Inverter Sizing Calculator - Running Watts, Surge Screen & DC Current

How It Works

1. Enter Loads From Nameplates

   Use actual nameplate or measured watts where available. The preset rows are placeholders only. Motor starting, compressor restart, power factor, and surge duration require selected-load and selected-inverter data.

2. Choose Battery Voltage and Output Display

   Select 12V, 24V, or 48V DC input and the AC output label you are planning around. Split-phase output is displayed only; the app does not balance legs, verify neutral loading, or design transfer equipment.

3. Review Local Running and Surge Screens

   The tool compares a local 125% running-watt calculator with a local surge calculator. Appliance mode uses simultaneous-start checkboxes and the largest single surge increment; manual mode uses a simple 1.5x surge assumption.

4. Use DC Current as a Review Trigger

   High DC current means conductor, fuse, disconnect, terminal, voltage-drop, enclosure, torque, and listing details need careful review. The small wire table is only a local prompt and is not NEC Table 310.16 or a final cable selection.

5. Resolve the Source Gaps

   Before purchasing or installing, verify inverter, battery, transfer, conductor, fuse, charger, utility, code, listing, and AHJ requirements with product manuals and qualified electrical or solar review.

Built For

• Building a preliminary off-grid or backup essential-load list before selecting exact hardware
• Comparing how 12V, 24V, and 48V battery voltage changes local DC current
• Flagging high-current battery-side paths that need detailed conductor and protection review
• Documenting which appliance wattage and surge assumptions still need nameplate or manufacturer confirmation
• Separating a quick calculator from final NEC, UL, interconnection, transfer, and battery-safety decisions

Features & Capabilities

Appliance Load Builder

Enter each load with running watts, hours per day, local surge multiplier, and simultaneous-start checkbox. The output keeps the preset rows labeled as placeholders that must be replaced with nameplate or measured data.

Manual Load Calculator

For rough early planning, enter one continuous wattage. Manual mode uses a local 1.5x surge assumption because no load-specific inrush data has been supplied.

DC Current Calculator

Converts running watts to local DC current using the selected battery voltage and efficiency input. This is a review trigger for conductor, fuse, terminal, voltage-drop, and equipment architecture, not final wiring design.

Battery Runtime Rows

Shows raw runtime amp-hours plus local 50% lead-acid and 80% lithium depth-of-discharge screens. Battery datasheets, BMS limits, C-rate, temperature, aging, enclosure, and fire-code requirements still control final design.

Modified Sine Boundary

Modified sine wave output is treated as a local 80% planning calculator and compatibility warning. The app does not approve motors, chargers, electronics, GFCI/AFCI equipment, medical loads, or critical loads.

Source Pointer Reporting

Reports and PDF exports carry NEC, UL 1741, UL 458, IEEE 1547, IEEE 1013, UL 1973, UL 9540, NFPA 855, UL Product iQ, and NIST source-gap pointers without reproducing protected standards text.

Assumptions

• Running watts, surge multipliers, efficiency, runtime, and depth-of-discharge rows are local user-entered or placeholder values.
• Manual mode assumes 1.5x surge because no load-specific starting data is provided.
• Appliance mode checks a local simultaneous-start scenario and the largest single surge increment.
• DC current is running watts divided by battery voltage and user-entered inverter efficiency.
• The output-voltage field is display context only and does not balance split-phase legs.

Limitations

• Does not verify selected inverter ratings, surge duration, thermal derating, no-load draw, charger behavior, or transfer behavior.
• Does not calculate power factor, locked-rotor current, harmonic effects, motor restart, or soft-starter performance.
• Does not select conductors, fuses, disconnects, lugs, terminals, busbars, conduit, or voltage-drop limits.
• Does not verify UL/NRTL certification, Product iQ records, utility interconnection, IEEE 1547 settings, or permission to operate.
• Does not approve battery chemistry, BMS limits, ESS listing, enclosure, fire separation, ventilation, or NFPA 855 installation details.

References

• NFPA 70 National Electrical Code source pointer.
• UL 1741, UL 458, UL 1973, and UL 9540 source pointers.
• IEEE 1547 and IEEE 1013 source pointers.
• NFPA 855 and UL Product iQ source pointers.
• Selected inverter, battery, transfer equipment, and load manufacturer manuals.

Frequently Asked Questions