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Inverter Sizing Calculator — Continuous Rating, Surge Capacity & DC Current Draw

Size Off-Grid and Hybrid Inverters Using Appliance Load Building, Surge Ratings, and Efficiency Curves

Free inverter sizing calculator for solar installers, electricians, and off-grid system builders. Build your appliance load list with running watts and surge watts for each device, and the calculator determines the minimum continuous power rating, peak surge capacity, and DC current draw from your battery bank. Accounts for inverter efficiency at partial and full load, power factor for inductive loads, simultaneous use factors, and ambient temperature derating. Supports 12V, 24V, and 48V DC input with 120V and 120/240V AC output configurations.

Pro Tip: Inverter sizing is not about adding up every load in the house. It is about identifying the worst-case simultaneous load plus the largest single surge. A 3,000W well pump starting surge on top of 2,000W of running loads requires a 5,000W surge rating, but the continuous rating only needs to cover the running loads. The mistake is buying an inverter rated for the total connected load of the entire panel, which wastes money and operates inefficiently at the typical partial load. Most off-grid homes with a 10,000W total connected load operate at 1,500-3,000W 90% of the time. Size the continuous rating for realistic simultaneous demand plus 25% headroom, and size the surge rating for the largest motor start on top of that.

Size the battery bank to supply the inverter's DC demand

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Read the complete charge controller guide

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Inverter Sizing Calculator
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How It Works

  1. Build Your Appliance Load List

    Enter each appliance with its running wattage and surge wattage. Running watts is the continuous draw during normal operation. Surge watts is the startup inrush, which is typically 2-3x running watts for resistive-start motors and 5-7x for capacitor-start motors like well pumps and air compressors. Check appliance nameplates or use the built-in load database for typical values.

  2. Set Simultaneous Use Factor

    Not all loads run at the same time. Assign a simultaneous use factor (0-100%) or use the default factors by load category. A typical off-grid home might have 60-70% of connected loads running simultaneously at peak demand. This factor prevents oversizing the inverter for a scenario that never occurs.

  3. Select System Voltage and Output

    Choose the DC input voltage (12V, 24V, or 48V) to match your battery bank. Select 120V single-phase or 120/240V split-phase AC output. The 120/240V option is required for 240V loads like well pumps, electric dryers, and central air handlers. Most off-grid homes with 240V loads need a split-phase inverter or two inverters stacked in series.

  4. Review Efficiency and Power Factor

    The calculator applies inverter efficiency curves showing efficiency at 25%, 50%, 75%, and 100% load. Most inverters peak at 92-96% efficiency around 50-75% load and drop to 85-90% at very light loads. For inductive loads (motors, compressors), the power factor adjustment increases the VA rating needed compared to pure resistive (unity power factor) loads.

  5. Review Inverter Sizing Results

    See the recommended minimum continuous power rating in watts, minimum surge/peak rating in watts, DC current draw at full load for battery cable sizing, estimated daily DC energy consumption in Ah for battery sizing, and recommended inverter class. The calculator flags loads that may require a dedicated soft starter or separate inverter circuit.

Built For

  • Off-grid solar installers selecting inverters that handle realistic simultaneous loads without oversizing for total connected load
  • Electricians sizing battery-to-inverter DC cables based on maximum current draw at the battery bank voltage
  • Homeowners building load lists to determine the minimum inverter size for their essential-loads backup panel
  • RV and van conversion builders sizing inverters for mobile living loads including microwave, induction cooktop, and AC units
  • Marine electricians selecting inverter/chargers for liveaboard sailboats and motor yachts with 120V shore power capability
  • Engineers designing hybrid solar systems where the inverter handles both grid-tie export and battery backup functions

Features & Capabilities

Appliance Load Builder

Interactive load list with pre-populated typical wattages for common appliances. Enter running watts, surge watts, hours of daily use, and simultaneous use probability for each device. The calculator totals continuous demand, peak surge demand, and daily energy consumption. Editable defaults let you customize values for your specific appliances.

Surge Rating Analysis

Identifies the worst-case surge scenario by analyzing which motor loads could start simultaneously. The largest single surge (e.g., well pump at 3,600W) on top of running loads determines the minimum surge rating. Flags loads with surge requirements that exceed typical inverter surge capabilities and recommends soft starters or dedicated circuits.

DC Current Draw Calculation

Converts the AC load demand to DC current draw at the battery bank voltage, accounting for inverter efficiency. A 3,000W AC load on a 48V bank with 93% efficiency draws approximately 67A DC. On a 12V bank, the same load draws 269A DC, requiring massive cables and creating significant I2R losses. This calculation is essential for DC cable sizing and battery discharge rate analysis.

Efficiency Curve Modeling

Applies realistic inverter efficiency curves rather than a single rated efficiency number. Most inverters are 92-96% efficient at 50-75% load, 88-92% at full load, and 80-88% at 10-25% load. The no-load power consumption (tare draw) of 15-50W is included because it runs 24/7 and can consume 360-1,200 Wh/day even with no AC loads running.

Power Factor Adjustment

Adjusts the VA rating for loads with power factors less than 1.0. A 1,000W motor with a 0.8 power factor draws 1,250VA from the inverter. The inverter must be rated for the VA demand, not just the watt demand. The calculator applies appropriate power factors by load type: 1.0 for resistive loads, 0.6-0.8 for inductive motor loads, and 0.95-0.99 for modern electronics.

12V/24V/48V Comparison

Shows the DC current draw, recommended cable gauge, and estimated cable cost for the same AC load at different battery voltages. This comparison often convinces system designers to upgrade from 12V or 24V to 48V when they see the wire size and cost difference. A 5,000W inverter on 12V draws 460A+ DC, which is impractical for most installations.

Frequently Asked Questions

A typical off-grid home with well pump, refrigerator, lighting, and modest appliances needs a 4,000-6,000W continuous rated inverter with 8,000-12,000W surge capability. The continuous rating covers normal simultaneous loads (refrigerator 150W + lights 200W + well pump running 1,000W + miscellaneous 500W = 1,850W typical, with peaks to 3,500W when multiple large loads coincide). The surge rating handles the well pump starting inrush (3,000-5,000W) on top of running loads. For homes with central AC, electric cooking, or electric water heating, the continuous rating may need to be 8,000-12,000W. Build your actual load list rather than guessing, because oversizing wastes money and undersizing causes nuisance shutdowns.
Continuous (or rated) power is the steady-state output the inverter can sustain indefinitely without overheating. Surge (or peak) power is the short-duration output the inverter can provide for motor starting, typically lasting 5-20 seconds. Most quality inverters have surge ratings of 2x their continuous rating. A 3,000W continuous / 6,000W surge inverter can power 3,000W of running loads continuously and handle a 6,000W motor start for up to 10 seconds. Some cheaper inverters have poor surge capability (1.2-1.5x continuous), which causes well pumps and compressors to fail to start.
Use the highest voltage your battery bank supports. For systems under 1,000W, 12V is acceptable because DC currents are manageable. For 1,000-3,000W, 24V is the minimum practical voltage. For 3,000W and above, 48V is strongly recommended. The reason is DC current. A 5,000W inverter on 12V draws over 460A from the battery at full load, requiring 4/0 AWG cables and creating enormous I2R losses. The same inverter on 48V draws 115A, using 2 AWG cables. The upfront cost of 48V batteries is offset by savings in cable, disconnects, fuses, and reduced energy waste. Nearly all professional off-grid installations over 3 kW use 48V.
Pure sine wave for any system powering modern electronics, variable speed motors, or medical equipment. Modified sine wave (MSW) inverters produce a stepped square wave that works for simple resistive loads (incandescent lights, basic heaters) but causes problems with anything containing a microprocessor, transformer, or motor controller. Specific problems include: motors run hot and loud, GFCI breakers may not trip correctly, audio equipment hums, battery chargers may overheat, and clocks run fast. The price premium for pure sine wave has dropped dramatically, and there is no reason to install MSW in a home or RV system. The only remaining MSW use case is temporary construction power for power tools.
Inverter no-load (tare) draw is the power consumed when the inverter is on but no AC loads are running. It typically ranges from 15W for small inverters to 50W+ for large split-phase units. This draws power 24 hours a day: a 30W tare draw consumes 720 Wh/day, which is 15 Ah on a 48V bank. Over a week of cloudy weather, that is 105 Ah consumed by the inverter doing nothing. For off-grid systems, this matters. Some inverters have a search/sleep mode that powers down the output stage and periodically pulses to detect load, reducing tare draw to 2-5W. This feature can save 500+ Wh/day. When comparing inverters, tare draw is as important as efficiency at rated load.
Disclaimer: This calculator provides inverter sizing estimates for planning purposes. Actual inverter selection must account for manufacturer specifications, environmental conditions, and code requirements. Inverter installations must comply with NEC Articles 690, 705, and 710 as applicable, plus local utility interconnection requirements for grid-tied systems. Improper inverter sizing or installation can cause equipment damage, fire, or electrocution. Consult a licensed electrician or solar installer for final system design. ToolGrit is not responsible for equipment selection or installation outcomes.

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