Inverter Load Calculator

Add your appliances from the 35-item preset library or enter custom loads. Get minimum inverter size, peak surge, required VA, DC current draw, and battery runtime instantly.

Add Appliances & Loads

Select from Preset Library

Custom Appliance

Appliance Name

Running Watts

Load Type Load type determines the startup surge multiplier. Motors and compressors draw 3–7× their running wattage at startup.

Hours / Day

Load List All fields are editable inline

Add appliances above to begin your load analysis

0 loads - Continuous: 0 W - Peak Surge: 0 W - Daily: 0.0 kWh

System Settings

Inverter Efficiency Found on your inverter spec sheet. Modern pure-sine inverters: 92–96%. Modified-sine inverters: 85–90%. Default 95% is appropriate for most quality inverters.

%

Battery Bank Voltage Higher voltage = less DC current, smaller wire, less heat. 48V is standard for residential systems. 12V creates very high current for any load over 500W.

Battery Capacity (Ah) Total Amp-hour capacity of your battery bank at the selected voltage. Example: two 100Ah batteries in parallel = 200Ah. Leave blank to skip runtime calculation.

Ah

Optional - enables battery runtime

Battery Type / DoD Depth of Discharge: the maximum fraction of battery capacity you should use. LFP lithium can safely use 80%. Lead-acid should stay above 50% to preserve lifespan.

Overall Power Factor Ratio of real power (W) to apparent power (VA). Inductive loads like motors draw more VA than their watt rating suggests. Mixed loads = 0.85. Motor-heavy = 0.75.

Check My Existing Inverter (Optional)

Enter your current inverter's ratings to see a Pass / Warning / Fail assessment against your calculated load.

My Inverter - Continuous W

My Inverter - Surge W Found on the inverter spec sheet as "surge," "peak," or "5-second rating." Leave blank and we'll estimate 2× continuous.

Add loads to see inverter sizing

Results update in real-time as you add appliances

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What This Inverter Load Calculator Covers

Everything you need to size a battery backup or off-grid inverter - from appliance entry to surge capacity, VA rating, DC current, and battery runtime. No signup required.

Appliance Load Entry

Choose from a 35-item preset library (kitchen, HVAC, laundry, electronics, pumps, EV chargers) or add custom appliances with running watts, quantity, hours/day, and load type.

Surge & Continuous Sizing

Calculates peak surge using worst-case startup (largest load surge + all others running). Applies NEC Article 690's 1.25× safety factor and rounds up to the next standard commercial inverter size.

Battery Runtime

Enter battery bank Ah and voltage to get runtime in hours and minutes. Accounts for depth of discharge by battery chemistry (LFP 80%, AGM 50%) and inverter efficiency.

Key Features

Built for solar installers, off-grid system designers, and homeowners sizing backup power - with the depth to match a professional load analysis.

35-Item Preset Appliance Library

Pre-loaded with real wattage, surge multiplier, and daily usage for common household appliances - from LED lights and refrigerators to well pumps, EV Level 2 chargers, and mini-splits.

5 Load Type Surge Multipliers

Resistive (1.0×), Capacitive/Electronic (1.5×), Inductive Small Motor (2.5×), Inductive Motor (4.0×), Inductive Compressor (5.0×). Each load type defaults to industry-standard LRA surge factor.

Inline-Editable Load Table

Edit any field directly in the table - name, watts, quantity, hours, load type, surge multiplier. Every change triggers an instant recalculation of all outputs.

Existing Inverter Assessment

Enter your current inverter's continuous and surge ratings to get a Pass / Warning / Fail status. Warning triggers when load exceeds 90% of capacity - leaving insufficient headroom for startup surges.

Power Factor Correction

Select from five power factor presets (1.00 to 0.65) to calculate required VA rating - not just watts. Motors and compressors draw significantly more apparent power than their watt rating suggests.

DC Current & Voltage Warning

Shows DC amps drawn from the battery bank at your selected voltage (12V / 24V / 48V). Flags a high-current warning when 12V systems exceed 150A - and recommends upgrading to 24V or 48V.

How to Use This Calculator

Four steps from a blank load list to a complete inverter specification.

1

Add your loads

Select appliances from the preset dropdown (refrigerator, AC, well pump, etc.) or click "Add Custom" to enter a name, running watts, hours/day, and load type. The tool pre-populates wattage and surge multipliers from the preset - adjust any field inline. Add all loads that may run simultaneously in a worst-case scenario.

2

Configure system settings

Set inverter efficiency (92–96% for pure-sine; 85–90% for modified-sine), battery bank voltage (12V / 24V / 48V), and power factor. For battery runtime, enter your bank's Ah capacity and select battery chemistry (LFP, NMC, AGM, or flooded lead-acid) to set depth of discharge.

3

Check existing inverter (optional)

Enter your current inverter's continuous and surge watt ratings from the spec sheet. The tool immediately flags Pass, Warning (above 90% utilization), or Fail - and tells you whether the surge rating handles startup loads.

4

Read your results

The results panel shows recommended minimum inverter size (watts), continuous load, peak surge, required VA, DC current draw, daily energy (kWh), and battery runtime. The bar chart breaks down the top 10 loads by wattage so you can spot the biggest contributors at a glance.

Surge Multipliers by Load Type

Startup current can be 2–5× the running current for motors and compressors. Getting surge wrong is the most common cause of inverter undersizing.

Load Type	Surge Multiplier	Common Appliances	Why Surge Occurs
Resistive	1.0×	Heaters, lights, toasters, kettles	No moving parts - no inrush current
Capacitive / Electronic	1.5×	TVs, computers, LED drivers, routers	Capacitor charging on power-up
Inductive - Small Motor	2.5×	Ceiling fans, box fans, small pumps	Locked-rotor current at start
Inductive - Motor	4.0×	Washing machines, well pumps, power tools	High LRA (locked-rotor amps) on startup
Inductive - Compressor	5.0×	Refrigerators, ACs, air compressors, mini-splits	Highest LRA - compressor must overcome back-pressure

Multipliers are editable in the load table (range 1.0–10.0×). Soft-start devices and variable-frequency drives reduce surge current and may warrant lower multipliers - consult the equipment spec sheet.

Calculation Methodology

All formulas are based on NEC Article 690, industry-standard surge data, and IEEE inverter sizing guidelines.

Continuous Load

Continuous Load (W) = Σ (Running Watts × Quantity) for all appliances

Sum of all running loads simultaneously. This is the sustained draw the inverter must handle without overheating.

Peak Surge Load

Largest single load surge = Running Watts × Quantity × Surge Multiplier

Peak Surge = Largest single load surge + Σ (continuous watts of all other loads)

Models the worst-case scenario: the highest-surge appliance starts while everything else is already running. This is the inverter's surge rating that matters most in practice.

Required VA Rating

Required VA = (Continuous Load ÷ Power Factor) × 1.25

Inverters are rated in both watts (W) and volt-amperes (VA). Inductive loads draw more VA than their watt rating - the power factor correction converts real power to apparent power. The 1.25× safety factor is per NEC Article 690.

Recommended Inverter Size

Minimum W = Continuous Load × 1.25

Recommended Size = next standard size above minimum (300W → 500W → 1000W → 2000W → 3000W → 4000W → 5000W → 6000W → 8000W → 10000W → 12000W)

Standard commercial inverter sizes are discrete - the tool snaps to the next size up rather than specifying an exact wattage that may not be commercially available.

DC Current Draw

DC Current (A) = AC Load (W) ÷ Battery Voltage (V) ÷ Inverter Efficiency (decimal)

DC amps determine battery cable sizing. At 12V with a 3,000W load and 95% efficiency, DC current is 263A - requiring very large cable cross-sections. The tool flags this and recommends higher voltage banks.

Battery Runtime

Usable Energy (kWh) = Battery Ah × Battery Voltage × DoD ÷ 1000

Runtime (hours) = Usable Energy ÷ (Continuous Load W ÷ Inverter Efficiency ÷ 1000)

DoD: LFP/NMC lithium = 80%; AGM/FLA lead-acid = 50%. Discharging lead-acid below 50% significantly reduces cycle life and voids most warranties.

Who Uses This Tool

Any project that needs an inverter needs a load calculation first.

Off-Grid System Designers

Build the full load list for a cabin, remote homestead, or rural property. Model peak surge from well pumps and refrigerators alongside continuous loads to right-size the inverter and avoid nuisance tripping or catastrophic undersizing.

Model all simultaneous loads including pump surge
Find the minimum battery bank for overnight autonomy
Compare 12V vs 24V vs 48V DC current requirements

Solar Installers Sizing Battery Backup

Before quoting a battery backup system, run a load analysis to determine minimum inverter capacity. Combine with the Battery Sizing Calculator to match the inverter to the battery bank - and with the AC Size Calculator to verify NEC 690 output circuit sizing.

Verify existing inverter pass/fail against customer loads
Justify upgrade recommendations with numbers
Calculate daily kWh to size the solar array

RV & Van Converters

Size the inverter/charger for a mobile power system. The 12V and 24V options and the high DC current warning make this tool directly applicable to RV and van builds where wire routing is constrained and DC current matters more than in a residential install.

RV/marine presets: 12V compressor fridge, mini fridge
DC current warning prevents cable undersizing
Runtime calculation for overnight autonomy planning

Homeowners Evaluating Backup Power

Add the loads you actually care about during an outage - refrigerator, lighting, phone charging, medical equipment, sump pump - and find out the minimum inverter size and how long your existing or planned battery bank will last.

No technical background required
Pre-populated presets for most household appliances
Runtime output answers "will this last through the night?"

12V vs 24V vs 48V: Choosing Battery Bank Voltage

Battery bank voltage is one of the most impactful design choices in an off-grid or backup system. Higher voltage means lower DC current for the same power output - which means smaller wire, less heat, and lower losses.

Factor	12V	24V	48V (Recommended)
DC Amps at 3,000W	263A	132A	66A
Wire Size Required	4/0 AWG or larger	2/0 AWG	1 AWG
Typical Application	Small RV, marine, portable	Medium off-grid, small backup	Residential, commercial, large off-grid
Max Practical Inverter Size	~3,000W	~6,000W	12,000W+
I²R Heat Loss (relative)	16× more than 48V	4× more than 48V	Baseline

DC amps calculated at 95% inverter efficiency. I²R losses are proportional to current squared - halving current reduces resistive losses by 75%.

Pro Tips

Don't add every appliance - add only what runs simultaneously

The calculator sums all loads in the table as if they run at the same time. For backup systems, only include loads you'd actually run during an outage. For off-grid, model your realistic peak usage window - not the theoretical all-on scenario.

Soft-start devices reduce surge multipliers significantly

Mini-splits and modern refrigerators with inverter compressors have variable-frequency drives that reduce startup surge to nearly 1.5–2×. Check the equipment spec sheet for "soft-start" or "inverter compressor" - if confirmed, manually lower the surge multiplier in the load table from the 5.0× default.

Use 48V for any system above 2,000W continuous

At 2,000W with 95% efficiency, a 12V system draws 175A DC. That requires multiple parallel 4/0 AWG cables and oversized fusing - adding cost and failure points. A 48V system pulls just 44A for the same load, handled by a single 6 AWG cable. The wire savings alone often offset the cost of the higher-voltage battery bank.

Match the inverter surge rating, not just the continuous rating

Most inverter spec sheets list a "surge," "peak," or "5-second" rating separately from continuous watts. A 3,000W continuous inverter might have a 6,000W surge rating - or only 4,500W. If the peak surge load from this calculator exceeds the inverter's surge spec, the inverter will shut down or trip when the highest-surge appliance starts. Always verify both ratings.

Frequently Asked Questions

What size inverter do I need for a 2,000W load?

For 2,000W continuous with NEC's 1.25× safety factor, the minimum inverter size is 2,500W - so you'd select a 3,000W inverter (next standard commercial size). If any of those loads are compressors or motors with high surge, the surge calculation may push the recommendation higher. Use the calculator with your actual appliance list for an accurate answer.

What's the difference between watts and VA on an inverter spec sheet?

Watts (W) is real power - what actually does work. Volt-Amperes (VA) is apparent power - what the inverter must supply to the load including reactive current from motors and capacitors. Inductive loads (motors, compressors) have a power factor below 1.0, meaning they draw more VA than their watt rating. A 1,000W motor load at 0.85 power factor draws 1,176 VA. Inverters must be rated for the VA draw, not just the watt draw.

Can I run a well pump on an inverter?

Yes - but the inverter must handle the well pump's surge. A 1 HP well pump runs at ~750W but surges to 3,000W (4× multiplier) at startup. If other loads are running simultaneously, total surge can easily exceed 4,000–5,000W. Use the Inverter Load Calculator with "Inductive - Motor" selected for the pump. Only pure-sine wave inverters reliably run inductive motor loads - modified-sine inverters can cause overheating and motor damage.

Pure-sine vs modified-sine inverter - does it matter?

Pure-sine inverters output a clean AC waveform identical to grid power - safe for all loads including motors, compressors, sensitive electronics, and medical equipment. Modified-sine inverters are cheaper but output a stepped waveform that can overheat motors, cause audible hum in audio equipment, and damage some electronics. For any system with motors, compressors, or sensitive electronics, use a pure-sine inverter. Set efficiency at 92–96% for pure-sine; 85–90% for modified-sine.

How do I find my inverter's surge rating?

Look at the inverter's spec sheet for "surge power," "peak power," or "5-second rating." It's usually listed separately from continuous watts - often 1.5–2.5× the continuous rating. If you can't find it, the calculator defaults to 2× continuous as a conservative estimate. Undersized surge capacity causes the inverter to shut down or beep when a high-surge appliance starts - the most common complaint about undersized inverters.

How long will my battery last with this load?

Enter your battery bank's Ah capacity and voltage in the System Settings panel, then select your battery chemistry to set depth of discharge. The Runtime card shows hours and minutes of backup at continuous load. Note that runtime is based on continuous watts - if loads cycle on and off (like a refrigerator compressor), actual runtime will be longer. Peukert's effect means high-current draws slightly reduce effective capacity in lead-acid batteries - the calculator uses a simplified model appropriate for planning purposes.

Inverter Load Calculator