---
title: "Size a DC-to-AC inverter for off-grid solar, battery backup, or mobile power systems. Computes the required continuous VA rating from total load watts and power factor, estimates surge capacity for motor-starting loads (3-7x rated power), applies NEC-recommended 20% derating for continuous operation, and selects the nearest standard inverter size. Also calculates DC input current at the system voltage for cable and fuse sizing. Chain from solar_load_audit to get peak_watts and largest_load_watts, then chain to wire_gauge or wire_ampacity to size the DC cables. Essential for off-grid cabins, RV/van builds, emergency backup, and solar+battery installations."
method: POST
path: "/api/inverter_sizing"
tags: ["power"]
---

# Size a DC-to-AC inverter for off-grid solar, battery backup, or mobile power systems. Computes the required continuous VA rating from total load watts and power factor, estimates surge capacity for motor-starting loads (3-7x rated power), applies NEC-recommended 20% derating for continuous operation, and selects the nearest standard inverter size. Also calculates DC input current at the system voltage for cable and fuse sizing. Chain from solar_load_audit to get peak_watts and largest_load_watts, then chain to wire_gauge or wire_ampacity to size the DC cables. Essential for off-grid cabins, RV/van builds, emergency backup, and solar+battery installations.

`POST /api/inverter_sizing`

## Request

- Base URL: `https://calc.engineer`
- URL: `https://calc.engineer/api/inverter_sizing`
- Auth: none declared

## Request body

- object
  - `continuous_watts` number, required: Total continuous load in watts. Sum of all appliances that may run simultaneously. Chain from solar_load_audit.peak_watts.
  - `surge_watts` number: Peak surge/starting load in watts. Motors and compressors draw 3-7x rated watts on startup. If omitted, estimated as 3x the largest single load or 2x continuous, whichever is greater.
  - `largest_load_watts` number: Wattage of the single largest appliance. Used to estimate surge if surge_watts is not provided. Chain from solar_load_audit.largest_load_watts.
  - `system_voltage` '12' | '24' | '48': DC system voltage. 12V for small/mobile, 24V for medium, 48V for large residential/commercial. Higher voltage = lower current = thinner cables.
  - `power_factor` number: Power factor (0.5-1.0). Resistive loads (heaters, lights) are ~1.0. Motor loads (fridge, AC, pump) are 0.6-0.8. Mixed loads typically 0.8.
  - `efficiency` number: Inverter efficiency (0.5-1.0). Good pure-sine inverters are 0.90-0.95. Modified-sine are 0.85-0.90. Budget units may be lower.
  - `derating_pct` number: Derating percentage for temperature, altitude, and safety margin. NEC recommends 20% (80% continuous rating). Range 0-50%.

## Response `200`

Computed result for inverter_sizing

- object
  - `result` object, required
    - `required_continuous_va` number, required: Required continuous VA rating accounting for power factor: continuous_watts / power_factor.
    - `required_surge_va` number, required: Required surge/peak VA capacity for motor starting loads.
    - `derated_continuous_va` number, required: Continuous VA after applying derating factor. The inverter must be rated at or above this value.
    - `recommended_size_watts` number, required: Nearest standard inverter size (watts) that meets the derated requirement.
    - `dc_current_continuous_a` number, required: Continuous DC input current draw at system voltage: VA / (voltage * efficiency). Sizes the DC wiring and fuses.
    - `dc_current_surge_a` number, required: Peak DC input current during surge. Sizes the DC fuse and battery cable.
    - `battery_cable_note` string, required: Guidance on DC cable sizing based on the continuous current draw.
    - `surge_margin_pct` number, required: How much surge headroom the recommended inverter provides above the estimated surge requirement.
    - `efficiency_loss_watts` number, required: Power lost to inverter inefficiency at continuous load, in watts.
  - `tool` string, required
  - `version` string, required
  - `computed_at` string, date-time, required
  - `chain` object, required
    - `feeds` unknown[]
      - unknown
    - `accepts_from` unknown[]
      - unknown

## Other responses

- `400`: Validation error or invalid JSON
- `404`: Tool not found

---

[API](https://skmtc.dev/calc/apis/calc-engineer-api.md) · [All operations](https://skmtc.dev/calc/apis/calc-engineer-api/llms.txt) · [OpenAPI document](https://skmtc.dev/calc/apis/calc-engineer-api/revisions/8830480ec488?raw)