Calculate Recommended Inverter Size
Enter the expected continuous electrical load and system details.
What Is an Inverter Size Calculator?
An Inverter Size Calculator estimates the inverter capacity needed to operate a selected group of electrical loads. Inverter ratings may be stated in watts, volt-amperes (VA) or kilovolt-amperes (kVA), depending on the product.
The calculator considers continuous load, power factor, surge demand and a capacity margin. It also estimates DC input current when battery voltage and inverter efficiency are provided.
Inverter Size Formula
If the load is expressed in real power and its power factor is known, apparent power can be estimated by dividing watts by power factor.
Design VA = Base VA × (1 + Safety Margin)
Surge VA = Base VA × Surge Multiplier
The inverter should satisfy both the continuous design requirement and the short-duration surge requirement allowed by the specific inverter model.
How to Use the Inverter Size Calculator
- Add the continuous wattage of appliances that may operate at the same time.
- Enter a surge multiplier for motors, compressors or other starting loads.
- Enter the expected overall power factor.
- Enter inverter efficiency for the battery-current estimate.
- Enter a desired safety margin.
- Enter the DC battery bank voltage.
- Click Calculate Inverter Size.
The calculator displays base VA, continuous capacity with margin, surge requirement, approximate battery current and a common reference inverter rating.
Inverter Sizing Example
Suppose the connected continuous load is 1,500 watts with a power factor of 0.90 and a 20% sizing margin.
= 1,666.67 VA
Design VA = 1,666.67 × 1.20
= 2,000 VA
If the load can require a 1.5× starting surge, the estimated surge demand would be approximately 2,500 VA. The inverter must have sufficient short-term surge capability in addition to its continuous rating.
Watts vs VA vs kVA
Watts
Watts represent real electrical power being converted into useful work, heat, light or another form of energy.
VA
Volt-amperes represent apparent power in an AC system and include the effect of power factor.
kVA
One kilovolt-ampere equals 1,000 VA. Larger inverter and UPS systems are commonly described in kVA.
Why Power Factor Matters
A 1,000-watt load does not always require only 1,000 VA. Loads containing motors, transformers or electronic power supplies can have a power factor below 1.
For example, 1,000 watts at a power factor of 0.80 equals 1,250 VA of apparent power.
Why Inverter Surge Rating Matters
Some appliances require substantially more power for a short period when starting. Refrigerators, pumps, air conditioners and other motor-driven equipment are common examples of loads that may have starting surge.
A system may have adequate continuous inverter capacity but still fail to start a motor if the inverter's surge rating or surge duration is insufficient.
Common Inverter Sizes
Available sizes vary among manufacturers. The following table shows common capacity classes that may be useful as general reference points.
| Calculated Design Requirement | Example Reference Size |
|---|---|
| Up to 300 VA | 300 VA |
| 300–500 VA | 500 VA |
| 500–800 VA | 800 VA |
| 800–1,000 VA | 1,000 VA |
| 1,000–1,500 VA | 1,500 VA |
| 1,500–2,000 VA | 2,000 VA |
| 2,000–3,000 VA | 3,000 VA |
| 3,000–5,000 VA | 5,000 VA |
| 5,000–8,000 VA | 8,000 VA |
| 8,000–10,000 VA | 10 kVA |
How to Estimate Inverter Battery Current
The DC current drawn from the battery can be estimated from AC load power, inverter efficiency and battery voltage.
For example, a 1,500-watt load with a 24-volt battery and 90% efficient inverter would draw approximately 69.44 amps from the battery under the simplified calculation.
12V, 24V and 48V Inverter Systems
For the same power level, increasing battery voltage reduces DC current. Lower current can reduce conductor current demand and voltage drop, although all equipment must be compatible with the selected battery voltage.
| Load | Battery Voltage | Approx. Current at 90% Efficiency |
|---|---|---|
| 1,000 W | 12 V | 92.59 A |
| 1,000 W | 24 V | 46.30 A |
| 1,000 W | 48 V | 23.15 A |
Pure Sine Wave vs Modified Sine Wave
Pure sine wave inverters produce an AC waveform designed to closely resemble utility power and are generally preferred for a wide variety of electronic and motor-driven equipment.
Modified sine wave inverters may work with some loads but can be unsuitable for certain electronics, motors or appliances. Always check the equipment manufacturer's compatibility requirements.
Factors That Affect Inverter Size
Simultaneous Loads
Only loads expected to operate simultaneously need to be combined for the continuous operating calculation.
Starting Loads
Motors and compressors may need significantly more short-term power when they start.
Operating Conditions
Temperature, ventilation and manufacturer derating rules can reduce the usable output of some inverter models.
Inverter Size for Solar Systems
In a solar installation, inverter sizing should consider both the loads being powered and the electrical characteristics of the solar and battery system. A grid-tied solar inverter may be sized using different criteria than a battery inverter.
For battery-based systems, also consider battery capacity, maximum discharge current, DC cable sizing, protective devices and the inverter's supported DC voltage range.
Frequently Asked Questions
How do I calculate what size inverter I need?
Add the running wattage of loads that operate together, convert to VA using power factor if necessary, add a capacity margin, and make sure the inverter also has sufficient surge power for starting loads.
What inverter size do I need for a 1,500 watt load?
At a power factor of 0.90, 1,500 watts equals about 1,667 VA. Adding a 20% margin gives approximately 2,000 VA, before checking surge requirements.
Can I use a larger inverter than my load?
An inverter can generally have a continuous capacity above the expected load, provided it is compatible with the battery system and other electrical requirements.
What is the difference between inverter watts and VA?
Watts represent real power while VA represents apparent AC power. Power factor determines the relationship between them.
Why do refrigerators need a larger inverter?
Refrigerator compressors can require a short starting surge that is higher than their normal running power, so inverter surge capability must be considered.
How much current does an inverter draw from a battery?
A simplified estimate divides the AC load watts by battery voltage multiplied by inverter efficiency. Actual current changes with load, voltage and operating conditions.
Can this calculator replace professional inverter sizing?
No. It provides a preliminary estimate. Final selection should consider actual appliance ratings, starting current, waveform requirements, inverter specifications, battery limits, wiring and applicable electrical requirements.