Inverter Size Calculator
Calculate the minimum continuous inverter rating and startup surge capability for your AC load, with optional battery-current, BMS, runtime, and existing-inverter checks.
Preliminary sizing only. Verify the inverter manufacturer’s continuous and surge-duration ratings, battery/BMS limits, wiring, protection, installation conditions, and applicable electrical requirements. Terms and Conditions
Headroom is applied to simultaneous running load; startup surge is checked separately because surge magnitude and allowable duration are inverter-model-specific.
Enter the load and battery values
Enter the simultaneous running watts and worst startup peak. Battery and compatibility details are optional.
Use nameplate or manufacturer input-power data when available. Startup peak should include the other loads that remain on while the highest-surge device starts.
Inverter sizing result
The primary answer is the calculated minimum continuous AC output rating; choose an actual manufacturer model at or above it and verify surge duration separately.
Sizing checks
- Check—
Show calculation steps Review load headroom, surge, DC current, runtime, and compatibility checks
- Enter valid values to see the complete calculation.
Running, continuous, and startup power comparison
Compare the simultaneous running load, calculated minimum continuous rating, and worst startup peak on one scale.
- Enter valid values to populate the chart.
Method, Sources, and Assumptions
This is a preliminary equipment-sizing model, not a code-compliance or installation design check.
Continuous inverter output is sized from simultaneous running watts plus the entered planning headroom. Startup peak is treated as a separate required surge capability. Optional battery-side estimates use AC watts divided by nominal DC voltage and inverter efficiency.
- The 25% headroom is an illustrative working-example value, not a universal design requirement. Optional battery, efficiency, capacity, BMS, and existing-inverter fields start blank.
- Startup surge must be verified for both magnitude and duration against the selected inverter and the actual load.
- Runtime ignores inverter idle consumption, battery voltage sag, temperature, aging, Peukert effects, and manufacturer-specific discharge limits.
- The calculator does not size conductors, overcurrent protection, grounding, disconnects, transfer equipment, or branch circuits.
- Final selection requires the inverter, battery, BMS, and installation manufacturer’s data plus applicable electrical requirements and qualified judgment.
Calculator guide
What Size Inverter Do I Need?
If you are asking what size inverter you need, choose one whose continuous rating covers everything that can run at the same time and whose surge rating covers the highest startup load for long enough to start it. This calculator determines the minimum continuous AC output requirement from simultaneous running load and headroom, then checks the separate startup peak. Optional battery values add DC-current, runtime, BMS, and existing-inverter wattage checks.
The main result is a calculated requirement, not a universal product size. Use it to screen real inverter models, then verify the manufacturer’s continuous-watt rating, surge magnitude and duration, output voltage and frequency, waveform suitability, battery compatibility, temperature derating, and installation requirements before making a final equipment decision.
- Core inputs
- Running watts, startup peak watts, and design headroom
- Main output
- Minimum continuous inverter AC output rating in watts
- Separate check
- Startup surge rating must also meet the entered peak for sufficient duration
How to Use the Inverter Size Calculator
Start with the loads that can actually run together. The calculator intentionally keeps battery and compatibility checks optional so you can get the inverter wattage requirement first, then add system details only when you have them.
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Enter simultaneous running load
Add the normal AC input watts of the equipment that can operate at the same time. Use measured input power or nameplate/manual input power when available. Do not add every appliance you own if those loads cannot realistically run together.
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Enter the worst startup peak
Use the highest total AC load expected during startup, not just the extra surge of one appliance. If a pump starts while lights and electronics remain on, the startup input should include those continuing loads plus the pump’s startup demand.
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Choose continuous headroom
The calculator’s example state uses 25% headroom as an illustrative planning value. It is editable and is not presented as a universal code requirement or manufacturer rule.
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Add battery values when you need DC-side checks
Battery voltage plus inverter efficiency activates DC-current estimates. Once those values are entered, adding battery capacity and usable capacity activates the runtime estimate. Enter BMS current limits to compare them with the estimated continuous and startup battery currents.
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Screen an inverter you already own
Enter its published continuous and surge watt ratings in Advanced Options. The calculator can compare the wattage numbers, but a passing result still requires a datasheet check for surge duration, temperature conditions, waveform, output configuration, and battery-side limits.
How to Calculate Inverter Size
The calculator treats continuous load and startup surge as two different constraints. Headroom is applied to the simultaneous running load; startup peak is checked separately rather than assuming every inverter has the same surge multiple or duration.
Running Watts vs Startup Watts
Running watts are the normal AC input watts of loads operating at the same time. Startup watts are the highest total AC watts expected during a starting event. Enter the total startup condition—not merely the extra surge above running power—because other loads may remain energized while a motor, compressor, or pump starts.
Minimum continuous inverter rating
Plain language: multiply the simultaneous running watts by one plus the selected headroom percentage.
This is a planning relationship for the inverter’s continuous AC output requirement. The calculator does not silently round the result to a supposedly universal product-size series.
Battery-side current estimate
Plain language: estimated battery current equals AC load power divided by nominal battery voltage and inverter efficiency.
Efficiency affects the estimated DC input current; it is not used by this calculator to inflate the AC continuous nameplate requirement. Actual current changes with battery voltage under load, inverter operating point, waveform, and transient behavior.
The startup requirement is a threshold check: the selected inverter’s published surge capability must be at least the entered worst startup peak, and the inverter must be able to sustain that power for long enough to start the load. Victron identifies compressors, pumps, motors, and some electronic power supplies as loads that can have substantial startup demand, while Samlex cautions that surge duration can be as important as the peak watt number.
- \(P_{\mathrm{cont} }\)
- Minimum continuous inverter rating Calculated continuous AC output requirement after the selected headroom is applied.
- \(P_{\mathrm{run} }\)
- Simultaneous running load Normal AC input power of all loads expected to operate at the same time.
- \(M\)
- Continuous design headroom User-selected planning margin added to the running load.
- \(P_{\mathrm{AC} }\)
- AC power used for the current check Running load, minimum continuous requirement, or startup peak depending on which current estimate is being calculated.
- \(I_{\mathrm{DC} }\)
- Estimated DC battery current Approximate current drawn from the battery side for the selected AC power level.
- \(V_{\mathrm{DC} }\)
- Nominal battery-system voltage Nominal DC voltage supplied to the inverter.
- \(\eta\)
- Inverter efficiency Fraction of DC input power represented by the AC output for this simplified current estimate.
Inverter Size Worked Example
This example reproduces the calculator’s verified example state, then adds optional battery inputs to show how the advanced results extend the core sizing answer without changing it.
Calculate the continuous requirement
The minimum continuous AC output requirement is 1,500 W. The startup requirement remains a separate 3,000 W peak check.
Estimate battery current
Estimate runtime
The simplified estimate is about 2 hours 53 minutes at a constant 1,200 W running load.
Result
1,500 W minimum continuous rating + 3,000 W minimum startup capability
Choose a real inverter whose published continuous-watt rating is at least 1,500 W and whose surge rating can supply at least 3,000 W for the actual startup duration. The battery-side numbers are planning estimates, not cable, fuse, or BMS design ratings.
How to Interpret Your Inverter Size Result
Treat the primary result as the minimum continuous AC output requirement under the load and headroom you entered. It does not replace the separate startup check, and it does not mean a product with the same marketing number will necessarily deliver that many continuous watts under your operating conditions.
Continuous rating
If the calculator returns 1,875 W, an inverter with only 1,800 W of published continuous AC output does not meet that calculated requirement. A larger actual product rating may be needed because manufacturers sell discrete models.
Startup rating
A passing continuous rating does not prove a compressor, pump, saw, or motor will start. Compare the calculator’s startup requirement with both the inverter’s surge watts and the time for which that surge is available.
Battery current
For the same AC power and efficiency, doubling nominal battery voltage approximately halves DC current. This is why high-power 12 V systems can create very large battery-side currents.
| Nominal battery voltage | Estimated DC current |
|---|---|
| 12 V | 111.1 A |
| 24 V | 55.6 A |
| 48 V | 27.8 A |
What Changes Inverter Performance in Real Use
A mathematically adequate watt rating can still underperform in the field because real inverter output depends on startup behavior, temperature, battery voltage, battery/BMS capability, waveform, and the manufacturer’s exact rating conditions.
Surge duration
Peak watts are only half the startup question. Samlex’s inverter guidance warns that some listed surge ratings apply for less than one second while certain motor loads can need several seconds to complete startup. Use the selected inverter’s own datasheet or manual for the actual surge time limit.
Temperature derating
Continuous output can fall as ambient temperature rises. For example, Victron lists 2,400 W continuous at 25°C, 2,200 W at 40°C, and 1,700 W at 65°C for its MultiPlus-II 3000 VA 120 V models. That is manufacturer-specific behavior, not a universal derating curve.
Battery voltage and sag
The calculator’s DC-current estimate uses nominal voltage. Real battery voltage changes with state of charge, load, chemistry, temperature, cable drop, and transient current, so actual current can differ from the simplified estimate.
BMS and battery discharge limits
An inverter can be large enough on the AC side while the battery or BMS is too small on the DC side. Compare both continuous and time-limited battery current capability with the expected inverter demand.
Watts versus VA
Do not assume an inverter advertised as 3,000 VA can necessarily supply 3,000 W continuously. Victron’s 3,000 VA MultiPlus-II example is published at 2,400 W continuous at 25°C, showing why the separate watt rating matters.
Waveform and load compatibility
Passing the wattage calculation does not prove every appliance will operate correctly. Samlex documents compatibility concerns for some loads on modified-sine-wave inverters, so check the appliance and inverter manufacturer requirements when waveform matters.
How to Choose an Actual Inverter
Separate the calculator’s exact requirement from the equipment you ultimately select. Upward rounding is not a mathematical requirement; it is a practical selection step because real inverters are sold in discrete ratings and may have manufacturer-specific derating or surge limits.
Calculated requirement
This is the minimum continuous wattage produced by the calculator from your running load and chosen headroom, plus a separate minimum startup-peak requirement. It represents what the load model requires.
Selected inverter
Choose a real model whose published continuous watts meet or exceed the calculated requirement and whose surge specification meets the startup requirement for adequate duration. Then verify voltage, frequency, waveform, battery-system compatibility, temperature rating, and installation constraints.
| Load type | Main sizing issue | What to verify |
|---|---|---|
| Lights and resistive heaters | Continuous watts | Actual input watts and simultaneous operation |
| Refrigerator or freezer | Compressor startup | Running watts, startup watts, and surge duration |
| Microwave | Electrical input can exceed cooking output | Nameplate or manual input power |
| Well or sump pump | Motor startup and output voltage | Startup demand, duration, and required AC voltage |
| Air conditioner | Compressor startup | Running input, startup demand, and any soft-start behavior |
| Power tools | Motor startup | Running input and startup demand |
| Sensitive electronics | Waveform compatibility | Manufacturer requirements and inverter waveform |
| Published rating condition | Published value |
|---|---|
| Continuous apparent power at 25°C | 3,000 VA |
| Continuous real power at 25°C | 2,400 W |
| Continuous real power at 40°C | 2,200 W |
| Continuous real power at 65°C | 1,700 W |
| Peak power | 5,500 W |
Common Inverter Sizing Mistakes
Most bad inverter selections come from using the wrong load number, treating surge as a generic multiple, or ignoring the battery side after the AC wattage appears to work.
Adding every load instead of simultaneous loads
Size from equipment that can actually operate together. Overstating simultaneous load can unnecessarily increase inverter size, cost, idle consumption, and battery-current capability requirements.
Using appliance output watts instead of electrical input watts
Microwave advertising commonly emphasizes cooking output. Samlex specifically advises checking the microwave’s electrical demand rather than assuming the cooking-power number equals inverter input demand.
Applying efficiency to the wrong side of the calculation
In this calculator, efficiency increases the estimated DC input current for a given AC load. It does not automatically turn a 1,500 W AC requirement into a larger AC nameplate requirement by dividing the result by efficiency.
Assuming a universal 2× surge rule
Some inverters can produce roughly twice continuous rating briefly, but startup requirements and available surge duration vary by load and inverter. Use real manufacturer data whenever the startup event matters.
Treating Ah as a power rating
Amp-hours describe charge capacity, not instantaneous AC output capability. Battery energy affects runtime, while battery/BMS discharge limits determine whether the pack can actually supply the required current.
Choosing cable or fuse size from inverter watts alone
The calculator intentionally does not prescribe conductor or overcurrent-protection sizes. Final selection depends on current, length, installation method, conductor and insulation ratings, voltage drop, terminals, protection device characteristics, manufacturer instructions, and applicable electrical requirements.
Assumptions and Limits
This is a Tier 3 equipment-selection planning tool. It can calculate and screen the power relationships you enter, but it cannot certify an inverter, battery system, or installation as safe, compatible, or code-compliant.
Startup peak must already be known or estimated
The calculator does not invent a refrigerator, compressor, pump, or motor surge multiplier. Enter manufacturer or measured startup data when possible because actual startup magnitude and duration vary.
Headroom is user-selected
The 25% example value is illustrative. The correct margin for a real project depends on the actual load uncertainty, operating conditions, expansion plans, inverter characteristics, and any requirements that apply to the installation.
DC current is approximate
The current model uses nominal DC voltage and entered efficiency. It does not model dynamic battery voltage, wiring losses, inverter current limits, crest factor, harmonic current, or detailed transient behavior.
Runtime is simplified
The runtime estimate uses nominal voltage, Ah capacity, usable fraction, efficiency, and constant running load. It does not fully model chemistry-specific discharge curves, battery aging, temperature, cutoff behavior, rate effects, or inverter idle consumption.
BMS checks are only current screens
Passing entered continuous and peak amp limits does not verify the duration allowed at those limits, state-of-charge restrictions, pack temperature, cell configuration, wiring, terminals, contactors, or protection coordination.
Wattage compatibility is not full compatibility
An existing inverter can pass the continuous and surge watt comparisons while still being unsuitable because of output voltage, frequency, waveform, temperature derating, overload duration, battery voltage, grounding/bonding, or installation constraints.
Sources and Calculation Basis
The calculator’s continuous sizing equation is algebraic and the current/runtime checks are derived from basic power and energy relationships. Manufacturer documentation is used here to show why real selection must also account for surge behavior, VA versus W ratings, temperature derating, waveform compatibility, and product-specific limits.
- Victron Energy — inverter/charger selection guidance — supports the need to consider continuous load, peak startup demand, VA versus W, temperature derating, battery capability, and system configuration.
- Victron Energy — MultiPlus-II 120 V technical specifications — provides the published 3,000 VA, continuous-watt, temperature-derated, efficiency, zero-load, and peak-power values used as the manufacturer example in this guide.
- Samlex America — inverter general-information FAQ — supports the discussion of starting surge, surge duration, microwave input-demand considerations, and motor/compressor/pump startup behavior.
- Samlex America — pure sine wave versus modified sine wave FAQ — supports the caution that waveform compatibility can matter for specific appliances and electronic controls even when wattage is adequate.
The worked example was independently recomputed from the calculator equations and reverse-checked using the original running load and the DC power balance.
Inverter Size Calculator FAQ
These questions address the most common sizing decisions that remain after the continuous and startup requirements have been calculated.
What size inverter do I need?
Use an inverter whose published continuous AC output rating is at least the calculator’s minimum continuous result and whose surge capability meets the entered startup peak for the required duration. Then verify the model’s voltage, frequency, waveform, battery compatibility, temperature rating, and installation requirements.
How much headroom should I add when sizing an inverter?
There is no single universal headroom percentage that applies to every inverter and installation. The calculator makes headroom an editable input. Use a value that reflects load uncertainty, expected continuous utilization, manufacturer guidance, operating temperature, and future load additions rather than treating 20% or 25% as a universal code rule.
Does inverter efficiency change the inverter watt rating I need?
In this calculator, no. The continuous AC rating is based on the AC running load plus selected headroom. Efficiency is used on the battery side to estimate the DC power and current required to deliver that AC output.
How many amps does a 2,000 W inverter draw?
Battery current depends on DC voltage and efficiency. At a 2,000 W AC load and 90% efficiency, the simplified estimate is about 185 A at 12 V, 92.6 A at 24 V, or 46.3 A at 48 V. Actual current varies with battery voltage under load and inverter operating conditions.
Should I use a 12 V, 24 V, or 48 V inverter system?
Higher nominal DC voltage reduces current for the same AC power, which can be useful at higher power levels. But system voltage is an architecture decision, not something the calculator selects by wattage alone. Confirm inverter availability, battery configuration, BMS capability, conductor/protection design, charging equipment, and the rest of the DC system.
What size inverter do I need for a refrigerator, pump, or air conditioner?
Use the equipment’s running input power and actual startup requirement if available. Motor and compressor loads can have large startup demand, and surge duration matters. Do not assume one universal multiplier; check the appliance and inverter manufacturer data whenever possible.
Can a 3,000 VA inverter always supply 3,000 W?
No. VA and W are not automatically interchangeable. As one manufacturer example, Victron lists certain MultiPlus-II 3,000 VA models at 2,400 W continuous output at 25°C. Always use the separate continuous-watt rating for a watt-based load check.
Can an inverter be too large?
Oversizing is not automatically unsafe, but it can increase cost and may increase no-load consumption and battery-side current capability requirements. A larger inverter also does not compensate for an undersized battery, BMS, wiring system, or incompatible output configuration.
Do I need a pure sine wave inverter?
Pure sine wave is generally the safer compatibility choice for sensitive electronics and for equipment whose manufacturer expects utility-like AC. Some motors, microwaves, electronic controls, audio equipment, and other loads may run hotter, noisier, less efficiently, or incorrectly on modified-sine-wave power. Follow the appliance manufacturer’s requirements.
Can I run a 240 V appliance from a 120 V inverter?
Not directly. The inverter must provide the AC voltage and output configuration required by the load, or the system must include appropriately designed conversion equipment. A sufficient watt rating alone does not establish voltage compatibility.
What can a 1,000 W inverter run?
A 1,000 W inverter can continuously supply a combination of compatible loads whose simultaneous electrical input remains within its actual published continuous-watt rating. Loads with motors, compressors, transformers, or high startup demand must also fit within the inverter’s surge rating for sufficient duration.
What can a 2,000 W inverter run?
A 2,000 W inverter can supply compatible loads whose simultaneous input remains within its actual continuous rating, but the 2,000 W label alone does not prove that a refrigerator, microwave, pump, air conditioner, or power tool will operate. Check startup demand, surge duration, voltage, waveform, and battery capability.
What can a 3,000 W inverter run?
A 3,000 W inverter can support a larger simultaneous AC load than a lower-rated unit when its published continuous-watt rating is truly 3,000 W, but VA and W ratings are not automatically interchangeable. High-starting-current loads still require a separate surge and duration check.