Solar Battery Bank Size Calculator
Size an off-grid or backup battery bank — usage, days of autonomy, voltage, and usable depth.
Updated
You need
260 Ahat 48 V
12.5 kWh of usable storage
- Bank capacity
- 12,500 Wh
- Energy to cover
- 10,000 Wh
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In short
How many amp-hours do I need for 10 kWh a day?
Bank Ah = daily kWh × 1,000 × days of autonomy ÷ depth of discharge ÷ system voltage. For 10 kWh a day with one day of autonomy at 80 percent usable, that is 12,500 Wh of bank capacity, which comes to about 260 Ah at 48 V, 521 Ah at 24 V, or 1,042 Ah at 12 V for exactly the same stored energy.
Sizing a bank on paper is not the same as designing an installation, and battery systems wired into a building or vehicle need a licensed electrician or installer.
How to use the solar battery bank size calculator
Enter the daily electricity the bank has to supply in kilowatt-hours, the days of autonomy you want, the system voltage the bank runs at, and the usable depth of discharge, and the tool returns the bank size in amp-hours plus the same capacity expressed in watt-hours and kilowatt-hours. Amp-hours leads because that is how batteries are sold, labelled and wired together.
The watt-hour figure is the one to trust when comparing options, because it is independent of system voltage and therefore the only fair basis for judging two different bank architectures against each other. Both numbers describe gross nameplate capacity, already scaled up so that the slice you actually use covers your energy need.
12,500 Wh
Gross bank capacity
10 kWh a day, one day, 80 percent usable
260 Ah
At 48 V
how that bank is bought and wired
1,042 Ah
At 12 V
the same energy, four times the amp-hours
Start with the daily usage, and be careful about which loads you count. This is the energy the battery must carry, not the energy your solar array produces and not necessarily your whole household consumption. For a grid-tied backup you might only be sizing for a fridge, a furnace fan, a well pump and some lighting, which could be two or three kilowatt-hours a day.
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For a full off-grid cabin it is everything, which is where figures around 10 kWh a day come from. If you do not have a number yet, add up each load wattage multiplied by the hours it runs, then divide by 1,000.
Checking one specific battery instead?
The battery watt-hours calculator handles the reverse direction when you want to check what a specific battery holds.
Open the battery watt-hours calculator →Days of autonomy is the input people underestimate, and it is a straight multiplier on the answer. It is how long the bank must carry the loads with no meaningful charge arriving: a run of overcast days when the array barely produces, or a grid outage you have to ride out entirely on stored energy.
One day is the common default for grid-tied backup where an outage is measured in hours. Off-grid systems in cloudier climates are frequently sized for two or three, because a single genuinely dark stretch can otherwise empty the bank and leave you with nothing. Every extra day multiplies the bank proportionally, so moving from one day to three triples both the cost and the physical space the batteries occupy.
Do
- Count only the loads that run off the battery, not your whole household usage.
- Match the depth of discharge to the chemistry you actually intend to buy.
- Compare two banks on watt-hours, the figure that does not shift with system voltage.
- Size for two or three days of autonomy where dark stretches are normal.
- Have a licensed electrician or installer sign off anything permanently wired in.
Don't
- Assume the daily solar production and the daily battery load are the same figure.
- Size a lead-acid bank at the 80 percent depth lithium is designed for.
- Read the amp-hour result as usable capacity; it is gross nameplate capacity.
- Expect the rated capacity in the cold or under a fast discharge.
- Take this for an array size, since it sizes storage only.
A ready-reckoner for bank sizing at 80 percent usable depth of discharge, covering everything from a weekend van fit-out to a large off-grid home. The three amp-hour columns are the same stored energy counted at three different system voltages, which makes the case for 48 V more vividly than any argument does: look at what happens to the 12 V column by the bottom of the table.
| Daily use | Days of autonomy | Bank capacity at 80 percent usable | Amp-hours at 48 V | Amp-hours at 24 V | Amp-hours at 12 V |
|---|---|---|---|---|---|
| 1 kWh (a van fridge and lighting) | 1 day | 1.25 kWh | 26 Ah | 52 Ah | 104 Ah |
| 2 kWh | 1 day | 2.5 kWh | 52 Ah | 104 Ah | 208 Ah |
| 3 kWh (essentials-only outage backup) | 1 day | 3.75 kWh | 78 Ah | 156 Ah | 313 Ah |
| 5 kWh (small cabin) | 1 day | 6.25 kWh | 130 Ah | 260 Ah | 521 Ah |
| 5 kWh | 2 days | 12.5 kWh | 260 Ah | 521 Ah | 1,042 Ah |
| 8 kWh | 1 day | 10 kWh | 208 Ah | 417 Ah | 833 Ah |
| 10 kWh (the tool default) | 1 day | 12.5 kWh | 260 Ah | 521 Ah | 1,042 Ah |
| 10 kWh | 2 days | 25 kWh | 521 Ah | 1,042 Ah | 2,083 Ah |
| 10 kWh | 3 days | 37.5 kWh | 781 Ah | 1,563 Ah | 3,125 Ah |
| 15 kWh | 1 day | 18.75 kWh | 391 Ah | 781 Ah | 1,563 Ah |
| 15 kWh | 2 days | 37.5 kWh | 781 Ah | 1,563 Ah | 3,125 Ah |
| 20 kWh (a fully electric off-grid home) | 1 day | 25 kWh | 521 Ah | 1,042 Ah | 2,083 Ah |
| 20 kWh | 2 days | 50 kWh | 1,042 Ah | 2,083 Ah | 4,167 Ah |
| 30 kWh | 1 day | 37.5 kWh | 781 Ah | 1,563 Ah | 3,125 Ah |
How much do chemistry and bus voltage change the bank?
Depth of discharge is the other big lever and it is set entirely by chemistry. Lead-acid banks are conventionally sized around 50 percent usable, and Trojan recommends discharging its deep-cycle batteries only 20 to 50 percent of rated capacity for optimum life. LiFePO4 is designed for far deeper cycling, with Battle Born rating its packs for 100 percent usable depth of discharge and a cycle life of 3,000 to 5,000.
Because you divide by that fraction, the same energy need produces a bank roughly 1.6 times larger on lead-acid than on lithium. System voltage, meanwhile, changes only the amp-hours and not the energy: a 48 V bank holds the same watt-hours as a 12 V one using a quarter of the amp-hours, which is why larger systems run at 48 V and move the same power at lower current.
Read it: Identical stored energy in every row; the higher bus simply moves the same power at a quarter of the current, which is why cables, fuses and busbars all shrink at 48 V.
Computed as bank Wh ÷ system voltage, the same arithmetic as the tool.
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Full guide
What Size Solar System Do I Need to Go Off Grid?
7 panels cover a 10 kWh day with 0.08 kWh to spare. Refilling the battery on that surplus takes 125 days.
Read the full guide →The formula, worked line by line
The bank has to hold a day of energy multiplied by your days of autonomy, then be scaled up so that the fraction you are willing to use covers that need, then converted into amp-hours at whatever voltage the bank runs. Daily kilowatt-hours are multiplied by 1,000 first, so the units line up with the volts and amp-hours batteries are actually rated in rather than the kilowatt-hours your bills are.
Two of the three terms are straight multipliers and behave predictably: double the autonomy and the bank doubles, halve the depth of discharge and the bank doubles again. The voltage term is different in character because it changes only how the same energy is counted. Watt-hours stay put while amp-hours scale inversely with voltage, which is the entire reason large systems move to 48 V.
needed Wh = daily kWh × 1000 × days of autonomy
bank Wh = needed Wh ÷ usable depth of discharge
bank Ah = bank Wh ÷ system voltage
10 × 1000 × 1 ÷ 0.80 ÷ 48 = 260 Ah- Daily energy
- 10 kWh × 1,000 = 10,000 Wh
- Days of autonomy
- × 1
- Usable depth of discharge
- ÷ 0.80 = 12,500 Wh gross
- System voltage
- ÷ 48 V
- Bank size
- 260.4 Ah
The division by 0.80 exists because you only intend to draw four-fifths of the bank, so a 48 V bank of roughly 260 Ah covers one cloudy day of a 10 kWh load. The same 12,500 Wh is 520.8 Ah at 24 V and 1,041.7 Ah at 12 V.
Move each lever separately to see its weight. Going from one day of autonomy to two doubles the bank; switching from 80 percent lithium to 50 percent lead-acid raises it by 60 percent for identical delivered energy; doing both at once compounds them.
- Two days of autonomy, lithium
- 25 kWh · about 521 Ah at 48 V
- One day, lead-acid at 50 percent
- 20,000 Wh · about 417 Ah at 48 V
- Three days of autonomy on lead-acid
- 60,000 Wh · 1,250 Ah at 48 V
Every row starts from the same worked default of 12,500 Wh and about 260 Ah at 48 V; the levers alone do the growing.
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Questions people ask
How many amp-hours do I need for 10 kWh a day?
For one day of autonomy at 80 percent usable depth of discharge, 10 kWh of daily use needs 12,500 Wh of gross bank capacity, which is about 260 Ah at 48 V, 521 Ah at 24 V, or 1,042 Ah at 12 V. Those three figures are the same stored energy counted at three different voltages, not three different bank sizes. Ask for two cloudy days of cover instead of one and every one of them doubles, and switching to lead-acid at 50 percent usable raises them by a further 60 percent.
Covered in depth in What Size Solar System Do I Need to Go Off Grid? →
What does days of autonomy actually mean?
It is how many days the bank must power your loads on its own with no meaningful charge arriving: an overcast stretch when the array barely produces, or a grid outage you have to ride out entirely on stored energy. One day is the usual default for grid-tied backup, where outages are measured in hours. Off-grid systems in cloudier climates are often sized for two or three days, because a single genuinely dark run can otherwise empty the bank. Each extra day multiplies the bank proportionally, in cost and in physical space.
How much does lead-acid versus lithium change the bank size?
Substantially, because you size around the usable slice rather than the nameplate. Lead-acid banks are conventionally designed at 50 percent usable, and Trojan recommends discharging its deep-cycle batteries only 20 to 50 percent of rated capacity for optimum life. LiFePO4 is designed for 80 percent, with Battle Born publishing 3,000 to 5,000 cycles at that depth. So a 10,000 Wh energy need becomes 20,000 Wh of lead-acid bank but only 12,500 Wh of lithium. Lead-acid costs less per kilowatt-hour but you have to buy and house about 60 percent more of it.
Why does a 48 V bank need fewer amp-hours than a 12 V one?
Because amp-hours only describe charge, and energy is charge multiplied by voltage. The same 12,500 Wh is about 260 Ah at 48 V but 1,042 Ah at 12 V, four times the amp-hours for identical stored energy. The practical consequence is current: a higher voltage moves the same power at a quarter of the amps, so cables, fuses and busbars can all be smaller and voltage drop along them is far less punishing. That is why larger off-grid systems are built at 48 V almost as a matter of course.
Does this size my solar array as well as the battery?
No, this sizes the storage only, which is the part that carries you through nights and cloudy spells. How much generation you need to refill that bank each day is a separate question the solar panel output calculator answers from your system size, peak sun hours and losses. Use the two together: this tool sets the storage, that one sets the production, and the battery watt-hours calculator checks what any individual battery on your shortlist actually holds. A licensed installer should size and sign off the finished system.
Sources
Where the constants and formulas on this page come from. Each line names the figure it backs.
That Trojan recommends discharging its deep-cycle batteries only 20 to 50 percent of rated capacity for optimum life, which is why lead-acid banks here are sized around 50 percent usable.
Frequently Asked Questions — Trojan Battery Company
The published cycle life of 3,000 to 5,000 and the 100 percent usable depth of discharge quoted for LiFePO4.
12V 100Ah LiFePO4 Deep Cycle Battery — specifications — Battle Born Batteries, August 2026
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