Inverter Size Calculator

An inverter size calculator establishes the minimum continuous rating, minimum surge rating, estimated apparent power, and estimated DC input needed to support a proposed backup or off-grid load. The results provide a starting point for inverter capacity planning before the battery system, DC conductors, overcurrent protection, disconnecting means, transfer equipment, and AC distribution are selected.

The primary sizing number is the inverter’s required watt rating under normal operation. A separate surge rating is needed when connected equipment draws substantially more power during starting or transient operation, such as motor-driven loads, compressors, pumps, refrigeration equipment, or certain power supplies.

For the example inputs below, the calculator produces a 2,250 W minimum continuous inverter rating and a 4,375 W minimum surge rating.

Load Rating Inputs

The calculator uses five inputs:

Input	Example	Electrical use
Running load	1,800 W	Total expected operating load supplied by the inverter
Surge load	3,500 W	Expected short-duration starting or transient load
Power factor	0.9 PF	Converts adjusted running watts to apparent power in VA
Planning margin	25%	Additional capacity above entered load values
Inverter efficiency	92%	Estimates DC-side input watts required to produce the AC running load

Running load is the real power expected while connected equipment operates normally. It should represent the simultaneous load that the inverter must support, not the sum of every nameplate wattage in a building unless all loads can operate at the same time.

Surge load is the anticipated peak demand during equipment starting or another short-duration event. Inverter surge capability must be evaluated against the actual load behavior and the manufacturer’s published time-duration rating. A unit that can deliver a high surge wattage for milliseconds may not support a motor-starting event that persists longer.

Power factor affects apparent power. Inverter equipment is commonly rated in both watts and volt-amperes because the AC current supplied to a load depends on VA, not watts alone. At a power factor below 1.0, a given real-power load requires more apparent power.

Planning margin adds capacity above both the entered running and surge loads. It is a design allowance selected by the user; it is not a substitute for load calculation, equipment nameplate review, or manufacturer-required sizing.

Inverter efficiency estimates the DC power drawn from the battery or DC source while serving the adjusted running load. Actual input current also varies with DC system voltage, inverter operating point, battery voltage under load, conductor losses, temperature, and waveform characteristics.

Inverter Sizing Formula

The calculator applies the planning margin to both load values:

\(\displaystyle \text{Minimum continuous rating} = \text{Running load} \times \left(1+\frac{\text{Planning margin}}{100}\right)\)

\(\displaystyle \text{Minimum surge rating} = \text{Surge load} \times \left(1+\frac{\text{Planning margin}}{100}\right)\)

It then estimates apparent power from the adjusted continuous load:

\(\displaystyle \text{Estimated apparent power} = \frac{\text{Minimum continuous rating}}{\text{Power factor}}\)

Finally, it estimates DC input watts from the adjusted continuous load:

\(\displaystyle \text{Estimated DC input} = \frac{\text{Minimum continuous rating}}{\text{Inverter efficiency}/100}\)

The DC-input result is based on the continuous operating condition. It does not calculate peak battery current during surge operation.

Calculation Example

Using the entered values:

• Running load: 1,800 W
• Surge load: 3,500 W
• Power factor: 0.9 PF
• Planning margin: 25%
• Inverter efficiency: 92%

The continuous rating calculation is:

\(\displaystyle 1{,}800\text{ W} \times 1.25 = \mathbf{2{,}250\text{ W}}\)

The surge rating calculation is:

\(\displaystyle 3{,}500\text{ W} \times 1.25 = \mathbf{4{,}375\text{ W}}\)

The apparent-power estimate is:

\(\displaystyle \frac{2{,}250\text{ W}}{0.9} = \mathbf{2{,}500\text{ VA}}\)

The estimated DC input is:

\(\displaystyle \frac{2{,}250\text{ W}}{0.92} = \mathbf{2{,}445.6522\text{ W}}\)

Result	Value	Application
Minimum continuous rating	2,250 W	Minimum planned inverter output for normal operation
Minimum surge rating	4,375 W	Minimum planned transient or starting capability
Estimated apparent power	2,500 VA	AC loading reference for inverter VA capacity and output-current review
Estimated DC input	2,445.6522 W	Starting point for battery-side power and current calculations

A listed inverter selected for this load must satisfy both its continuous watt capability and its applicable VA rating, while also providing sufficient surge capacity for the connected equipment over the duration required.

DC Conductors and Battery Current

The estimated DC input watts are used next to evaluate battery-bank current, DC feeder ampacity, DC overcurrent protection, disconnect ratings, conductor AWG or kcmil size, and voltage drop.

For a nominal DC system voltage, the basic current relationship is:

\(\displaystyle \text{DC current} = \frac{\text{DC input watts}}{\text{DC system voltage}}\)

For example, a 2,445.6522 W estimated DC input at 48 V corresponds to approximately:

\(\displaystyle \frac{2{,}445.6522\text{ W}}{48\text{ V}} \approx 50.95\text{ A}\)

That value is not a final conductor ampacity. The DC conductor design must account for actual voltage under load, terminal ratings, insulation temperature rating, correction factors, adjustment factors where applicable, installed raceway conditions, current-carrying conductors, connection limitations, and voltage drop. The inverter’s specified maximum DC input current can exceed a simple wattage-based estimate, particularly during surge demand or low battery-voltage conditions.

A low-voltage battery system can require large DC conductors even when the AC load appears modest. At the same power level, a 12 V system carries roughly four times the current of a 48 V system. That affects conductor AWG or kcmil selection, lug compatibility, raceway fill, bending space, voltage-drop performance, and available fault-current considerations.

AC Output Load Review

The 2,250 W continuous result can be converted to approximate AC output current after the system voltage is established:

\(\displaystyle \text{AC current} = \frac{\text{VA}}{\text{AC voltage}}\)

At 120 V, the 2,500 VA estimate corresponds to approximately 20.8 A. At 240 V, it corresponds to approximately 10.4 A. Actual branch-circuit and feeder design depends on the inverter output configuration, whether loads are line-to-neutral or line-to-line, load characteristics, continuous-load treatment, neutral loading, and the listed output rating of the inverter.

The inverter rating does not establish the final ampacity of AC conductors. Conductors, breakers, transfer equipment, panelboards, and branch circuits must be selected for the actual system design and listed equipment requirements. Voltage-drop review should include both the DC path from battery to inverter and the AC path from inverter to loads, especially where the inverter is remote from the battery bank or critical-load panel.

Field Verification

Confirm the following before final equipment selection or installation:

• Load actual operating watts and simultaneous demand rather than relying solely on connected nameplate ratings.
• Motor, compressor, pump, and refrigeration starting requirements, including starting method and surge duration.
• Inverter continuous watt rating, VA rating, surge rating, surge-duration curve, and manufacturer installation instructions.
• Battery voltage range, maximum inverter DC input current, battery discharge capability, and battery-management-system limits.
• DC conductor ampacity, insulation temperature rating, terminal rating, overcurrent protection, disconnecting means, and voltage drop.
• AC branch-circuit and feeder ampacity, transfer equipment rating, grounding and bonding arrangement, and panel capacity.
• Applicable NEC requirements, local amendments, utility interconnection rules where applicable, and AHJ requirements.

The worksheet supports early backup and off-grid inverter planning. It does not select a listed inverter or approve wiring, protection, transfer equipment, battery configuration, or interconnection.

Related calculations: Battery Capacity Calculator, Generator Size Calculator.