SizingKitPower & current
Amp hour calculator
Drawn at 25 A, a 100 Ah flooded lead-acid block delivers 66.9 Ah, and stopping at the usual 50% floor leaves 33.4 Ah you may actually take out — a third of the figure on the case. This page applies the three reductions that produce that gap: Peukert's rate law at the current you are really pulling, the discharge floor your cycle-life target implies, and a temperature correction from your own datasheet. Five chemistry presets fill the fields in, and all four numbers behind them stay editable, because the exponent for one AGM is not the exponent for the next.
- 100% free
- No signup
- 5 chemistry presets
- Peukert exponent editable
- 6 rates, printable
The battery, and how hard you are pulling on it
Chemistry sets the four figures below it. Every one of them is a starting point from a datasheet, and every one is meant to be overwritten by yours.
North American deep-cycle capacity is stated at the 20-hour rate; European traction cells are usually stated at the 5-hour rate instead, and the two figures for one battery differ by around 15%.
The number on the case, before any of this.
Default 20 h for flooded lead-acid, deep cycle. Source: cell and battery manufacturers' published datasheets and cycle-life curves — the figures move between products of the same chemistry, so the datasheet for your own battery wins.
Steady average, not the inrush. Milliamps are accepted written out.
Default 50%. This is an economic choice about cycle life, not a boundary the battery enforces.
Default 1.25. The exponent is fitted per product and drifts upward as plates sulphate, so a five-year-old battery behaves worse than its datasheet.
A multiplier from your own datasheet. 1.00 applies no correction.
77.0 °F / 25.0 °C — recorded with the answer, not used to compute it.
Only used to restate the answer as energy.
Usable capacity
USABLE, AMP-HOURS
33.4 Ah
THE SAME FIGURE AS ENERGY
401 Wh
- Delivered at this current
- 66.9 Ah
- Share of the rating
- 66.9%
- Rate, in C notation
- 0.25C (C/4)
What decided this number: the discharge floor. Stopping at 50% depth of discharge costs more than the rate does here, and raising it trades cycle life for capacity. The other two reductions are still applied — they are simply smaller.
On temperature. IEEE 485 requires a temperature correction when lead-acid cells are sized, and takes the factor from the cell manufacturer's own table. This site does not reproduce that table — it differs per product and per cell design — so the field is yours to fill from the datasheet, and 1.00 leaves the correction out.
The same battery at six discharge rates
One nameplate, six answers. Rated 100 Ah over 20 hours, exponent 1.25, then 50% depth of discharge and a 1 temperature correction on top.
| Rate | Current | Delivered | Of rating | Usable |
|---|---|---|---|---|
| 0.05C (C/20) | 5 A | 100 Ah | 100% | 50 Ah |
| 0.1C (C/10) | 10 A | 84.1 Ah | 84.1% | 42 Ah |
| 0.2C (C/5) | 20 A | 70.7 Ah | 70.7% | 35.4 Ah |
| 0.5C (C/2) | 50 A | 56.2 Ah | 56.2% | 28.1 Ah |
| 1C | 100 A | 47.3 Ah | 47.3% | 23.6 Ah |
| 2C | 200 A | 39.8 Ah | 39.8% | 19.9 Ah |
Rate column generated from the rating, not tabulated. Peukert's relation, published by Wilhelm Peukert in Elektrotechnische Zeitschrift in 1897 and still the way lead-acid rate dependence is expressed. Chemistry defaults come from cell and battery manufacturers' published datasheets and cycle-life curves — the figures move between products of the same chemistry, so the datasheet for your own battery wins.
How to find the amp-hours a battery will really give up
Three of the four fields under the chemistry selector are assumptions. Replacing them with your battery's own figures is the whole exercise.
Enter the rating and the rate it was measured at
Both halves matter equally. A 225 Ah golf-cart battery quoted at the 20-hour rate and the same cell quoted at the 5-hour rate are around 15% apart, and copying the capacity across without the hours turns that difference into a silent error you will never see again.
Put in the current you actually draw
Not the inverter's maximum and not the surge — the steady average, which for most installations sits far above the gentle current the rating was taken at. That distance is what Peukert's exponent converts into lost capacity, and it is why a battery bank feels smaller in service than on paper.
Set the floor and the correction, then read the binding line
Choose the depth of discharge your cycle-life target implies and enter a temperature factor from the manufacturer's table, leaving it at 1.00 if you have none. The note under the answer names which of the three reductions took the largest bite, so you know whether to change the wiring, the discharge setting or the location of the box.
Technical specifications
| Rate law | Peukert's relation, anchored on the rating rather than on an absolute constant, so the result equals the nameplate exactly when the draw equals the rating current. Published by Wilhelm Peukert in Elektrotechnische Zeitschrift in 1897. |
|---|---|
| Chemistry presets | 5 — flooded deep-cycle, AGM, gel, LiFePO4 and lithium-ion NMC. Each fills in a rating basis, a discharge floor and an exponent; none of the three is locked. |
| Exponent range accepted | 1.00 to 1.60. At 1.00 capacity is indifferent to current, which is roughly true of lithium iron phosphate at 1.02; a well-used flooded cell behaves nearer 1.30. |
| Rating basis defaults | 20 hours for the lead-acid presets and 5 hours for the lithium ones, matching how each is normally published. Any value from 0.1 to 100 hours is accepted. |
| Discharge floor defaults | 50% for lead-acid, 80% for lithium. Both are cycle-life economics rather than limits, and the field goes to 100% for the emergency case. |
| Temperature handling | A correction multiplier between 0.30 and 1.20 that you supply, shown beside the reading it applies at in °F and °C. No correction table is reproduced on this site — IEEE 485 takes that figure from the cell manufacturer. |
| Printed chart | 6 rows at C/20, C/10, C/5, C/2, 1C and 2C, each generated from the rating you entered rather than looked up, with delivered capacity, its share of the rating and the usable figure. |
| Where the figures stay | In the page. Nothing you copy off a datasheet or a nameplate is transmitted, so a site survey stays on the device that made it. |
Frequently asked questions
Why does a 100 Ah battery not give 100 Ah?
Because the rating is a measurement taken under conditions your installation does not reproduce. It is the capacity delivered at one specific gentle current, at 25 °C, down to a specific end voltage, on a new battery; draw harder, run colder, stop higher, or use it for three years, and each of those departures takes a slice. On this page the three you can quantify are separated so you can see which one is costing you the most.
What does C/20 mean, and where do I find the rate my battery was rated at?
C/20 means the current that would empty the rated capacity in twenty hours — 5 A for a 100 Ah battery — and it is the basis most North American deep-cycle capacities are published at. It is printed on the datasheet next to the capacity, usually as a column header of a table with 5, 10, 20 and 100-hour columns in it; if the sheet gives several, the largest number of hours is the largest capacity, which is why marketing quotes it.
Is 50% depth of discharge a rule or a habit?
It is an economic choice, not a limit the battery enforces. Lead-acid cycle life falls steeply with depth — the same cell that survives a few hundred cycles at 80% will manage several times that at 50% — so 50% is roughly where the curve stops punishing you per kilowatt-hour delivered. In a genuine outage you can take a lead-acid bank well below it once; you will simply have spent some of its remaining life doing so.
Does the Peukert effect apply to lithium batteries?
Barely, and that is the single biggest practical difference between the chemistries. A lithium iron phosphate cell sits near a Peukert exponent of 1.02 against 1.25 for a flooded lead-acid, so at a 1C draw the lithium pack returns almost its full rating while the lead-acid pack returns roughly half of its. Compare the two chemistries at the rating current and they look similar; compare them at the current an inverter actually pulls and they are not in the same class.
My bank lives in an unheated garage — how much capacity does the cold cost?
Enough to matter, and this page will not guess the number for you. Capacity falls with temperature for every chemistry and the shape of the fall is specific to the cell design, which is why IEEE 485 requires a temperature correction when lead-acid cells are sized and then sends you to the manufacturer's table for the value. Find that table, enter the factor, and the answer carries the temperature it was taken at so the assumption travels with the result.
Can I add capacity by paralleling a new battery with an old one?
Not reliably, and the arithmetic on this page will overstate what you get. Batteries in parallel share current in inverse proportion to their internal resistance, so the newer, lower-resistance unit takes a disproportionate share of every discharge and ages faster while the tired one contributes less than its rating. Paralleling is for matched units of the same age, chemistry and rating, and a bank that has drifted apart is replaced together rather than topped up.
How many hours will this usable capacity actually run for?
This page deliberately stops at amp-hours and does not convert them into time. The hours depend on the load, on the inverter between the battery and the load, and on where the low-voltage cutoff is set, and those belong to the battery runtime calculator; keeping the derating and the runtime apart is what stops one of them from quietly assuming the other's inputs.
About the rating conditions nobody reads
An amp-hour rating is a measurement, not a property, and like every measurement it comes with the conditions it was taken under. The current is the one that matters most: lead plates deliver charge as fast as the electrolyte can diffuse into them, so pull harder than the diffusion can keep up with and the cell reaches its end voltage with usable material still unreacted. Wilhelm Peukert described the relationship in Elektrotechnische Zeitschrift in 1897 as a fixed product of current raised to an exponent and time, and more than a century later it remains how lead-acid rate dependence is written down. The exponent is fitted per product, it is not a constant of nature, and it rises as plates sulphate — which is why a battery that met its specification when new does not five winters later.
The second reduction is a decision rather than a physical effect. Depth of discharge sets how far into the pack you are willing to go, and lead-acid cycle life is so sensitive to it that the conventional 50% is best understood as the point where the cost per delivered kilowatt-hour stops improving. Lithium iron phosphate moves that point to 80% or higher because its cycle-life curve is far flatter, which is a bigger part of the lithium argument than the weight is. Neither figure is a boundary the battery defends: the remaining charge is still there in an emergency, and taking it is a withdrawal from cycle life rather than a fault. That trade is also why the same physical bank appears in two sizes depending on who specified it — see the battery capacity calculator for the design end of the same argument.
The third is temperature, and it is the one this site refuses to tabulate. Correction factors differ per cell design and per manufacturer, IEEE 485 requires one when stationary lead-acid cells are sized and then explicitly takes the value from the cell maker's table, so inventing a curve here would produce a confident number with nothing behind it. The field is therefore yours to fill and the reading it applies at is printed beside the answer. Once you have the usable capacity, converting it to stored energy is the watt hour calculator and converting it to hours under a load is the battery runtime calculator. If what you know about the load is its resistance rather than its current, the Ohm's law calculator gives you the amps this page asks for, and a UPS quotes the whole problem in volt-amperes instead — that translation is on the UPS runtime calculator.
Where your datasheet figures go
Every number on this page is worked out by JavaScript running in the tab you are reading it in. Nothing you type — loads, lengths, nameplate ratings, the rates your utility charges you — is uploaded, logged or kept, which is also why the calculators carry on working in a mechanical room with no signal.
Chemistry, exponent, discharge floor and temperature are read and discarded in the tab, so the make-up of a client's battery room is not something this site could disclose even if it were asked.