Solar Battery Calculator Widget
Help off-grid and backup buyers size a battery bank. Readers enter their daily energy, the days the bank must last, the battery chemistry and system voltage, and get the nominal kWh and Ah needed and how many batteries to wire in series and parallel.
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How it works
The bank must deliver the daily energy for the chosen days of autonomy. Because charging, storage and the inverter lose some energy, the energy drawn from the batteries is the delivered energy divided by the round-trip efficiency. Batteries should not be emptied completely, so the nominal capacity is that usable energy divided by the depth of discharge you allow - 50% is the usual ceiling for lead-acid (Trojan recommends discharging 20-50% for long life), while lithium iron phosphate banks are rated for 80-100%. The nominal kWh is turned into amp-hours at the system voltage. Batteries are then arranged: system voltage divided by battery voltage gives how many go in series, and the Ah needed divided by one battery's Ah, rounded up, gives the parallel strings. 5 kWh a day for one day at 90% efficiency and 80% DoD needs 6.94 kWh - 145 Ah at 48 V, which is eight 12 V 100 Ah batteries (4 in series x 2 strings).
Calculation method
- Delivered = daily kWh x days of autonomy
- Drawn from batteries = delivered / efficiency; nominal = drawn / depth of discharge
- Ah = nominal Wh / system voltage
- Series = system V / battery V (must be a whole number); strings = ceil(Ah / battery Ah); total = series x strings
Worked examples
Backup for evening loads
Inputs: 5 kWh a day; 1 day; LFP 80% DoD; 90% efficiency; 48 V; 12 V 100 Ah batteries
Result: 6.94 kWh nominal (145 Ah at 48 V); 8 batteries, 4 in series x 2 strings
5 / 0.9 = 5.56 kWh; / 0.8 = 6.94 kWh; x 1000 / 48 = 144.7 Ah.
Off-grid cabin on lead-acid
Inputs: 10 kWh a day; 2 days; 50% DoD; 100% efficiency; 24 V; 12 V 200 Ah
Result: 40 kWh (1,667 Ah at 24 V); 18 batteries, 2 in series x 9 strings
10 x 2 / 0.5 = 40 kWh; 40,000 / 24 = 1,667 Ah; / 200 = 8.3 -> 9 strings.
A sizing estimate for planning - battery wiring, fusing and charging should be designed by a qualified installer.
Limitations
- Ignores temperature: cold batteries deliver noticeably less capacity, and some chemistries must not be charged below freezing.
- Assumes nominal capacity at the manufacturer's rated discharge rate; high currents reduce lead-acid capacity (Peukert effect).
- Many parallel lead-acid strings balance poorly; follow the maker's limit on strings.
Where publishers use it
- Off-grid solar kit sellers and battery shops
- Van-life, RV and boat electrical guides
- Home-backup articles for regions with long power cuts
- Solar installers explaining hybrid-system options
- Rural electrification and telecom-site planning blogs
Questions
How many days of autonomy do I need?
Grid-tied backup systems often plan for one day or less; off-grid homes in cloudy climates commonly plan for two to three days, because the bank must carry the load through poor-sun spells. Each extra day multiplies the bank size.
Why is lead-acid limited to 50% depth of discharge?
Deep cycling shortens lead-acid life. Trojan Battery recommends discharging 20-50% of rated capacity for optimum life even though the battery can cycle to 80%. A 50% DoD bank is twice the size of the energy you use.
What DoD can lithium iron phosphate use?
LFP banks are routinely rated for 80-100% usable capacity; some makers quote usable energy at 100% DoD. Choosing 80-90% keeps margin for ageing and cold weather.
Why do I need 12 V batteries in series for 48 V?
Series wiring adds voltages: four 12 V batteries make 48 V, at the Ah of one battery. Parallel strings add capacity. Eight 12 V 100 Ah batteries wired 4S2P give 48 V 200 Ah, 9.6 kWh.
Which system voltage should I choose?
Higher voltage means lower current for the same power: 3 kW is 250 A at 12 V but 62.5 A at 48 V, so cables are thinner and losses smaller. Small systems under about 1 kW often use 12 V; homes usually use 48 V.
What does C-rate mean for a battery bank?
C-rate is the charge or discharge current as a fraction of capacity: a 200 Ah bank discharged at 20 A runs at C/10. Lead-acid capacity is usually quoted at C/20, and heavy inverter loads at C/5 or faster deliver less; LFP holds its capacity better at high rates.
Should I mix old and new batteries?
No. In series the weakest battery limits the string and gets driven into deep discharge; in parallel unequal batteries share current badly. Build each bank from identical batteries of the same age, and keep the cable lengths in parallel strings equal.
Sources
- Battery FAQs: depth of discharge - Trojan Battery Company . For optimum life and performance, a discharge of 20 to 50% of rated capacity is recommended even though the battery can be cycled to 80%.
- Battery-Box Premium HVS datasheet - BYD . Lithium iron phosphate (LiFePO4) cells; usable energy stated at 100% depth of discharge.
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A2Z Tools Solar Battery Calculator https://a2z.tools/embed/solar-battery-calculator
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Solar Battery Calculator by A2Z Tools - https://a2z.tools/embed/solar-battery-calculator
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