Battery Backup Calculator

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Full battery sizing chain — connected load, diversity, inverter and battery efficiency, DoD, temperature derating and design margin — to a practical series/parallel bank with current checks.

Inputs

Solve for

Connected load (W)

Sum of every appliance that could be connected

Load operating factor (%)

Share of the connected load actually running at the same time

Required backup (h)

Battery bank voltage (V)

System DC bus voltage — 12/24/48 V typical

Battery unit voltage (V)

Datasheet nominal voltage of one battery

Battery unit capacity (Ah)

Datasheet C10/C20 capacity of one battery

Battery chemistry

Sets the default DoD, round-trip efficiency and continuous C-rate

Inverter efficiency (%)

Battery operating temperature (°C)

Capacity falls below 25 °C — this may legitimately be negative

Design margin (%)

Ageing and future load allowance

State of health (%)

100% for new batteries; use 80% to size for end of life

How to use this calculator: Battery Backup Calculator

Full battery sizing chain — connected load, diversity, inverter and battery efficiency, DoD, temperature derating and design margin — to a practical series/parallel bank with current checks. The example below is calculated by this page's real engine from the displayed inputs.

1Select whether you need a battery bank for a required backup duration or want to estimate runtime from batteries already installed.
2Add the wattage of all appliances that may be connected, then use the operating factor for the share likely to run at the same time.
3Enter the required hours and the inverter DC bus voltage. Use the actual 12 V, 24 V or 48 V system arrangement.
4Select LiFePO4, lead-acid, AGM or custom datasheet values. Chemistry changes usable depth of discharge, efficiency and current capability.
5Review required Ah, series batteries, parallel strings, installed energy, discharge current and every PASS/WARN check before purchasing equipment.

Input guide and example values

Use values from the same measurement basis and time period. Conditional fields appear only when the related option is selected.

Input	Example value	Why it matters
Solve for	Battery bank size for a required backup time	Select the option that matches the real installation or scenario.
Connected load	1500 W	Sum of every appliance that could be connected
Load operating factor	70 %	Share of the connected load actually running at the same time
Required backup	4 h	Measured or known required backup used by the calculation engine.
Battery bank voltage	48 V	System DC bus voltage — 12/24/48 V typical
Battery unit voltage	12 V	Datasheet nominal voltage of one battery
Battery unit capacity	100 Ah	Datasheet C10/C20 capacity of one battery
Battery chemistry	LiFePO₄	Sets the default DoD, round-trip efficiency and continuous C-rate
Inverter efficiency	92 %	Measured or known inverter efficiency used by the calculation engine.
Battery operating temperature	25 °C	Capacity falls below 25 °C — this may legitimately be negative
Design margin	10 %	Ageing and future load allowance
State of health	100 %	100% for new batteries; use 80% to size for end of life

Formula inputs & variables for Battery Backup Calculator

These are the named quantities used by this calculator. When the source formula does not define a mathematical symbol, OneCalcApp keeps the real input label instead of inventing one.

Variable / input	Unit	Meaning in this calculation
Solve for	—	Select the option that matches the real installation or scenario.
Connected load	W	Sum of every appliance that could be connected
Load operating factor	%	Share of the connected load actually running at the same time
Required backup	h	Measured or known required backup used by the calculation engine.
Battery bank voltage	V	System DC bus voltage — 12/24/48 V typical
Battery unit voltage	V	Datasheet nominal voltage of one battery
Battery unit capacity	Ah	Datasheet C10/C20 capacity of one battery
Batteries installed	nos	Total number of units in the existing bank

Formula, derivation and worked example

Nameplate capacity is not usable capacity. The chain runs from the connected load through diversity, inverter conversion, battery round-trip efficiency, allowable depth of discharge, temperature derating and a design margin — and only then converts to amp-hours and a real series/parallel arrangement. The discharge current is checked against the battery's continuous rating, because a bank that has enough energy can still fail if it cannot deliver the current.

P_sim = P_connected × operating factor

E_AC = P_sim × t_backup

E_DC = E_AC / η_inverter

E_nominal = E_DC / (DoD × η_battery × k_temp × SoH) × (1 + margin)

Ah = E_nominal / V_bank; N_series = V_bank / V_unit; N_parallel = ⌈Ah / Ah_unit⌉

I_discharge = P_sim / (η_inverter × V_bank) ≤ C_rate × Ah_installed

Substitution steps

1. Connected load
Σ appliance ratings
= 1,500 W
2. Simultaneous load
P_connected × operating factor
= 1,050 W
3. Inverter efficiency
η_inv
= 92 %
4. Temperature factor
k_temp at 25 °C
= 1
5. Usable capacity factor
DoD × η_batt × k_temp × SoH
= 0.855
6. AC energy required
P_sim × t
= 4.2 kWh
7. DC energy required
E_AC / η_inv
= 4.565 kWh
8. Nominal energy required
E_DC / usable factor × (1 + margin)
= 5.873 kWh
9. Required capacity
E_nominal / V_bank
= 122.4 Ah
10. Series count
V_bank / V_unit
= 4
11. Parallel strings
⌈Ah_req / Ah_unit⌉
= 2
12. Installed capacity
N_parallel × Ah_unit
= 200 Ah (9.6 kWh)
13. Discharge current
P_sim / (η_inv × V_bank)
= 23.8 A

Computed example results

Battery bank required: 4S × 2P — 8 × 12 V 100 Ah; theoretical 122.4 Ah, selected 200 Ah
Theoretical capacity: 122.4 Ah at 48 V; nearest catalogue unit 150 Ah
Installed nominal energy: 9.6 kWh
Usable energy: 8.21 kWh; 85.5% of nameplate
Simultaneous load: 1,050 W; 1,500 W connected × 70%
Energy needed at the load: 4.2 kWh
Discharge current: 23.8 A; C-rate 0.12C, limit 1C
Achievable backup with this bank: 7.19 h

Understanding the result

Read the main result together with supporting checks, assumptions, limits and intermediate values.

For a manual check, repeat the first equation, confirm the units and change one input at a time.

Common mistakes when using Battery Backup Calculator

Do not mix units for Connected load (W), Load operating factor (%), Required backup (h). A unit mismatch changes the input magnitude even when the typed number looks reasonable.
Do not leave Solve for on the default choice unless that choice matches the real scenario; the selected option can change the calculation path or factor.
Do not replace the displayed P_sim = P_connected × operating factor relationship with a different convention without also changing the underlying assumptions; compare like-for-like methods when checking the result.
Do not treat Battery bank required = 4S × 2P — 8 × 12 V 100 Ah from the worked example as a universal answer. It belongs to the displayed example inputs and must be recalculated for the actual case.

When the Battery Backup Calculator is useful

Battery Backup Calculator is designed for cases where Solve for, Connected load, Load operating factor, Required backup are known and you need Battery bank required, Theoretical capacity, Installed nominal energy. The page keeps the live calculator, calculation method and worked example together so the result can be checked instead of treated as a black-box number.

Use the calculator for the scope described by its inputs and notes. The displayed method is P_sim = P_connected × operating factor. If the real project or decision needs factors that are not represented here, treat the result as an estimate and add the missing checks separately.

Solve for and Connected load: what changes the answer

The worked example uses Solve for = Battery bank size for a required backup time, Connected load = 1500 W, Load operating factor = 70 %, Required backup = 4 h. With those values, Battery bank required is 4S × 2P — 8 × 12 V 100 Ah. Changing an input should be interpreted according to that field's unit, range, option and hint rather than by the number alone.

For this calculator, the main input roles are: Solve for: Select the option that matches the real installation or scenario. Available choices include Battery bank size for a required backup time, Backup time from a known battery bank. Connected load (W): Sum of every appliance that could be connected Load operating factor (%): Share of the connected load actually running at the same time Required backup (h): Measured or known required backup used by the calculation engine.

How to sanity-check a Battery Backup Calculator result

Start by confirming the entered values and units, then compare the substituted working with the displayed formula or calculation steps. Pay particular attention to Battery bank required, because it is the first worked-example output shown by the live engine.

Finally, compare the result with the assumptions, warnings and related calculators on this page. A nearby calculator can be useful as a cross-check when it measures the same workflow from a different input or output direction.

Next logical calculator

Continue with Solar Generation Calculator

Solar Generation Calculator is directly connected from Battery Backup Calculator as a source-defined continuation or comparison.

Open Solar Generation Calculator

Standards, source trail and limitations

References show the method used. Check the current local edition, amendments and project specification before a regulated decision.

Formula

P_sim = P_connected × operating factor
E_AC = P_sim × t_backup
E_DC = E_AC / η_inverter
E_nominal = E_DC / (DoD × η_battery × k_temp × SoH) × (1 + margin)
Ah = E_nominal / V_bank; N_series = V_bank / V_unit; N_parallel = ⌈Ah / Ah_unit⌉
I_discharge = P_sim / (η_inverter × V_bank) ≤ C_rate × Ah_installed

Worked example

11 500 W connected, 70% operating factor → 1 050 W simultaneous, 4 h backup.
2E_AC = 4.2 kWh; E_DC = 4.57 kWh; usable factor 0.9 × 0.95 × 1.0 = 0.855 → 5.34 kWh nominal, +10% margin = 5.88 kWh.
3At 48 V that is 122 Ah → 2 parallel strings of 4 × 12 V 100 Ah = 8 units, 9.6 kWh installed.

Assumptions

Load is constant over the backup period; motor starting surges are covered by the inverter, not the energy calculation.
Lead-acid Peukert losses at high discharge rates are represented through the C-rate check, not by re-rating capacity.

Tips

Size on the worst month's load, not the annual average.
Keep parallel strings to four or fewer and use equal-length cables to keep them balanced.

Warnings

Discharging lead-acid beyond 50% DoD roughly halves cycle life.
Below 10 °C a lead-acid bank loses 20–30% of its usable capacity; lithium may refuse to charge below 0 °C.

Standards & references

IEEE 1013 (PV battery sizing)
IEEE 485
IEC 62619
IEC 61427-1

Frequently asked questions

How long will a 200 Ah battery last?

At 48 V with a 1 000 W load, roughly 8 hours for LiFePO₄ and about 4 hours for lead-acid once depth of discharge, battery efficiency and inverter losses are applied.

Why is the operating factor needed?

Because the connected load is almost never running all at once. Sizing on connected load alone typically oversizes the bank by 30–50%.

Does temperature really matter?

Yes. Usable capacity falls roughly 1% per °C below 25 °C for lead-acid, which is why the calculator applies an explicit temperature factor.

What inputs does the Battery Backup Calculator use?

It uses Solve for, Connected load, Load operating factor, Required backup, Battery bank voltage, Battery unit voltage, Battery unit capacity, Battery chemistry, Inverter efficiency, Battery operating temperature, Design margin, and State of health. Follow the unit printed for each field and choose any selectable option to match the real scenario.

What does the Battery Backup Calculator calculate?

It calculates Battery bank required, Theoretical capacity, Installed nominal energy, Usable energy, Simultaneous load, Energy needed at the load, Discharge current, and Achievable backup with this bank. With the displayed default inputs, Battery bank required is 4S × 2P — 8 × 12 V 100 Ah.

Which formula does the Battery Backup Calculator use?

The primary relationship is P_sim = P_connected × operating factor. The page also shows substituted values and calculation steps so the result can be checked independently.

How can I verify a Battery Backup Calculator result?

First verify the units for Solve for, Connected load, and Load operating factor. Then compare the substituted formula steps with Battery bank required and its displayed precision.

What are the limitations of the Battery Backup Calculator?

Battery Backup Calculator: Use this result for preliminary design and cross-checking. Confirm the applicable code edition, manufacturer data, site conditions and qualified-engineer approval before final design or installation.

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