Calculator Inputs
Formula used
DC: Ah = (kW × 1000 × hours) ÷ (voltage × efficiency)
Single phase AC: Ah = (kW × 1000 × hours) ÷ (voltage × power factor × efficiency)
Three phase AC: Ah = (kW × 1000 × hours) ÷ (1.732 × voltage × power factor × efficiency)
Rated bank Ah: Load Ah × loss factor × reserve factor × aging factor ÷ (depth of discharge × temperature capacity)
How to use this calculator
- Enter the power draw and choose the correct power unit.
- Add the planned runtime and select minutes, hours, or days.
- Enter system voltage and choose the correct system type.
- Set power factor for AC loads, if known.
- Add efficiency, discharge depth, losses, reserve, and aging values.
- Press the calculate button to view amp hours above the form.
- Use CSV or PDF buttons when you need a saved copy.
Example data table
| Power | Runtime | Voltage | Base formula | Load Ah |
|---|---|---|---|---|
| 0.5 kW | 4 hours | 12 V | 500 × 4 ÷ 12 | 166.67 Ah |
| 1 kW | 5 hours | 24 V | 1000 × 5 ÷ 24 | 208.33 Ah |
| 2 kW | 3 hours | 48 V | 2000 × 3 ÷ 48 | 125.00 Ah |
| 3.5 kW | 2 hours | 120 V | 3500 × 2 ÷ 120 | 58.33 Ah |
Understanding kW to Amp Hour Conversion
Kilowatts show the rate at which a device uses power. Amp hours show how much electric charge a battery can deliver over time. These values look different, but they connect through voltage and runtime. This calculator joins them with practical battery sizing choices.
Why Voltage Matters
A kilowatt value cannot become amp hours by itself. Voltage is the bridge between power and current. A higher voltage system needs fewer amps for the same load. A lower voltage system needs more amps. That is why a 1 kW load can require very different battery capacities on 12 volt, 24 volt, and 48 volt systems.
Runtime Changes the Result
Amp hours grow with time. A load running for one hour needs less stored charge than the same load running for eight hours. The calculator accepts minutes, hours, or days. It then converts the runtime into hours before the main calculation starts. This keeps the result consistent.
Efficiency and Losses
Real systems waste some energy. Inverters, wiring, converters, and battery chemistry all create losses. The efficiency field accounts for useful delivered energy. The extra loss field can cover wiring drops, inverter standby use, or harsh operating conditions. These settings make the estimate more realistic.
Power Factor and Phase Options
For direct current systems, power factor is not used. For alternating current systems, power factor can affect current. A single phase load uses voltage, power factor, and efficiency. A three phase load also uses the square root of three. These options help when the load is connected through an inverter, generator, or industrial supply.
Battery Depth of Discharge
Most batteries should not be drained completely. Depth of discharge defines the usable share of the rated capacity. If you use only 80 percent of a battery, the rated amp hour bank must be larger than the load amp hours. The calculator increases the required capacity when a lower discharge limit is entered.
Reserve and Aging Margin
A reserve margin gives extra capacity for cloudy weather, startup surge, colder days, or future load growth. An aging factor helps cover capacity loss over time. Batteries often deliver less capacity as they age. Adding these margins can prevent an undersized system.
Using the Result
The main amp hour value shows the estimated rated battery bank size. The current value helps check wires, fuses, and inverter input limits. The energy result shows watt hours and kilowatt hours. Use the per string value when several parallel strings share the load. Always compare the estimate with equipment manuals, safety codes, and manufacturer limits before final installation.
A Helpful Planning Workflow
Start with the steady running load. Add surge loads separately. Choose the battery voltage used by the bank. Enter a runtime target. Then apply conservative efficiency, discharge, reserve, and aging values. Review the final amp hours beside available battery models. Round upward for safer planning.
FAQs
What does this calculator convert?
It converts power and runtime into estimated amp hours. The calculation uses voltage, efficiency, power factor, losses, discharge depth, reserve margin, and aging allowance.
Can kW convert to amp hours without voltage?
No. Voltage is required because amp hours depend on current. Current comes from power divided by voltage, with adjustments for efficiency and system type.
What is the basic DC formula?
The basic formula is Ah = kW × 1000 × hours ÷ volts. This gives load amp hours before practical battery sizing adjustments.
Should I use battery voltage or appliance voltage?
Use battery voltage when sizing a battery bank. Use appliance or line voltage when estimating current on an AC supply side.
Why does efficiency change the answer?
Efficiency accounts for energy lost in inverters, converters, wiring, and devices. Lower efficiency means the source must provide more amp hours.
Does power factor affect DC battery sizing?
Power factor is not used in the DC mode. It matters in AC modes because apparent current changes when power factor is below one.
What is depth of discharge?
Depth of discharge is the percentage of rated battery capacity you plan to use. Lower discharge limits require a larger battery bank.
Why include reserve margin?
A reserve margin protects against unexpected runtime, startup surge, colder conditions, future expansion, or small errors in load estimates.
What does temperature capacity mean?
Temperature capacity estimates reduced battery output in difficult conditions. A lower percentage increases the required rated amp hour capacity.
How do parallel strings affect results?
Parallel strings share the required amp hour capacity. The calculator divides total required amp hours by the number of strings entered.
Is this result safe for final installation?
Use it for planning only. Final battery, wire, fuse, inverter, and charger choices should follow equipment manuals and local electrical rules.