Battery & UPS Sizing Calculator
Size UPS and battery banks for backup power, runtime, and load profile per AS 62040 and AS/NZS 3000.
Inputs
Typical: 15 to 60 minutes for graceful shutdown
▶Advanced parameters
Lead-acid: 0.5, AGM: 0.6, Lithium: 0.8+
Results
Required UPS Rating
2000
VA
Total Load: 1600W
UPS Rating
VA (Limit: 2000 >=)
Battery Runtime
minutes (Limit: 30 >=)
Show the working
| Step | Working | Result | Reference |
|---|---|---|---|
| Calculate total connected load | 2 x 500 W + 1 x 200 W + 4 x 100 W = 1600 W | 1600 W | AS 62040:2015 |
| Calculate critical load | 2 x 500 W + 1 x 200 W = 1200 W of 1600 W flagged critical | 1200 W | AS/NZS 3000:2018 |
| Convert load to apparent power | 1600 W / 0.8 UPS power factor = 2000.00 VA | 2000.00 VA | AS 62040:2015 |
| Select UPS rating | Table lookup: smallest standard UPS rating >= 2000.00 VA is 2000 VA (line_interactive topology) | 2000 VA | AS 62040:2015 |
| Calculate energy required over the backup period | 1600 W x (30 min / 60) / 0.9 inverter efficiency = 888.89 Wh | 888.89 Wh | AS 62040:2015 |
| Convert energy to battery capacity | 888.89 Wh / 48 V = 18.52 Ah | 18.52 Ah | AS 62040:2015 |
| Apply depth of discharge, temperature and aging derating | 18.52 Ah / (0.8 DOD x 1 temperature x 0.8 aging) = 18.52 / 0.6400 = 28.94 Ah | 28.94 Ah | AS 62040:2015 |
| Determine number of batteries | ceil(28.94 Ah / 100 Ah per lithium_ion battery) = 1 | 1 batteries | AS 62040:2015 |
| Estimate actual runtime at full load | 28.94 Ah x 48 V x 0.8 DOD x 0.9 efficiency x 1 temperature x 0.8 aging / 1600 W x 60 = 30.0 min | 30.0 minutes | AS 62040:2015 |
Standards referenced
- AS 62040:2015. Power electronic systems for industrial use
- AS/NZS 3000:2018. Electrical installations (Safety Standard)
Lookup values used by these calculators are indicative and awaiting validation against the current standards. Confirm against your own licensed copy before relying on a result for design.
Parameters
Every field this calculator exposes, what it means, the range it accepts, and the traps worth knowing before you rely on the answer.
Per load fields
- Load nametext
- A label only. It appears in the load breakdown and the exported report and changes no result. It cannot be blank.
- Wattswatts per unit
- Real power drawn by one unit of that load. Use the measured or nameplate input power of the equipment, not the power supply badge rating, which is usually a maximum rather than a working figure. Cannot be negative.
- Quantitywhole number
- How many of that item. Watts multiplied by quantity gives the row total. Must be at least 1.
- Criticalcheckbox
- Gotcha, and it is the big one on this page: this flag is reported only. The critical load total is displayed for information, but the uninterruptible power supply rating, the battery capacity and the runtime are all sized on the full load total, critical and non critical together. If you only want the critical load backed up, delete the non critical rows rather than unticking them.
System fields
- Backup timeminutes
- How long the battery has to carry the load. Must be greater than 0; the form accepts 1 to 1,440 minutes. Fifteen to sixty minutes is typical for a controlled shutdown or to cover a generator start. Battery capacity scales directly with this figure.
- Battery voltagevolts direct current
- The direct current bus voltage. Only 12, 24 and 48 volts are accepted. Amp hour capacity is inversely proportional to this, so moving from 12 volts to 48 volts cuts the required amp hours to a quarter for the same stored energy.
- Battery typelead acid, AGM or lithium ion
- Gotcha: this only supplies a default depth of discharge when depth of discharge is not given. Those defaults are 0.5 for lead acid, 0.6 for AGM and 0.8 for lithium ion. Because the form always sends an explicit depth of discharge, changing the battery type on its own will not change any result. Set the depth of discharge to match the chemistry you picked.
- Inverter efficiencyratio, 0.8 to 0.98
- Conversion efficiency from the battery to the alternating current output. Default 0.9. The battery has to supply the load energy plus the conversion loss, so a lower value increases required capacity.
- Depth of dischargeratio, 0.5 to 1.0
- The fraction of rated capacity you are willing to use in one discharge. Default 0.8 on the form. Lower values protect cycle life and increase the battery bank size. Note the engine will not accept anything below 0.5.
- UPS typestandby, line interactive or online
- Recorded for the report only. It does not change the rating, the capacity or the runtime, and no transfer time or efficiency penalty is applied per type.
- Temperature factor and ageing factorratio, engine defaults
- Two derating factors the engine applies but the web form does not expose. Temperature factor defaults to 1.0, meaning no allowance for a hot or cold battery room. Ageing factor defaults to 0.8, meaning the bank is sized so it still meets the runtime at 80 percent of rated capacity near end of life. Both multiply with depth of discharge to convert the required capacity into the recommended capacity.
Assumptions and limits
This is an energy balance estimate. It is useful for a first pass on uninterruptible power supply rating and battery bank size, and it is not a battery system design.
What the tool assumes
- A fixed uninterruptible power supply power factor of 0.8 converts watts to volt amperes. Modern units are often 0.9 or unity rated, which would give a smaller volt ampere figure than shown here.
- The standard rating ladder is 650, 1,000, 1,500, 2,000, 3,000, 5,000, 6,000, 10,000, 15,000, 20,000, 30,000, 50,000 and 100,000 volt amperes. It is an indicative ladder, not a manufacturer catalogue, and there is no growth headroom added before the size is chosen.
- The load is assumed constant at its full wattage for the whole discharge, with no inrush, no startup surge and no profile.
- The battery count assumes a 100 amp hour block and divides the recommended capacity by 100, rounding up. It takes no account of the cells needed in series to reach the direct current bus voltage, so on a 48 volt bus the real block count will be higher than the number shown.
- The estimated runtime is derived by reversing the same equations that sized the bank, so it always returns the backup time you asked for and the runtime check always passes. It is a consistency readout, not an independent verification. Treat a passing runtime check as confirming the arithmetic, not the design.
What it does not do
- No discharge rate correction. A battery rated in amp hours at a 20 hour discharge delivers considerably less when emptied in 15 or 30 minutes. That effect is not modelled, so for short runtimes the capacity here is optimistic. Size from the manufacturer constant power discharge tables at your actual runtime, in watts per cell, rather than from an amp hour figure.
- No end of discharge voltage, no cell count, no series and parallel string arrangement and no charger or recharge time sizing.
- No temperature effect unless you supply the factor directly, and the web form does not offer it. Lead acid life roughly halves for every 10 degrees Celsius above 25.
- No battery room ventilation or hydrogen evolution rate, no seismic or shelf restraint, no direct current fusing or isolation, and no battery short circuit current.
- No statutory backup duration. Essential services, medical installations and emergency lighting under AS 2293 all carry minimum durations set elsewhere, and this tool does not know about them.
- No selectivity between the uninterruptible power supply and its downstream protection, and no bypass or maintenance bypass arrangement.
Treat the output as an indicative sizing figure for early planning. Check it against the current edition of the AS 62040 series and AS/NZS 3000 together with the battery manufacturer discharge data, and have the responsible person for the installation review and sign off the final design. Nothing here is validated or certified.
Worked examples
Three examples covering a 48 volt lithium bank, a 48 volt lead acid bank at a shallower depth of discharge, and a 12 volt bank that needs more than one block. All three use the engine default temperature factor of 1.0 and ageing factor of 0.8. Every figure comes from the same arithmetic the calculator runs.
Example 1. Small server room, 48 volt lithium bank
Two servers, a network stack and some non critical lighting, with 30 minutes of runtime for a controlled shutdown.
Inputs
- Server: 500 W x 2, critical
- Networking: 200 W x 1, critical
- Lighting: 100 W x 4, not critical
- Backup time: 30 minutes, battery voltage: 48 V
- Inverter efficiency: 0.90, depth of discharge: 0.80
Working
- Total load = 1,000 + 200 + 400 = 1,600 W (critical portion 1,200 W, reported only)
- Apparent power = 1,600 / 0.8 = 2,000 VA
- Next standard rating at or above 2,000 VA = 2,000 VA
- Energy required = 1,600 x (30 / 60) / 0.90 = 888.89 Wh
- Required capacity = 888.89 / 48 = 18.52 Ah
- Recommended capacity = 18.52 / (0.80 x 1.00 x 0.80) = 28.94 Ah
- Blocks = 28.94 / 100 rounded up = 1 x 100 Ah
- Estimated runtime = 30 minutes
Pass: the 2,000 VA rating meets the 2,000 VA load exactly and the bank meets the 30 minute target. Note the whole 1,600 W is backed up, including the 400 W of non critical lighting, because the critical flag does not reduce the sizing.
Example 2. Laboratory instruments, 48 volt lead acid bank
Analytical instruments plus bench equipment needing 15 minutes of ride through while a generator starts. Lead acid chemistry, so a conservative 0.5 depth of discharge, and a better grade inverter at 0.92.
Inputs
- Analyser: 2,000 W x 3, critical
- Bench equipment: 400 W x 2, not critical
- Backup time: 15 minutes, battery voltage: 48 V
- Inverter efficiency: 0.92, depth of discharge: 0.50
Working
- Total load = 6,000 + 800 = 6,800 W (critical portion 6,000 W, reported only)
- Apparent power = 6,800 / 0.8 = 8,500 VA
- Next standard rating at or above 8,500 VA = 10,000 VA (6,000 VA is too small)
- Energy required = 6,800 x (15 / 60) / 0.92 = 1,847.83 Wh
- Required capacity = 1,847.83 / 48 = 38.50 Ah
- Recommended capacity = 38.50 / (0.50 x 1.00 x 0.80) = 96.24 Ah
- Blocks = 96.24 / 100 rounded up = 1 x 100 Ah
- Estimated runtime = 15 minutes
Pass: both checks pass. The shallower 0.5 depth of discharge multiplies the required capacity by 2.5 rather than the 1.56 that lithium at 0.8 would give, which is the single biggest driver of bank size here. A 15 minute discharge is also exactly where the missing discharge rate correction bites, so verify against the manufacturer constant power table.
Example 3. Retail point of sale, 12 volt bank over one hour
Four point of sale terminals and a router that must stay up for a full hour on a 12 volt bank. AGM chemistry at 0.6 depth of discharge.
Inputs
- POS terminal: 150 W x 4, critical
- Router: 50 W x 1, critical
- Backup time: 60 minutes, battery voltage: 12 V
- Inverter efficiency: 0.90, depth of discharge: 0.60
Working
- Total load = 600 + 50 = 650 W (all critical)
- Apparent power = 650 / 0.8 = 812.5 VA
- Next standard rating at or above 812.5 VA = 1,000 VA
- Energy required = 650 x (60 / 60) / 0.90 = 722.22 Wh
- Required capacity = 722.22 / 12 = 60.19 Ah
- Recommended capacity = 60.19 / (0.60 x 1.00 x 0.80) = 125.39 Ah
- Blocks = 125.39 / 100 rounded up = 2 x 100 Ah
- Estimated runtime = 60 minutes
Pass: both checks pass, but read the block count carefully. Two 100 amp hour blocks on a 12 volt bus means two blocks in parallel, giving 200 amp hours against the 125.39 amp hours needed. The same energy on a 48 volt bus would need only 31.35 amp hours, which is why higher bus voltages are preferred once the load grows.
Battery & UPS Sizing for AS 62040 and AS/NZS 3000
Uninterruptible power supplies protect critical loads from mains failures, voltage sags, and transient disturbances. Sizing a UPS correctly requires matching the UPS kVA rating to the connected load, then calculating the battery bank capacity needed for the required backup runtime. Undersizing results in insufficient runtime or UPS overload during a mains failure. Oversizing wastes capital and floor space. This calculator takes your load profile, desired runtime, battery voltage, and chemistry, then outputs the required UPS rating and battery bank capacity in Ah.
The calculation follows the principles in AS 62040.1 (general and safety), AS 62040.2 (EMC), and AS 62040.3 (performance and test), alongside the installation wiring rules in AS/NZS 3000:2018. Battery sizing accounts for inverter efficiency losses and depth of discharge limits to protect battery cycle life.
Key concepts
- Load rating: VA vs watts. UPS units are rated in both VA (apparent power) and watts (real power). The VA rating accounts for reactive power drawn by the load. A typical UPS has a power factor of 0.8 to 0.9, meaning a 10 kVA UPS delivers 8 to 9 kW of real power. Always check both the VA and watt rating when selecting a UPS to ensure neither is exceeded.
- Depth of discharge (DOD). The percentage of total battery capacity used during each discharge cycle. Lead-acid batteries should not be discharged below 80% DOD (leaving 20% remaining) to preserve cycle life. Lithium-ion batteries tolerate 90 to 100% DOD. Using a shallower DOD extends battery life but requires a larger, more expensive battery bank.
- Inverter efficiency. The UPS inverter converts DC battery power to AC output. Typical efficiency is 90 to 95% for modern double-conversion units. The battery bank must supply extra energy to compensate for these losses, so the required Ah increases as efficiency decreases.
- Battery chemistry. Valve-regulated lead-acid (VRLA) batteries are the most common in UPS applications: lower upfront cost, 3 to 5 year lifespan, and well-understood behaviour. Lithium-ion batteries last 8 to 15 years, tolerate deeper discharge, and weigh less, but cost significantly more. Temperature affects both chemistries; every 10 degrees Celsius above 25 degrees roughly halves lead-acid battery life.
Common scenarios
- Server room backup. A small business server room has 4 servers, 2 network switches, and a NAS drawing a total of 3.2 kW (4.0 kVA at 0.8 PF). The client requires 30 minutes of runtime to allow a clean shutdown. Using a 48 V lead-acid battery bank at 90% inverter efficiency and 80% DOD, the calculator determines a minimum battery capacity of approximately 58 Ah, leading to a 5 kVA UPS with a matched battery cabinet.
- Medical facility essential services. A pathology lab needs 15 minutes of UPS backup for 8 kW of analytical instruments while the diesel generator starts. The load is sensitive to power quality, requiring a double-conversion (online) UPS. The calculator sizes a 10 kVA online UPS with a 96 V battery string, factoring in the higher ambient temperature (28 degrees Celsius) of the plant room.
- Retail point-of-sale protection. A retail store wants 10 minutes of backup for 4 POS terminals and a router (total 600 W). A small 1 kVA line-interactive UPS with an internal 12 V sealed lead-acid battery is sufficient. The calculator confirms the standard internal battery provides adequate runtime without an external battery pack.
Common questions
How do I size a UPS for a server room?+
Calculate the total load in VA or watts (sum all equipment nameplate ratings times a loading factor of 0.6 to 0.8). Add 20 to 30 percent headroom for future growth. Select the next standard UPS rating: 1, 2, 3, 5, 6, 10, 15, 20, 30, 40, 60, 80, 100, 120, 160, 200 kVA. The runtime depends on the battery bank size.
How do I calculate battery bank size for a given runtime?+
Battery capacity in Ah = (load in watts times runtime in hours) / (battery voltage times efficiency times depth of discharge). For a 5 kW load, 30 minutes runtime, 48 V battery, 0.90 efficiency, 80 percent DOD: Ah = (5000 times 0.5) / (48 times 0.90 times 0.80) = 72.3 Ah. Select the next standard battery capacity.
What is depth of discharge and why does it matter?+
Depth of discharge (DOD) is the percentage of battery capacity used during a discharge cycle. Lead-acid batteries should not be discharged below 80 percent DOD (20 percent remaining) to preserve cycle life. Lithium batteries tolerate deeper discharge (90 to 100 percent DOD). Lower DOD means longer battery life but requires a larger battery bank.
What is the difference between online and line-interactive UPS?+
An online (double-conversion) UPS continuously converts AC to DC and back to AC, providing complete isolation from mains disturbances. A line-interactive UPS normally passes mains power through and only switches to battery on a mains failure or voltage excursion. Online provides better protection but is less efficient and more expensive.
How often should UPS batteries be replaced?+
Lead-acid UPS batteries typically last 3 to 5 years in a temperature-controlled environment (20 to 25 degrees Celsius). Higher ambient temperatures reduce life significantly (halved for every 10 degrees above 25). Lithium batteries last 8 to 15 years. Test battery capacity annually and replace when capacity drops below 80 percent of rated.
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