SizingKitElectronics
Battery capacity calculator
Four loads drawing 2,543 Wh a day off a 48 V bus, held for two days and stopped at a 50% floor, need 212 Ah — which becomes 300 Ah the moment you buy it in 100 Ah modules, three strings of four. This page starts from the load schedule rather than a single wattage, charges the inverter's losses only to the rows that pass through it, and returns the arrangement, the installed capacity and how long a day's use takes to put back. It also names which of the three multipliers — the autonomy, the discharge floor or the module size — actually decided the number, because that is the one worth arguing about.
- 100% free
- No signup
- 12 load rows
- Series and parallel worked out
- Recharge hours
What has to run, and for how long each day
Run time is the hours the load is actually drawing, not the hours it is switched on. A fridge that cycles a third of the time is eight hours, and getting that wrong moves the bank size more than any other field on this page.
| Load | Power | Hours | Supply | Wh/day | Remove |
|---|---|---|---|---|---|
| 533 | |||||
| 1,067 | |||||
| 583 | |||||
| 360 |
The decisions that turn a load list into a bank
Doubling this halves the current and quarters the copper loss for the same power.
No standard sets this. It is a decision about how long the site has to run itself, and the honest way to choose it is from the longest outage or the longest run of dull days the location actually sees.
The fraction you are willing to spend, straight into the bank size as its reciprocal.
Applied only to rows marked AC. DC rows come straight off the bus untouched.
A battery you can actually order, not the ideal size.
The bus has to be a whole multiple of this, or the arrangement does not exist.
What the array, the alternator or the generator's charger can actually deliver.
Default 1.15 — the charge acceptance stated on lead-acid datasheets: more charge has to be returned than was taken, because some of it goes into gassing rather than into the plates. Lithium iron phosphate is nearer 1.02 — its coulombic efficiency is high enough that the difference disappears into the wiring losses. Take the figure from your own datasheet.
Bank size
BANK REQUIRED
212 Ah
THE SAME BANK AS ENERGY
10.2 kWh
- Taken from the bank each day
- 2,543 Wh · 53 Ah
- At the loads themselves
- 2,325 Wh
- Arrangement
- 4S3P · 12 modules
- Installed capacity
- 300 Ah · 14.4 kWh
- Replacing one day's use
- 1.02 h
- Floor to full
- 2.88 h
What set the size: The discharge floor did. Stopping at 50% multiplies the bank by 2 against 2 for the autonomy, so raising the floor buys more here than adding a day would.
What this is not. A bank sized here is arithmetic on the load list you typed, not a design anybody has signed. Conductor sizes between the modules, overcurrent protection, disconnects, ventilation for a flooded bank and the room it lives in are all decided by the electrical code your jurisdiction has adopted and, past a certain size, by an engineer who stamps the result.
How to size a bank from what the site has to run
The bank size is a load schedule multiplied by three decisions. Only the schedule takes any real work.
List the loads with their real run times
Give each load its steady draw and the hours a day it actually spends drawing — a compressor that cycles a third of the time gets eight hours, not twenty-four, and a well pump that runs four minutes an hour gets 0.7. Mark each row AC or DC, because a DC load takes its energy straight off the bus and an AC one pays the inverter first.
Set the bus voltage, the autonomy and the floor
Bus voltage decides how much current the cable between bank and inverter carries; autonomy is how long the site must run itself unassisted; the usable share is how deep into the bank you are prepared to go. Together they multiply the daily figure by anything between two and eight, so this is where a bank quietly triples in price.
Buy it in modules and check the recharge
Enter a battery you can order and the page works out how many go in series to make the bus and how many strings go in parallel to make the capacity, rounding strings up because half a battery is not a thing. Then read the recharge line: a bank the array or the generator cannot refill inside the hours available is a bank that drifts steadily downwards.
Technical specifications
| Load schedule | Up to 12 rows, each with its own daily run time and its own AC or DC supply. Power fields read whatever unit is written into them, so 12,000 BTU/h enters as 3,517 W and 1.5 kW as 1,500 W. |
|---|---|
| Where the inverter is charged | Only against rows marked AC. With the four default loads at 90%, 2,325 Wh at the appliances becomes 2,543 Wh taken off the bus. |
| Series arrangement | Computed, and refused rather than rounded when the bus is not a whole multiple of the module. 48 V from 12 V modules is four in series; 48 V typed against a 51.2 V lithium module is refused, because that module is itself the 48 V class and the bus field should say 51.2 V. |
| Parallel arrangement | Rounded up to whole strings, never down: a 212 Ah requirement from 100 Ah modules is three strings and 300 Ah installed, and the 41.5% overshoot is printed rather than absorbed. |
| Recharge | Amp-hours taken multiplied by a charge factor, over the charge current, reported both for one day's use and for a climb from the floor to full. |
| Charge factor default | 1.15 for lead-acid, because part of the returned charge goes into gassing rather than the plates. Lithium iron phosphate is nearer 1.02, and the field is editable to anything from 1.00 to 1.60. |
| Autonomy range accepted | 0.1 to 30 days, with no standard behind any value in it. Grid backup is usually measured in hours and an off-grid site in the longest overcast run its location sees. |
| What is retained | Nothing. The load list lives in the page's own memory while the tab is open and is written to no server, no account and no file. |
Frequently asked questions
How many days of autonomy should I design for?
There is no standard that answers this, and any calculator that supplies a default without saying so is guessing on your behalf. The question is really how long the site has to look after itself: a grid-tied backup that only has to cover the outages the utility actually has may need six hours, while an off-grid cabin is sized against the longest run of dull days its location sees, which is a weather question rather than an electrical one.
Why does the bank come out at four times the energy I use in a day?
Because the multipliers stack. Two days of autonomy doubles it, a 50% floor doubles it again, the inverter adds about a tenth to everything that passes through it, and rounding up to whole modules adds whatever is left over — which is how 2,325 Wh of appliance consumption arrives as 14.4 kWh of installed battery. Each of those factors is visible and editable on this page precisely so you can see which one you are paying for.
Do I apply the inverter efficiency to DC loads as well?
No, and doing so is one of the commonest ways a bank ends up oversized. A DC load — LED lighting on the bus, a controller, a DC fridge — takes its energy straight off the battery terminals and never sees the inverter, so charging it a 10% conversion loss invents consumption that does not happen. That is why every row here carries its own supply marker instead of the page applying one efficiency to the total.
Can I build a 48 V bank out of 51.2 V lithium modules?
You already have: a 51.2 V lithium iron phosphate module is what the industry sells as a 48 V battery, so the bus voltage field should read 51.2 V rather than 48 V. Series strings set the voltage and parallel strings set the capacity, and there is no arrangement of whole batteries that turns 51.2 into 48 — which is why the page refuses the combination instead of rounding it to one string and quietly understating the bus by 7%.
What size charger or array does a bank this size need?
Large enough to return one day's consumption inside the hours you actually have, which is the recharge line under the answer. A site using 53 Ah a day needs about 61 Ah put back after charge losses, and at 60 A that is an hour of full current — trivial for a generator and a real constraint for an array, where the useful window is the peak sun hours rather than daylight. Sizing that array is the solar panel output calculator's job.
Does a bigger bank need a bigger charger?
It needs one large enough to refill it, and small enough not to exceed the maximum charge current the cells allow. Manufacturers state that limit as a C-rate — a figure like 0.2C or 0.5C of the bank's rating — and it is a ceiling on the charger, not a target; below it, charger size is an economic question about how quickly you want to be full again rather than a safety one.
Will an inspector accept a bank sized here?
No, and nothing on this page pretends otherwise. This is arithmetic on the load list you typed; the conductors between modules, the overcurrent protection at both ends, the disconnects, the ventilation a flooded bank needs and the room it is allowed to occupy are all decided by the electrical code your jurisdiction has adopted, and past a certain size by an engineer who puts a stamp on the drawing.
About load schedules, and the three multipliers that follow them
Every honest bank sizing starts as a list, and the list is where the errors live. Not the wattages — those are printed on the equipment — but the hours, because almost nothing in a building runs continuously and almost everybody enters it as though it did. A refrigerator is a 120 W load for roughly a third of the hour, so its true figure is around eight hours a day; a well pump is a 750 W load for the few minutes an hour it takes to refill a pressure tank. Enter those two as 24 hours each and the schedule reads 21,000 Wh a day instead of 1,485, and every decision downstream inherits the mistake. The second structural point is that the inverter is not in every path: loads wired to the DC bus never pass through it, so its losses belong to individual rows rather than to the total.
What happens next is pure multiplication, and it is worth seeing as multiplication. Autonomy multiplies the daily figure by the number of days; the discharge floor multiplies it again by the reciprocal of the usable fraction, so 50% doubles it and 80% adds only a quarter; the inverter adds its share to the AC rows; and the module catalog rounds whatever is left up to the next whole string. Four ordinary decisions therefore turn 2.3 kWh of consumption into 14.4 kWh of installed battery, and the note under the answer names whichever of them did the most damage — because arguing about the one that moved the number is productive and arguing about the other two is not. Whether that floor should be 50% or 80% at all is a cycle-life argument, and it belongs to the amp hour calculator.
The last line is the one most sizing tools omit entirely: a bank you cannot refill is a bank you own exactly once. Returning charge costs more than taking it did — lead-acid wants something like 15% more amp-hours back than it gave, because part of the current goes into gassing instead of the plates — and the charge source has to deliver that inside whatever window exists, which for an array is the peak sun hours rather than daylight and is worked out on the solar panel output calculator. Once the bank exists, asking how long it lasts under a particular load is the battery runtime calculator, converting between its amp-hours and its watt-hours is the watt hour calculator, and turning an appliance nameplate into the watts this schedule wants is the volts amps watts calculator.
Where your load schedule lives
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.
The load list is held in the page's own memory for as long as the tab is open and nowhere else, so a survey of what a client's site runs and when it runs it leaves with you rather than with us.