gear and two people. Mid-size trucks and SUVs frequently have under 1,200 lb of payload to start with. The payload and GVWR guide explains what the sticker actually means.

When the current gets silly. Above roughly 400Ah of storage or 2,000W of inverter, 12V starts costing more in copper than the alternative saves. A 3,000W inverter at 12V draws roughly 280A, which needs 4/0 cable and a 350A class T fuse. The same inverter at 24V draws 140A and runs happily on 2/0. The 12V versus 24V comparison covers when the switch pays for itself.

How do you wire and protect a bank once you have sized it?

The bank is the highest energy component in the vehicle, so the protection goes on first and it goes on at the source. A RED WOLF 4 Way ANL Fuse Holder and Distribution Block (12V) or a terminal-mounted fuse holder sits within a few inches of the positive post, sized at or below the ampacity of the cable leaving it, and everything else in the system lives downstream of it. Nothing gets connected directly to the battery post except that fuse and the shunt on the negative side.

Cable is sized by two independent tests and has to pass both: ampacity, which is the safety limit, and voltage drop, which is the performance limit. Run the numbers in the wire gauge calculator for your own run length rather than copying a build video. A pair of tinned 4 AWG Marine Grade Battery Cable, 20 ft Red and 20 ft Black runs covers most main connections in a single battery build, and the wiring and fusing guide is the page to read twice before you cut anything.

Batteries in parallel need matched cable lengths so they share current evenly, and they need a common busbar rather than a daisy chain from one battery to the next, because a daisy chain works the first battery harder than the last. Two in parallel is routine. Four is a sign the build should be looked at as 24V.

Finally, the sizing on this page is arithmetic you can check, drawn from published manufacturer specifications and standards documents. It is researched guidance rather than an electrical certification, it does not replace ABYC E-11 or the installation manual that came with your components, and any lithium installation should be inspected by a qualified installer before it carries load.

Where to go next

• Power consumption calculator, for a single appliance in detail
• Solar array calculator, to see what the roof can put back
• How to size a battery bank, the long form reasoning
• Best lithium battery for a van build
• Three complete builds with every component priced