Solar Battery Bank Sizing Calculator
Include inverter and other system losses in efficiency. This is a constant planning efficiency, not an inverter curve.
Battery sizing inputs
Capacity planning from load energy. Fractions of an hour or day are supported. Chemistry changes restore its planning usable-capacity default.
Required battery capacity
555.56 Ah at 12 V
- Daily load energy
- 4,800 Wh
- Required nominal energy
- 6.67 kWh
- Usable battery energy, before system losses
- 5,333.33 Wh
- Energy delivered to load over backup
- 4,800 Wh
1 days · 90% efficiency · 80% usable capacity · LiFePO4.
Nominal rated capacity is not a guarantee of deliverable energy or runtime.
How many batteries do I need?
Use identical batteries. Match nominal voltage conventions: a “12 V class” LiFePO4 battery may be rated 12.8 V. This tool requires exact arithmetic voltage compatibility; it does not equate different ratings automatically. Check the selected bank and manufacturer documentation.
Changing inputs clears this confirmation. It is your confirmation, not independent product verification.
Check the exact battery manufacturer’s series AND parallel limits for the calculated requirement, then confirm support. No battery count is shown until confirmed.
Chemistry comparison
Explicit planning assumptions: LiFePO4 80% usable versus lead-acid 50% usable, keeping load, voltage, duration and efficiency equal. These are editable defaults, not universal manufacturer limits.
LiFePO4: 555.56 Ah (6.67 kWh). Lead-acid: 888.89 Ah (10.67 kWh).
Existing battery bank
Enter total bank Ah at bank voltage, not the sum of Ah of batteries in series. Load must be greater than zero.
Estimated runtime
25.92 hours · 1.08 days
7,200 Wh nominal · 5,760 Wh usable before losses · 5,184 Wh delivered.
Estimated running current: 18.52 A at nominal voltage.
No recharge is assumed. Constant load and efficiency; temperature, aging, discharge rate and cutoff can shorten runtime. Check surge, inverter and BMS current limits.
Daily appliance loads
Use running watts, whole-number quantity and hours per day (0–24). Up to 30 appliances. This sum does not model startup surge or simultaneous demand.
400 Wh/day
How to use this calculator
Estimate solar battery bank capacity in Ah and kWh, calculate how many batteries you need, and check backup runtime using realistic efficiency and usable-capacity assumptions.
Start with Battery Sizing for a continuous load, Battery Runtime for an existing bank, or Appliance Load Builder for a daily profile. Choose the load measurement side and use efficiencies and capacity limits appropriate to your equipment. All calculations run locally; copy tools include the assumptions.
Sizing and runtime formulas
With efficiency e and usable fraction u expressed as fractions, the capacity method is:
dailyWh = loadWatts × hoursPerDayrequiredNominalWh = dailyWh × backupDays / (e × u)requiredAh = requiredNominalWh / bankVoltagerequiredKWh = requiredNominalWh / 1000usableBatteryWh = requiredNominalWh × udeliveredLoadWh = usableBatteryWh × e
Existing-bank runtime is bankVoltage × bankAh × u × e / loadWatts hours. Days are hours divided by 24. Estimated running current is load watts divided by nominal bank voltage and efficiency; it is not a cable or fuse sizing rule.
Series, parallel and battery counts
Series count equals bank voltage divided by individual battery voltage and must be an exact positive integer. Identical batteries in series increase voltage but do not add Ah. Parallel strings equal the ceiling of required bank Ah divided by individual battery Ah. Total batteries equal series count times parallel strings.
For a 24 V bank requiring 300 Ah from 12 V 100 Ah batteries, capacity arithmetic gives two in series per string and three parallel strings: six batteries, 300 Ah at 24 V, or 7.2 kWh nominal. The arrangement is shown only after user confirmation of the exact manufacturer’s support. No battery model, maximum parallel count or BMS is independently verified here.
Some LiFePO4 specifications use 12.8 V and 25.6 V for products marketed as 12 V and 24 V class. Choose consistent nominal voltage conventions; the tool does not silently reinterpret a 12.8 V rating as 12 V. Manufacturer support is required for both series and parallel connections, matched batteries, total count and BMS compatibility.
Worked battery sizing example
A 200 W continuous load over 24 hours consumes 4,800 Wh/day. One backup day at 90% efficiency and 80% usable capacity requires 4800 / (0.90 × 0.80) = 6,666.6667 Wh nominal. At 12 V this is 555.5556 Ah, displayed as 555.56 Ah and 6.67 kWh. Usable battery energy is 5,333.33 Wh before losses, delivering 4,800 Wh to the load.
If the manufacturer supports the arrangement, six 12 V class 100 Ah batteries in parallel provide 600 Ah and 7.2 kWh nominal. With the same assumptions, the bank delivers 5,184 Wh, giving 25.92 hours at 200 W. At 50% usable capacity, the sizing requirement becomes 888.89 Ah or 10.67 kWh. Counts use unrounded capacity, never the two-decimal display.
Chemistry and efficiency assumptions
LiFePO4 starts at 80% usable capacity and lead-acid at 50%. These are transparent planning choices, not universal limits or product specifications. Changing chemistry restores that mode’s default; you can then edit it. The comparison always states 80% versus 50%, independent of your edited usable fraction.
Trojan describes discharge recommendations in the context of life and performance. Victron describes depth-of-discharge effects, model-specific connection limits and temperature considerations for its own batteries. Neither reference approves an arbitrary battery arrangement or guarantees runtime. Use the exact battery documentation.
The default 90% efficiency is a planning assumption. AC-side loads require allowance for inverter and other system losses. DC-side loads without an inverter should use DC-system losses. Constant efficiency excludes load-dependent inverter curves and separate standby loads unless you explicitly add those loads.
Build a daily appliance profile
For each appliance, multiply running watts by quantity and daily hours. Four 20 W lights for five hours use 400 Wh/day. Add a 60 W appliance for eight hours to obtain 880 Wh/day. Transferring stores the exact total and displays its average load over 24 hours. The sum of listed running watts is used only for an all-appliances-on current estimate.
Duty cycles should be represented by equivalent operating hours. The tool does not model overlapping schedules, power factor or surge. A transferred profile is a snapshot; after editing the builder, transfer again to update sizing. Appliance names and values are not stored or sent to a calculation service.
For a simple continuous DC load, check ideal DC power from voltage and current separately. The ideal model does not account for AC power factor, surge or a complete appliance profile, and it provides no wiring or installation approval.
Limitations and installation checks
This is an illustrative capacity estimator, not electrical design, product selection, solar generation simulation or a runtime guarantee. It assumes a fully available bank, constant load/efficiency and no solar recharge during backup. Aging, cold or hot conditions, discharge rate, low-voltage cutoff and battery-management protection can reduce capacity. Check surge, inverter ratings and continuous/peak battery discharge current.
Check manufacturer series/parallel and charge/discharge limits, matched battery requirements, wiring, fusing, disconnects, BMS and professional installation requirements. High parallel counts need particular attention to current sharing and protection. Manufacturer confirmation in the form is supplied by the user and resets when inputs change.
Input planning limits include 0–1,000,000 W, 0–24 daily hours, 0.01–365 backup days, 1–100% efficiency and usable capacity, and 30 appliances. Runtime requires positive watts. Individual capacities are 0.01–100,000 Ah, existing banks up to 10,000,000 Ah, and calculated energy is limited to 10¹² Wh. Negative/nonfinite values, expressions and overflow are rejected. Zero daily demand gives zero required capacity and no arrangement. Display rounding does not drive counts.
No account, external calculation request, persistent input storage or tracking is added. Copying uses the clipboard only on request and provides selectable text when access is unavailable.
For a separately specified circuit, estimate conductor voltage drop separately using the relevant conductor and circuit assumptions. Battery capacity does not establish safe conductor selection, protection-device suitability, electrical-code compliance or installation certification.
FAQ
How do I size a solar battery bank?
Multiply running watts by hours per day and backup days. Divide that delivered energy by system efficiency and usable capacity fraction to estimate nominal Wh. Divide by bank voltage for Ah. Solar recharge during backup is not included.
How many 100 Ah batteries do I need for a 12 V bank?
For the worked 555.56 Ah requirement, six compatible 12 V class 100 Ah batteries in parallel provide 600 Ah. The exact manufacturer must support six parallel strings, including BMS and model limits. This is a capacity illustration, not an installation design.
Do batteries in series add their Ah?
No. Identical batteries in series add voltage while string Ah remains the Ah of one battery. Parallel strings add Ah. A 24 V bank made from 12 V 100 Ah batteries needs two batteries per series string.
Are 80% and 50% depth of discharge universal limits?
No. These are editable planning assumptions for LiFePO4 and lead-acid respectively. Select usable capacity from your exact battery specification, desired cycle life and operating conditions. Chemistry alone does not verify a product.
Is usable battery energy the same as energy delivered to the load?
No. Usable battery Wh is nominal Wh times the usable fraction. Delivered Wh also includes the selected system efficiency. For an AC load, include inverter losses; for a DC load without an inverter, use DC conversion and wiring losses.
Can I estimate existing battery runtime?
Yes. Runtime hours equals bank voltage times bank Ah times usable fraction times efficiency, divided by load watts. Enter total bank Ah at its bank voltage. A positive load is required, and no recharge is assumed.
Can I size from several appliances?
Use the Appliance Load Builder to sum watts times quantity times hours for each appliance, then transfer the exact daily Wh total. The sizing fields show an equivalent 24-hour average. Editing load or hours returns to continuous-load sizing.
Why is no battery count shown?
Counts are withheld for incompatible voltage ratios, invalid inputs, no energy demand, unconfirmed manufacturer support or arrangements over the tool’s planning limit. Input changes clear manufacturer confirmation. No product has been independently verified by this calculator.
Does this calculate solar panels or recharge time?
No. It estimates storage capacity and no-recharge runtime. Solar yield, weather, shading, charging losses, charge controllers and recharge time require separate calculations.
Will the estimated runtime be achieved?
It is not guaranteed. Temperature, age, discharge rate, voltage cutoff, inverter standby draw and startup surge can reduce runtime. Check manufacturer charge/discharge limits, BMS, wiring, fusing and professional installation requirements.
Sources and method notes
References provide manufacturer-specific context. The formulas and worked examples above define this tool; the 80%, 50% and 90% defaults are planning assumptions. Review the exact product manuals before using any estimated arrangement.