Free battery runtime & sizing calculator · No sign-up · Runs entirely in your browser
Work out how long a battery will actually run your load, or what capacity you need for a given runtime — with usable capacity, depth of discharge and inverter losses accounted for.
Your load
Sizing — how much do I need?
| Measure | Value |
|---|
| Chemistry | Usable DoD | Cycles |
|---|---|---|
| LiFePO4 (lithium) | 80–90% | 3,000–6,000 |
| AGM | 50% | 500–800 |
| Gel | 50% | 500–1,000 |
| Flooded lead acid | 50% | 300–700 |
Indicative. Cycle life depends heavily on how deeply and how often you discharge.
This is the single most common and most expensive misunderstanding in battery sizing. A battery's rated capacity is the total energy it physically contains. The amount you can actually take out without wrecking it is considerably less.
| Chemistry | Usable depth of discharge | From a 100 Ah battery at 12 V |
|---|---|---|
| LiFePO4 (lithium) | 80–90% | 80 Ah = 960 Wh |
| AGM | ~50% | 50 Ah = 600 Wh |
| Gel | ~50% | 50 Ah = 600 Wh |
| Flooded lead acid | ~50% | 50 Ah = 600 Wh |
Amp-hours are how batteries are sold. Watt-hours are how you should think.
The same "100 Ah" is four times the energy at 48 V as at 12 V. This is why comparing battery banks by amp-hours across different system voltages is meaningless, and why larger off-grid systems run at 24 V or 48 V — the same energy at a quarter of the current, which means thinner cable and far less loss. Our voltage drop calculator shows exactly what that current costs you over a cable run.
| Term | What it means |
|---|---|
| Capacity (Ah) | The rating on the battery, multiplied by how many you have in parallel |
| Voltage | System voltage — 12, 24 or 48 V |
| DoD | Depth of discharge you are willing to use, as a percentage |
| Inverter efficiency | Typically 85–95%. Only applies to AC loads |
Sizing a battery starts with knowing your daily load in amp-hours. These are typical figures for a 12 V system — adjust the hours to match how you actually live.
| Load | Power | Daily use | Energy | Per day at 12 V |
|---|---|---|---|---|
| 12 V compressor fridge | 55 W | 24 h @ 40% | 528 Wh | 44.0 Ah |
| LED lighting (6 lights) | 36 W | 5 h | 180 Wh | 15.0 Ah |
| Water pump | 60 W | 0.5 h | 30 Wh | 2.5 Ah |
| CPAP machine | 40 W | 8 h | 320 Wh | 26.7 Ah |
| Diesel heater | 30 W | 6 h | 180 Wh | 15.0 Ah |
| Phone + tablet charging | 20 W | 3 h | 60 Wh | 5.0 Ah |
| Laptop (via inverter) | 60 W | 4 h | 273 Wh | 22.7 Ah |
| TV, 24-inch 12 V | 45 W | 3 h | 135 Wh | 11.2 Ah |
| Starlink (continuous) | 60 W | 24 h | 1440 Wh | 120.0 Ah |
| Microwave (via inverter) | 1200 W | 0.15 h | 205 Wh | 17.0 Ah |
Running everything in that table comes to roughly 279 Ah a day. On LiFePO4 at 80% usable that needs about 349 Ah of rated capacity; on AGM at 50% it needs about 558 Ah — and considerably more weight. Most people run a subset, which is why measuring your own consumption over a few days beats any table.
A 12 V compressor fridge drawing 55 W, running off a single 200 Ah LiFePO4 battery at 80% DoD.
| Step | Working | Result |
|---|---|---|
| Usable capacity | 200 × 0.80 | 160 Ah |
| Usable energy | 160 × 12 | 1,920 Wh |
| Current draw | 55 ÷ 12 | 4.6 A |
| Runtime | 1,920 ÷ 55 | 34 h 55 min |
Nearly a day and a half — but a fridge compressor cycles rather than running continuously, so real-world runtime is usually longer. The honest way to plan is to measure actual daily consumption in amp-hours over a few days rather than assuming the rating runs constantly.
Identical setup, but a 200 Ah AGM battery at 50% DoD.
| Measure | LiFePO4 | AGM |
|---|---|---|
| Rated capacity | 200 Ah | 200 Ah |
| Usable capacity | 160 Ah | 100 Ah |
| Usable energy | 1,920 Wh | 1,200 Wh |
| Runtime on 55 W | 34 h 55 min | 21 h 49 min |
| Weight | ~20 kg | ~55 kg |
A 100 W AC appliance — a laptop and a few lights — from a 100 Ah LiFePO4 battery at 12 V through an 88% efficient inverter.
| Step | Working | Result |
|---|---|---|
| Usable energy | 100 × 0.80 × 12 | 960 Wh |
| Draw from battery | 100 ÷ 0.88 | 113.6 W |
| Runtime with inverter | 960 ÷ 113.6 | 8 h 27 min |
| Runtime if the load were DC | 960 ÷ 100 | 9 h 36 min |
The inverter costs you over an hour of runtime on this load alone. Where a 12 V version of an appliance exists — fridge, lights, fans, water pump — running it directly off the battery avoids that loss entirely. It is the main reason caravan systems are built around 12 V appliances rather than an inverter and household ones.
Working the other way — you know the load and the runtime, and need the capacity. A 500 W average household load on a 48 V LiFePO4 system, through a 92% inverter, wanting 24 hours of autonomy.
| Step | Working | Result |
|---|---|---|
| Draw from battery | 500 ÷ 0.92 | 543.5 W |
| Energy for 24 h | 543.5 × 24 | 13,043 Wh |
| Allowing for 80% DoD | 13,043 ÷ 0.80 | 16,304 Wh rated |
| At 48 V | 16,304 ÷ 48 | 340 Ah |
Rough capacities for the batteries most people are actually working with, and what each is built to do.
| Application | Typical capacity | Type |
|---|---|---|
| Motorcycle | 7–20 Ah | Starting — short high-current bursts |
| Car / light vehicle | 45–100 Ah | Starting |
| 4WD auxiliary (dual battery) | 75–120 Ah | Deep cycle |
| Caravan — AGM | 100–200 Ah | Deep cycle |
| Caravan — LiFePO4 | 100–300 Ah | Deep cycle |
| Marine house bank | 200–600 Ah | Deep cycle |
| Off-grid solar (12/24 V) | 200–1,000+ Ah | Deep cycle |
| Home storage (48 V) | Usually quoted in kWh — 5–20 kWh | Deep cycle lithium |
| Portable power station | Quoted in Wh — 250–3,000 Wh | Deep cycle lithium |
| Electric bike | Quoted in Wh — 300–750 Wh | Lithium-ion |
| LiFePO4 | AGM | Gel | Flooded | |
|---|---|---|---|---|
| Usable DoD | 80–90% | 50% | 50% | 50% |
| Typical cycles | 3,000–6,000 | 500–800 | 500–1,000 | 300–700 |
| Weight per usable kWh | Lowest | High | High | Highest |
| Purchase cost | Highest | Moderate | Moderate | Lowest |
| Cost per usable cycle | Lowest | Moderate | Moderate | Highest |
| Charge speed | Fast | Moderate | Slow | Moderate |
| Mounting | Any orientation | Any orientation | Any orientation | Upright, ventilated |
| Maintenance | None | None | None | Top up electrolyte |
| Cold weather | Cannot charge below 0 °C without a heater | Tolerates cold | Tolerates cold | Tolerates cold |
The row that decides most purchases is cost per usable cycle. A lithium battery costing three times as much but delivering six times the cycles at nearly twice the usable depth works out cheaper over its life — provided the installation lasts long enough to realise it.
Not every battery is a bank of 12 V blocks. These are sold differently and need converting before you can compare them with anything on this page.
| Type | How it is rated | Rough 12 V equivalent | What to watch |
|---|---|---|---|
| Portable power station | Watt-hours (250–3,000 Wh) | 1,000 Wh ≈ 83 Ah at 12 V | The inverter is built in, so the Wh figure is usually already usable energy |
| Electric bike | Watt-hours (300–750 Wh) | 500 Wh ≈ 42 Ah at 12 V | Range depends far more on terrain, rider weight and assist level than on capacity |
| Electric scooter | Watt-hours (250–1,000 Wh) | — | Same — quoted range assumes flat ground and a light rider |
| Camping / slimline battery | Amp-hours at 12 V | Direct | Check whether the quoted capacity is rated or usable |
| Home storage | Kilowatt-hours (5–20 kWh) | 13.5 kWh ≈ 280 Ah at 48 V | Usable versus total capacity is often quoted separately — read both |
For home storage, the number that matters is usable kilowatt-hours rather than total, and how much of it you can realistically recharge each day. A battery larger than your solar array can refill in winter is capacity you paid for and cannot use.
Battery capacity ratings assume a slow discharge — typically the C/20 rate, meaning the battery is drained evenly over twenty hours. Draw harder than that and a lead-acid battery delivers noticeably less than its rating. The effect is described by Peukert's law.
| Discharge rate | Lead acid delivers roughly | LiFePO4 delivers roughly |
|---|---|---|
| C/20 (slow — the rating condition) | 100% of rating | 100% |
| C/10 | ~90% | ~100% |
| C/5 (heavy) | ~75–80% | ~98% |
| C/1 (very heavy, e.g. an inverter surge) | ~50–60% | ~95% |
Most battery failures are avoidable, and most come down to the same handful of things.
| Check | Why it matters | How often |
|---|---|---|
| Avoid deep discharge on lead acid | Below about 50%, cycle life falls away sharply. Below 20% you may cause permanent damage in a single event | Every cycle |
| Recharge lead acid promptly | Left partially discharged, sulfate crystals harden on the plates and permanently reduce capacity | After every discharge |
| Keep terminals clean and tight | Corrosion adds resistance, causing voltage drop and heat at the connection. A loose terminal can arc | Every few months |
| Use a charger matched to the chemistry | Lithium and lead acid need different voltage profiles. The wrong one under- or overcharges | Once, at setup |
| Watch temperature | Heat accelerates ageing in all chemistries. LiFePO4 must not be charged below 0 °C without a heater | Seasonally |
| Monitor state of charge | Voltage alone is a poor indicator under load. A shunt-based monitor counting amp-hours is far more accurate | Ongoing |
| Check electrolyte (flooded only) | Plates exposed to air are permanently damaged. Top up with distilled water only | Every 1–3 months |
| Store at partial charge | Lead acid should be stored fully charged and topped up periodically; LiFePO4 stores best around 50–60% | Before any long layup |
How do I calculate battery runtime?
Multiply capacity in amp-hours by system voltage to get watt-hours, multiply by your usable depth of discharge, then divide by the load in watts. A 100 Ah LiFePO4 at 12 V with 80% usable gives 960 Wh, which runs a 100 W load for about 9 hours 36 minutes. For AC loads, divide the load by inverter efficiency first.
How long will a 100 Ah battery last?
It depends on chemistry and load. At 12 V running a 100 W load: a 100 Ah LiFePO4 at 80% usable lasts about 9.5 hours; a 100 Ah AGM at 50% usable lasts about 6 hours. Through an inverter, subtract roughly another 12%. The same label produces very different results.
What is depth of discharge?
Depth of discharge is how much of a battery's capacity you actually use before recharging. Lead-acid chemistries are generally limited to about 50% to preserve cycle life; LiFePO4 tolerates 80–90%. Discharging lead acid deeper than recommended shortens its life dramatically — a battery rated for 700 cycles at 50% may manage only a couple of hundred at 80%.
What is an amp-hour?
An amp-hour is one amp of current flowing for one hour. A 100 Ah battery can theoretically supply 5 A for 20 hours. Amp-hours only describe energy when combined with voltage — 100 Ah at 24 V holds twice the energy of 100 Ah at 12 V, which is why watt-hours are the better comparison.
How many batteries do I need?
Work out your daily consumption in watt-hours, decide how many days of autonomy you want, divide by your usable depth of discharge, then divide by the watt-hours each battery provides. For off-grid use, two to three days of autonomy is a common target because it covers consecutive overcast days.
Should I choose lithium or AGM?
Lithium costs more upfront but delivers roughly 60% more usable capacity from the same rating, several times the cycle life, and about a third of the weight. AGM is cheaper to buy and tolerates cold charging. For caravans, boats and off-grid systems that cycle daily, lithium usually wins on cost per usable cycle. For occasional standby use, AGM can be the sensible choice.
Does an inverter reduce battery runtime?
Yes. A typical inverter is 85–95% efficient, so a 100 W AC appliance draws around 105–118 W from the battery. On a 100 Ah LiFePO4 at 12 V that is the difference between about 9 hours 36 minutes and 8 hours 27 minutes. Running 12 V appliances directly avoids the loss entirely.
Why does my battery deliver less than its rating under heavy load?
Capacity ratings assume a slow discharge, usually over twenty hours. Lead-acid batteries deliver progressively less as the discharge rate rises — an effect described by Peukert's law — so a 100 Ah AGM under a heavy inverter load may yield little more than half its rating. LiFePO4 is far less affected, which is why it suits high-draw applications.
Should I wire batteries in series or parallel?
Parallel keeps the voltage the same and adds amp-hours; series raises the voltage and keeps amp-hours the same. Total energy is identical either way, but a higher system voltage means lower current for the same power — thinner cable and much smaller losses. That is why larger off-grid systems run at 24 V or 48 V rather than 12 V.
What size battery do I need for a caravan?
Add up your daily consumption in watt-hours: a 55 W fridge running roughly half the time is about 660 Wh, plus lights, water pump, phone charging and any inverter loads. Divide by your usable depth of discharge to get rated capacity. Many Australian caravan setups land between 100 and 300 Ah at 12 V, with lithium at the lower end because more of it is usable.
Can I charge a lithium battery in cold weather?
Not below 0 °C without an integrated heater — charging LiFePO4 below freezing causes permanent damage. Discharging in the cold is fine. Many quality lithium batteries include low-temperature charge protection or a built-in heater, which matters for alpine and inland-winter use in Australia.
How much solar do I need to recharge my battery?
Enough to replace a full day's consumption during the shortest winter days, not the longest summer ones. A panel's rated wattage is achieved only in ideal conditions, so real daily output is a fraction of rating multiplied by usable sun hours. Sizing solar to average conditions is the most common reason off-grid systems end up running a generator in winter.
Almost every battery sizing mistake comes back to one thing: treating the rated capacity as though it were available. Convert to usable watt-hours, allow for the inverter if you have one, and design for the worst week rather than the average day. Do that and the arithmetic on this page will hold up in the field.
From here, check cable losses with the voltage drop calculator — critical on 12 V systems — convert between volts, amps and watts with the Ohm's Law calculator, and price mains consumption with the electricity cost calculator.