Battery Calculator

How to use this calculator

1. Enter watts if the load runs through an inverterMains appliances do. The inverter efficiency is then applied, because converting 12 V DC to 230 V AC loses around 10%.
2. Enter amps for DC loadsLights, fridges and pumps wired directly to the battery draw DC and bypass the inverter entirely, which is why they are so much more efficient.
3. Set the Peukert exponent for lead acidAround 1.2 models the capacity loss at high discharge rates. Leave it at 1.0 for lithium, which is largely immune.
4. Compare usable capacity, not nominalA 100 Ah lithium and a 100 Ah lead acid are not comparable products. The breakdown shows the usable figure for both.

Nominal capacity is not what you get

A battery's amp hour rating is the headline number and it is close to useless on its own, for three separate reasons.

The first is depth of discharge. Taking a lead acid battery below about 50% dramatically shortens its life — a battery routinely drained to 20% might last 200 cycles instead of 700. Lithium iron phosphate tolerates 80 to 90% routinely, and some manufacturers specify 100%.

So a 100 Ah lead acid battery gives you 50 Ah of usable capacity, while a 100 Ah lithium gives 80 to 90. The lithium delivers nearly twice as much energy from the same nominal rating.

That single fact makes comparing amp hours across chemistries meaningless, and it is why lithium batteries that look twice the price are often no more expensive per usable amp hour.

The second reason is Peukert's effect. Lead acid batteries deliver less than their rated capacity when discharged quickly. The rating is typically quoted over 20 hours, and drawing the same battery flat in five hours might yield only 80% of the rated figure.

Lithium is largely immune to this, delivering close to its rated capacity across a wide range of discharge rates. For a high-current application — an inverter running a kettle, a winch, a starter motor — that difference is substantial and often decisive.

The third is the inverter. Any mains appliance runs through one, and converting 12 V DC to 230 V AC costs around 10% of the energy. A 100 W load draws about 111 W from the battery, which is 9.3 A at 12 V rather than 8.3.

That is why DC appliances are so much better in an off-grid or leisure setup. A 12 V compressor fridge draws maybe 45 W directly; the equivalent mains fridge through an inverter draws 50 W from the battery for the same cooling, and the inverter's own standby draw runs continuously on top.

Stack the three effects together and the gap between the headline number and reality is large. A 100 Ah lead acid battery running a 100 W mains load has 50 Ah usable, faces a 9.3 A draw, and loses further capacity to Peukert — perhaps four and a half hours rather than the twelve the nominal rating might suggest.

Worked example: 100 Ah running a 100 W load

Take a 100 Ah 12 V battery and a 100 W mains load through a 90% efficient inverter.

The battery holds 100 × 12 = 1,200 Wh nominally.

The load draws 100 ÷ 0.9 = 111.1 W from the battery, which at 12 V is 9.26 A.

For lithium iron phosphate at 90% depth of discharge, usable capacity is 90 Ah, or 1,080 Wh. Runtime: 90 ÷ 9.26 = 9.7 hours.

For AGM lead acid at 50%, usable capacity is 50 Ah, or 600 Wh. Runtime: 50 ÷ 9.26 = 5.4 hours.

Apply a Peukert exponent of 1.2 to the lead acid and it falls further, to 4 hours 46 minutes, because 9.26 A is a faster discharge than the 5 A the 20-hour rating assumes.

So the same nominal capacity gives 9 hours 43 minutes or 4 hours 46 minutes depending on chemistry — a factor of more than two from a headline figure that is identical.

Now run the load directly as DC. A 100 W 12 V appliance draws 8.33 A rather than 9.26. The lithium runtime rises to 10.8 hours and the lead acid to 6 hours, purely by removing the inverter.

Now size for a requirement instead. To run 100 W for 8 hours through an inverter, you need 9.26 × 8 = 74.1 Ah usable. In lithium at 90% that is an 82 Ah battery; in lead acid at 50% it is 148 Ah.

The lead acid battery is also roughly three times the weight for that capacity — around 45 kg against 15 — which matters enormously in a campervan, a boat or anything portable.

On charging: putting the lithium's 90 Ah back at 20 A is nominally 4.5 hours, and about 4 hours 57 minutes once the absorption stage is allowed for. The lead acid needs only 50 Ah replaced, so it finishes in about 3 hours 30 minutes — but it is storing barely half the energy, and the last 20% is disproportionately slow in lead chemistries.

That charging difference matters for solar. A lead acid bank that only sees five hours of useful sun may never reach full charge, and chronic undercharging sulphates the plates and destroys capacity — a failure mode lithium does not share.

The same 100 Ah battery by chemistry

The last two rows show the trade-off within lead acid: you can have the capacity or the cycle life, but not both. Discharging to 80% gives lithium-like runtime and destroys the battery in a couple of years of daily use. That trade-off does not exist in the same way for lithium, which is the real reason it dominates leisure and off-grid applications despite the purchase price.

Temperature, ageing and sizing a real system

Cold reduces capacity in every chemistry, typically 20% or more near freezing and considerably worse below. A battery bank sized on a mild-weather calculation will disappoint in January, which is exactly when a heating load is highest.

Lithium has an additional constraint: it must not be charged below 0°C without a built-in heater, though it can be discharged. Charging a cold lithium cell causes lithium plating, which is permanent damage. Most quality LiFePO4 batteries include low-temperature charge protection, and it is worth confirming rather than assuming.

Ageing reduces capacity gradually in both chemistries. A battery at the end of its stated life typically retains 70 to 80% of its original capacity, so sizing with no margin means the system stops meeting the requirement well before the battery is dead.

On cycle life and cost, the comparison that matters is cost per usable kilowatt-hour over the life of the battery, not the purchase price. A lead acid battery at half the price delivering a fifth of the cycles at half the usable depth is considerably more expensive over ten years, and that calculation is what has shifted the leisure and off-grid market.

For sizing a real system, work from an energy audit rather than a single load. List every appliance, its power draw and its hours of use per day, and total the watt hours. Add the inverter's standby consumption, which runs continuously and is easy to forget — 10 W of standby is 240 Wh a day, which can exceed the fridge.

Then add margin. Two or three days of autonomy is normal for an off-grid system with solar, because consecutive dull days happen and running a bank flat is what kills it.

On charging sources: alternator charging in a vehicle needs a DC-to-DC charger for lithium, because a standard alternator's voltage profile will not fully charge a lithium bank and may overheat if the battery accepts current indefinitely. Solar needs an MPPT controller sized for the array and configured for the chemistry.

A battery management system is essential for lithium and does not exist for lead acid. It protects against over-charge, over-discharge, cell imbalance and temperature extremes, and a lithium battery without one is a fire risk. Quality LiFePO4 batteries have it built in.

Finally, wiring. High currents need appropriately sized cable and correctly rated fusing close to the battery terminal. A 100 A draw through undersized cable is a fire, and the battery will supply hundreds of amps into a short circuit without complaint.

Assumptions and limitations

• Depth of discharge figures are typical — manufacturers specify their own
• Assumes a new battery at moderate temperature
• Peukert modelling is a simplification and applies only to lead acid
• Does not model voltage sag under load or the cut-off voltage of the inverter
• Does not account for inverter standby consumption, which runs continuously
• Charge time is an estimate — the absorption stage varies by charger and chemistry
• Does not size cable, fusing or charge controllers, all of which need separate design

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