Battery runtime calculator

Battery bank sizing looks complicated until you break it into three questions: how much energy do you use in a day, how much of the battery's capacity you can actually use, and how many days you want to ride out without sun or generator help. The calculator above does the math, but here is the reasoning behind every number it spits out, so you can sanity check it or size a bank by hand if you are away from a screen.

Watt-hours vs amp-hours and system voltage (12/24/48V)

Batteries are usually labeled in amp-hours (Ah), but the number that actually matters for sizing is watt-hours (Wh), because watt-hours account for voltage. A 100Ah battery at 12V holds 1,200Wh. The same 100Ah at 24V holds 2,400Wh, and at 48V it holds 4,800Wh, even though the amp-hour rating on the label is identical. This is why comparing two battery banks by amp-hours alone is meaningless unless you also know the voltage.

The formula is simple: Wh = Ah x V. Flip it around and Ah = Wh / V, which is how you convert a daily energy need back into a battery size once you have picked a voltage.

Voltage choice depends mostly on system size. 12V is fine for small setups, a camper van, a shed, a weekend cabin. 24V starts making sense once you are running a few thousand watts of load, because it cuts amperage in half, which means thinner, cheaper wire and less voltage drop over distance. 48V is the standard for larger off-grid homes and anything with a serious solar array, since it keeps current low enough that wiring and breakers stay a manageable size even at several kilowatts. If you are unsure which voltage fits your load, the sizing tool in /tools will show you the amp-hour equivalent at each voltage so you can compare battery options apples to apples.

Usable capacity and depth of discharge

A battery's total capacity and its usable capacity are two different numbers, and mixing them up is the single most common sizing mistake. Depth of discharge (DoD) is the percentage of a battery you can draw down before you need to stop and recharge it, if you want it to live a normal lifespan.

• LiFePO4 (lithium iron phosphate): typically rated for 80 to 90 percent DoD, sometimes advertised as 100 percent usable. Lithium chemistry tolerates deep discharges far better than lead-acid because it does not suffer the same sulfation damage.
• Lead-acid, AGM, and gel: generally limited to around 50 percent DoD for regular use. Some manufacturers allow occasional discharges to 80 percent, but routinely draining past 50 percent shortens cycle life dramatically.

Practically, this means a 200Ah lithium battery gives you roughly 160 to 180Ah of real, usable capacity, while a 200Ah lead-acid battery only gives you about 100Ah before you should stop pulling from it. This is why lithium banks are often sized smaller in raw Ah terms than the lead-acid systems they replace, and still outperform them: you are comparing usable energy, not nameplate capacity.

How to estimate runtime from capacity and load

Runtime estimation starts with usable watt-hours divided by your load in watts, but two real-world factors shave that number down: inverter losses and battery efficiency losses.

Basic formula: Runtime (hours) = Usable Wh / Load (W).

Example: a 2,400Wh usable lithium bank running a 200W load would, in theory, run for 12 hours. In practice, expect somewhat less once you factor in:

• Inverter losses: converting DC battery power to AC through an inverter typically wastes 5 to 15 percent as heat, depending on the inverter's efficiency and how lightly or heavily it's loaded. Cheap modified-sine inverters and inverters running well below their rated capacity tend to be less efficient.
• Battery round-trip efficiency: lithium batteries are usually 95 percent efficient or better, lead-acid closer to 80 to 85 percent, meaning some energy is lost simply charging and discharging the chemistry itself.
• Wiring and connection losses: usually small if wire gauge is sized correctly, but worth a rough 1 to 3 percent buffer.

A reasonable rule of thumb is to apply a combined 15 to 20 percent derate to your theoretical runtime for an inverter-based AC system, and closer to 5 to 10 percent for DC-only loads that skip the inverter entirely (like 12V lighting or a DC fridge). The calculator bakes in a default derate, but you can adjust it if you know your specific inverter's efficiency curve.

Sizing a bank from daily watt-hours, days of autonomy, and DoD

Once you know your daily energy use, sizing the bank is a matter of deciding how many days you want to cover without any charging input, then correcting for depth of discharge.

The formula: Battery capacity (Wh) = Daily Wh use x Days of autonomy / DoD (as a decimal).

Say your cabin uses 3,000Wh per day and you want 2 days of autonomy (to cover a stretch of cloudy weather) on a lithium bank with 85 percent usable DoD:

• 3,000 x 2 = 6,000Wh needed before the DoD correction
• 6,000 / 0.85 = about 7,060Wh of total battery capacity

Do the same math with lead-acid at 50 percent DoD and you'd need 12,000Wh of total battery capacity, nearly double, to get the same usable energy and days of autonomy. This is the core tradeoff: lithium costs more up front per kWh of nameplate capacity, but you buy less total capacity to get the same usable result, and it typically lasts several times longer in cycle count too.

Days of autonomy is a judgment call, not a fixed number. Two days is common for a solar-charged system in a moderately sunny climate. Homes in cloudier regions, or setups without a backup generator, often size for 3 to 5 days. More autonomy means a bigger, more expensive bank, so it's worth pairing this decision with a realistic look at your generation capacity, covered in more depth over in /blog/category/solar-power.

Temperature effects and battery care

Batteries are chemistry, and chemistry is temperature sensitive. Cold reduces the amount of usable capacity you get out of any battery, and it reduces charge acceptance rates even more sharply. Lead-acid batteries lose meaningful capacity below freezing and can be permanently damaged if charged while frozen. Lithium batteries are worse in one specific way: most LiFePO4 cells should not be charged at all below freezing (around 32°F / 0°C), though many can still discharge in the cold. This is why lithium batteries used in cold climates often need an internal heating pad or a battery box kept above freezing, especially if solar charging is happening on cold mornings.

Heat is a slower killer but a killer nonetheless. High temperatures accelerate the chemical degradation that shortens both lead-acid and lithium battery lifespans, which is why battery banks do best in a stable, moderate-temperature space rather than a hot shed or an uninsulated rooftop box.

General care habits that extend the life of any bank:

• Avoid parking a lead-acid battery at low state of charge for extended periods, sulfation sets in fastest when a battery sits partially discharged.
• Keep terminals clean and connections torqued to spec, loose connections cause resistance, heat, and voltage sag under load.
• Store batteries at a partial state of charge (around 50 percent for lithium) if taking a system offline for the season, rather than fully charged or fully dead.

The role of the BMS and charge sources

Every lithium battery relies on a battery management system (BMS), a small onboard computer that monitors individual cell voltages and temperature, and steps in to prevent overcharge, over-discharge, over-current, and cell imbalance. The BMS is what allows lithium to be pushed to a high DoD safely, it will cut off discharge before the cells are damaged, and cut off charging before they're overfilled. A cheap or poorly designed BMS is one of the most common failure points in budget lithium batteries, so it's worth understanding that not all "LiFePO4" batteries are built to the same internal quality standard even if the outside label looks similar.

Lead-acid batteries don't have a BMS in the same sense, they rely entirely on the charge controller and the user to avoid over-discharging, which is part of why they're less forgiving of mistakes.

On the charging side, off-grid banks often draw from more than one source: solar panels through a charge controller, a generator through a battery charger or hybrid inverter, and sometimes vehicle alternators through a DC-DC charger. Each source needs to be matched to the battery chemistry's charge profile, lithium and lead-acid have different absorption and float voltages, and running the wrong charge profile on the wrong chemistry will shorten battery life or trip the BMS repeatedly.

Common mistakes

• Undersizing the bank: basing capacity on best-case sunny-day usage instead of a realistic daily average, which leaves no buffer for cloudy stretches or unexpected loads.
• Deep-cycling lead-acid batteries: routinely draining past 50 percent DoD because the calculator says there's "more capacity available," which quietly cuts cycle life from thousands of cycles down to a few hundred.
• Mixing old and new batteries: wiring a fresh battery in parallel with older, degraded ones. The weaker battery drags down the whole bank's performance and can even be reverse-charged by the healthier ones, damaging both.

Common questions

Do I need to use the same brand and age of battery in a bank?
It's strongly recommended. Batteries in parallel should be matched in chemistry, capacity, and ideally age and charge history, since mismatched batteries wear unevenly and can shorten the life of the whole bank.

Is a bigger battery bank always better?
Not necessarily. Oversizing costs more upfront and, for lead-acid especially, can lead to batteries sitting under-cycled and undercharged if your solar array can't fully recharge such a large bank on an average day. Size the bank to your actual daily use and realistic autonomy needs, not the biggest number you can afford.

Why does my runtime never match the calculator exactly?
Real-world runtime is affected by temperature, battery age, inverter efficiency at your specific load level, and how accurately you estimated your daily watt-hour use. Treat calculator results as a solid planning estimate, then adjust based on what you observe once the system is running.