Battery Energy and Backup Duration Calculator
Size the battery your critical load needs at the end of the battery's life — or find how long a battery you have will carry it — from the datasheet figures that decide it. Power is checked separately: an inverter can have the energy and still trip on a motor start.
- Expansion
- Operating cost
What this tool estimates
The nominal battery energy (kWh) that carries a critical AC load (kW) for the hours you need at the end of the battery's life — or the backup time a given battery gives — with the inverter's continuous and short-time power and the battery's discharge power checked separately against the load and its starting transients.
What it cannot decide
- How the battery is recharged, how long that takes, or whether storage pays for itself — charging and tariffs are outside this sizing result.
- Thermal design, fire protection, ventilation and the battery's location — specialist design and the OEM's installation limits decide those.
- Whether your critical circuits can be separated from the rest of the installation, or how the system islands — that is a switchgear and protection design.
- Which datasheet figures apply to your conditions — the OEM's warranty terms do.
Result
| Check | Needed | Rating | Margin |
|---|
| Step | Value |
|---|
| Scenario | Nominal kWh |
|---|
Method version 6 October 2026. A screening estimate, not a design: see the limits below.
Datasheet figures to pin down before you size
Each input is a definition as much as a number. Two datasheets can quote the same chemistry and give different answers here because they define these figures differently.
| Figure | Ask for | The trap |
|---|---|---|
| Usable depth of discharge | The depth the warranty allows, and whether "usable capacity" on the datasheet already includes it. | Applying a depth to a figure that is already "usable" counts the reserve twice. |
| Discharge-path efficiency | Battery discharge efficiency and the inverter's efficiency at your load, not at its best point. | An AC round-trip figure includes charging losses; a battery-only DC figure leaves out the inverter. |
| End-of-life capacity | The capacity the warranty guarantees, and after how many years or cycles. | Sizing for a new battery leaves the backup short later in its life. |
| Inverter ratings | Continuous rating in kW and kVA, and the short-time rating with its duration. | Comparing a kW load with a kVA rating, or a motor's starting kVA with a kW rating. |
| Battery discharge power | Maximum continuous discharge power (or C-rate) at your temperature. | A battery with enough kWh can still be unable to deliver the kW. |
A grid-tied solar plant is not backup: its inverter shuts down when the grid fails. Backup needs storage and an inverter that can island the backed-up circuits safely — see battery energy storage systems. Electrical safety for the installation is governed by the CEA (Measures relating to Safety and Electric Supply) Regulations, 2023 and your state's rooftop regulation; this calculator applies no clause from either.
How this calculator works — and where it stops
What it calculates
The nominal battery energy (kWh) that carries a critical AC load (kW) for a required time at the end of the battery's life — or the backup time a battery of a given nominal energy gives at the end of its life and when new — from the usable depth of discharge, the discharge-path efficiency and the end-of-life capacity. The inverter's continuous and short-time ratings and the battery's discharge power are checked separately against the load and its starting transients.
Inputs
- Critical AC load, kW — what must run together during an outage, not the connected load.
- Backup time needed in hours — or, the other way round, the battery's nominal energy in kWh.
- Usable depth of discharge, discharge-path efficiency and capacity left at the end of life, each as a % from the datasheet and warranty.
- Optional: module or rack size in kWh, to round up to whole modules.
- Optional power check: the inverter's continuous rating (kW or kVA), the load power factor when a rating is in kVA, the largest starting step and the inverter's short-time rating in the same unit, and the battery's maximum continuous discharge power (kW DC).
Method
- Energy at the load (kWh) = critical load (kW) × backup time (h).
- Energy out of the battery = energy at the load ÷ discharge-path efficiency.
- Usable share of nominal at the end of life = usable depth × end-of-life capacity.
- Nominal battery energy = energy out of the battery ÷ usable share — that is, kW × h ÷ (depth × efficiency × end-of-life fraction).
- Backup time from a nominal energy = nominal × depth × efficiency × end-of-life fraction ÷ load at the end of life; without the end-of-life fraction, when new.
- Whole modules = nominal ÷ module size, rounded up.
- Power, separately: continuous rating ≥ critical load (against a kVA rating the load is kW ÷ power factor); short-time rating ≥ load + largest starting step (with no short-time rating entered, the continuous rating is used — the conservative case); battery discharge power ≥ load ÷ discharge-path efficiency. A shortfall is reported as a shortfall.
- Sensitivity: usable depth 10 points lower and higher, efficiency 5 points lower, end-of-life capacity 10 points lower, and no ageing allowance — one at a time, never outside 0–100%.
Assumptions
- The critical load is constant for the backup period; a varying load needs its energy (kWh) summed over the period instead of kW × h.
- The efficiency is the discharge path only. An AC round-trip figure would oversize the battery and a battery-only DC figure would undersize it; the tool converts neither.
- The datasheet figures hold at your operating temperature and discharge rate — the OEM confirms that.
Limitations
- Excludes charging: how the battery is recharged (solar, grid or DG), recharge time and charging power, tariffs and payback.
- Thermal and fire design, ventilation, the battery's location and the OEM's integration limits are specialist design work.
- No islanding or protection design. A grid-tied solar inverter shuts down when the grid fails; backup needs an inverter and switchgear that isolate the backed-up circuits, under your state's rooftop regulation.
- The starting check uses the single largest step; simultaneous starts, harmonics and voltage dip need a detailed study.
- The battery's own short-time discharge capability is not checked — only its continuous rating.
Worked example
Produced by running this calculator with the inputs below.
Inputs
- Critical AC load 20 kW for 2 hours
- Usable depth of discharge 80%; discharge-path efficiency 90%; capacity left at end of life 80%
- Example power figures, not recommendations: inverter 25 kW continuous and 30 kW short-time; largest starting step 8 kW; battery discharge power 25 kW
Working
- Energy at the load = 20 kW × 2 h = 40 kWh.
- Energy out of the battery = 40 ÷ 0.9 = 44.44 kWh.
- Usable share at the end of life = 0.8 × 0.8 = 64%.
- Nominal battery energy = 44.44 ÷ 0.64 = 69.44 kWh — that is, 20 × 2 ÷ (0.8 × 0.9 × 0.8).
- When new, the same battery carries 20 kW for 69.44 × 0.8 × 0.9 ÷ 20 = 2.5 hours.
- Power: continuous 25 kW against 20 kW — 5 kW spare; short-time 30 kW against 20 + 8 = 28 kW — 2 kW spare; battery 25 kW against 20 ÷ 0.9 = 22.22 kW — 2.78 kW spare.
Result. 69.44 kWh nominal, with the example inverter and battery covering the load and its largest starting step.
Sensitivity — what moves the answer
- Usable depth 70% instead of 80%: 79.37 kWh; at 90%: 61.73 kWh.
- Discharge-path efficiency 85% instead of 90%: 73.53 kWh.
- End-of-life capacity 70% instead of 80%: 79.37 kWh. Sizing for a new battery (100%) gives 55.56 kWh — 13.89 kWh less, and short of the 2 hours later in life.
How engineers use the result
To fix the order of magnitude of the battery and the inverter before an OEM is chosen: the nominal kWh to ask for, and whether the inverter and battery can deliver the power — including the largest motor start — that the energy figure assumes.
When a professional design must replace it
Before ordering: measured critical loads and starting currents, the OEM's warranty figures at your temperature and duty, a protection and switchgear design that islands the backed-up circuits, and the thermal and fire design of the battery location.
Sources
- PSERC (Grid Interactive Rooftop Solar Photo Voltaic Systems) Regulations, 2021, consolidated up to the 3rd Amendment (2026), PSPCL memo of 8 Jul 2026 — Punjab, checked 6 Oct 2026 — reg 6.4: with battery backup, the inverter must prevent battery power flowing into the grid when grid supply is absent, and a manual isolation switch is required; reg 6.6: anti-islanding protection is mandatory; reg 6.3: the interconnection must conform to the CEA safety regulations, 2023. Other states: confirm with your regulator
- CEA (Measures relating to Safety and Electric Supply) Regulations, 2023 — notified 8 Jun 2023; electrical installation safety — this calculator applies no clause from it
- Energy = power × time; nominal = energy ÷ (depth × efficiency × end-of-life fraction) — definitions; the figures come from the OEM's datasheet and warranty
Questions people ask before using it
What information do I need before sizing a battery for backup?
The critical load in kW — what must run together during an outage, measured or taken from the circuit schedule, not the connected load — and how long it must run. From the battery and inverter datasheets: the usable depth of discharge the warranty allows, the discharge-path efficiency (battery discharge × inverter at that load × cabling), the capacity the warranty guarantees at the end of the design life, the inverter's continuous and short-time ratings, and the battery's maximum continuous discharge power. For the transient check, the extra power the largest motor or compressor draws for a few seconds when it starts.
Which site conditions could change the battery size?
What actually runs together during an outage, and how the largest motors start — direct-on-line starting draws far more for those seconds than a soft starter or drive. The power factor of the backed-up load, wherever a rating is in kVA. The temperature where the battery sits, which the OEM's datasheet ties to available capacity and ageing. How often and how long outages last, which drives cycling and so the end-of-life figure you design for. Loads you plan to add. And whether the critical circuits can be separated from the rest of the installation at all — that is a switchgear question.
What should I send an engineer for a project-specific battery design?
A list of the critical loads with ratings, starting method and duty; logged load data for those circuits if you have it; the outage record — frequency and duration, from DG run logs or the DISCOM; the single-line diagram; details of any existing solar inverter; where the battery could go, with its ventilation and fire-separation constraints; and any battery or inverter datasheets you are considering. You can attach this calculation to the optional review request below; it shows exactly what it would send first.
Will my grid-tied solar plant keep running in a power cut?
No. A grid-connected solar inverter must stop feeding the grid when the grid fails — anti-islanding protection — so a plain grid-tied plant shuts down in an outage, however sunny it is. Backup needs a battery and an inverter that can isolate the backed-up circuits from the grid before supplying them. State rooftop regulations set the conditions: in Punjab, for example, a system with battery backup must prevent battery power flowing into the grid when grid supply is absent and must have a manual isolation switch, and anti-islanding protection is mandatory (regs 6.4 and 6.6 of the PSERC Grid Interactive Rooftop Solar Photo Voltaic Systems Regulations, 2021, consolidated up to the 3rd Amendment, 2026). Check your own state's regulation and your DISCOM.
Why discharge-path efficiency and not round-trip efficiency?
Backup time depends only on the losses between the battery terminals and the load: battery discharge, the inverter at that load and the cabling. A round-trip efficiency also includes charging losses, so using an AC round-trip figure would oversize the battery; using a battery-only DC figure would leave out the inverter and undersize it. The calculator does not convert one into the other — take the discharge-path figure from the datasheets, or ask the OEM.
Does adding a battery change the solar plant's design?
Yes. Storage needs a hybrid inverter or a separate power conversion system, a decision between AC and DC coupling, and switchgear that can island the backed-up circuits. Adding storage to a plain grid-tied plant later is possible but involves more change than a plant designed with storage in mind from the start, so say so early if storage is likely. The battery's location, ventilation, fire separation and the OEM's installation limits are specialist design work beyond this calculation.
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