Inverter Size Calculator
Calculate inverter size based on continuous and surge power requirements. Compare pure sine vs modified sine wave with battery current draw calculations.
Power Requirements
Motor loads have high startup surge (3x running watts)
Related Calculators
About This Calculator
The Inverter Size Calculator helps you choose the right inverter for your off-grid, RV, or backup power system. Calculate the continuous and surge power requirements, understand pure sine vs modified sine wave differences, and determine the battery current draw at different voltages. Choosing an undersized inverter leads to shutdowns and potential damage, while oversized inverters waste money and reduce efficiency. With 2026 inverter technology advancing rapidly—featuring higher efficiency ratings, built-in MPPT controllers, and LiFePO4 battery compatibility—proper sizing is more important than ever for maximizing your investment.
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How to Use the Inverter Size Calculator
- 1Enter your total continuous load - all devices running at the same time.
- 2Optionally enter your peak/surge load, or let the calculator estimate based on motor loads.
- 3Check the motor loads box if you have refrigerators, AC units, pumps, or power tools.
- 4Select pure sine wave (recommended) or modified sine wave based on your devices.
- 5Enable Advanced mode to adjust safety margin, efficiency, and view cost estimates.
- 6Review the recommended inverter size and battery current requirements.
Understanding Inverter Sizing
Proper inverter sizing requires understanding two different power ratings:
Continuous (Running) Power: The maximum power the inverter can deliver indefinitely. Your total load should be 75-80% of this rating.
Surge (Peak) Power: The maximum power for short bursts (typically 5-30 seconds). Motors require 2-3x their running watts to start.
Sizing Formula: Recommended Size = Total Running Watts × 1.25 (25% safety margin)
Example Load Calculation:
| Device | Running | Starting |
|---|---|---|
| Refrigerator | 150W | 450W |
| Well Pump | 750W | 2,250W |
| Lights | 200W | 200W |
| Total | 1,100W | 2,900W |
Recommendation: 2,000W inverter (handles 1,100W continuous, 4,000W surge)
2026 Inverter Pricing and Options
The off-grid inverter market has matured significantly, with excellent options at every price point:
2026 Pure Sine Wave Inverter Prices:
| Size | Basic PSW | Inverter-Charger | Split-Phase |
|---|---|---|---|
| 1,000W | $80-150 | $150-250 | N/A |
| 2,000W | $150-300 | $300-500 | N/A |
| 3,000W | $250-450 | $445-700 | $600-900 |
| 5,000W | $400-700 | $690-1,100 | $900-1,400 |
| 8,000W | $800-1,200 | $1,200-2,000 | $1,500-2,500 |
| 10,000W+ | $1,200-2,000 | $2,000-3,500 | $2,500-4,500 |
Popular 2026 Brands by Price Tier:
| Tier | Brands | Warranty | Features |
|---|---|---|---|
| Budget | VEVOR, BestSun, Power Queen | 1-2 years | Basic PSW, manual transfer |
| Mid-Range | Renogy, SunGold, AMPINVT | 2-3 years | Built-in charger, LCD display |
| Premium | Victron, Outback, Schneider | 5-10 years | Remote monitoring, programmable |
| Top-Tier | Sol-Ark, EG4, Growatt | 10 years | Hybrid, MPPT, smart load management |
Key 2026 Advancements:
- LiFePO4-native charging profiles (no adapter needed)
- 97-98% peak efficiency (up from 90-93% in 2020)
- Built-in Wi-Fi monitoring on mid-range models
- Split-phase 240V output for well pumps and AC units
Pure Sine vs Modified Sine Wave
The waveform type affects device compatibility, efficiency, and cost:
Pure Sine Wave (PSW):
- Produces smooth, utility-grade power (THD <3%)
- Required for sensitive electronics
- Motors run cooler and quieter
- Maximum efficiency
- Now only 20-30% more expensive than modified sine
Modified Sine Wave (MSW):
- Produces stepped, approximate sine wave (THD 25-40%)
- Works for simple resistive loads
- Causes humming in motors
- Can damage or destroy some electronics
- Use only for budget/temporary applications
Device Compatibility Matrix:
| Device Category | Pure Sine | Modified Sine | Notes |
|---|---|---|---|
| CPAP/Medical | Required | Will fail | Medical certification requires PSW |
| Laser Printers | Required | Will damage | Internal power supplies fail on MSW |
| Variable Speed Tools | Required | May not work | Speed controllers malfunction |
| Desktop Computers | Required | May work* | PSU stress and potential failure |
| Laptops | Required | May work* | Charging inefficiency, heat |
| Gaming Consoles | Required | May work* | Risk of damage |
| Refrigerators | Required | Works (inefficient) | 10-30% efficiency loss, noise |
| Microwave Ovens | Required | Works (inefficient) | Reduced power, longer cook times |
| Window AC | Recommended | Works (less efficient) | Compressor strain |
| Power Tools (AC) | Recommended | Works | Motor heating |
| Incandescent Lights | Either | Either | No difference |
| Phone Chargers | Either | Either | Slight efficiency loss on MSW |
| Fans (simple) | Either | Either | May hum slightly on MSW |
| Battery Chargers | Either | Either | Slower charging on MSW |
*Modified sine may reduce power supply lifespan and void warranties.
The 2026 Recommendation: Pure sine wave prices have dropped 40-50% since 2020. The price difference no longer justifies the risks of modified sine wave for any application except emergency backup or very simple loads.
Battery Voltage and Current Draw
Higher battery voltage means lower current draw and smaller wires:
Current Draw Formula: Amps (DC) = Watts (AC) ÷ (Battery Voltage × Inverter Efficiency)
Current Draw Comparison (at 95% Efficiency):
| Load | 12V System | 24V System | 48V System |
|---|---|---|---|
| 500W | 44A | 22A | 11A |
| 1,000W | 88A | 44A | 22A |
| 2,000W | 175A | 88A | 44A |
| 3,000W | 263A | 131A | 66A |
| 5,000W | 439A | 219A | 110A |
| 10,000W | 877A | 439A | 219A |
Wire Sizing Requirements (6-foot run to inverter):
| Current | Min Wire (Cu) | Wire Cost/ft | 6ft Total |
|---|---|---|---|
| <50A | 6 AWG | $1.00-2.00 | $12-24 |
| 50-100A | 2 AWG | $2.50-4.50 | $30-54 |
| 100-150A | 1/0 AWG | $4.00-7.00 | $48-84 |
| 150-200A | 2/0 AWG | $5.50-9.00 | $66-108 |
| 200-250A | 3/0 AWG | $7.00-11.00 | $84-132 |
| 250-300A | 4/0 AWG | $8.50-14.00 | $102-168 |
| 300-400A | Parallel runs | $17.00-28.00 | $204-336 |
Voltage Selection Guidelines:
- 12V: Systems under 1,500W. Best for RV/marine compatibility with existing 12V equipment.
- 24V: Systems 1,500-4,000W. Good balance of compatibility and efficiency. Common for mid-size RV/cabin systems.
- 48V: Systems over 3,000W. Most efficient, smallest wires, lowest cost per kWh. Required for large systems, standard for home backup.
Critical Note: Match inverter voltage to your battery bank voltage exactly. A 48V inverter will not work with 12V batteries and could be damaged or cause a fire if connected incorrectly.
Motor Startup Surge Management
Motors draw significantly more power when starting. This "inrush current" or "locked rotor current" is the most common cause of inverter shutdown.
Typical Surge Multipliers:
| Motor Type | Running Watts | Surge Multiple | Peak Watts |
|---|---|---|---|
| Refrigerator | 100-200W | 3x | 300-600W |
| Chest Freezer | 60-100W | 3-4x | 240-400W |
| Window AC (5,000 BTU) | 500W | 3-5x | 1,500-2,500W |
| Window AC (12,000 BTU) | 1,200W | 3-5x | 3,600-6,000W |
| RV AC (15,000 BTU) | 1,800W | 5-7x | 9,000-12,600W |
| Central AC (2-ton) | 2,400W | 4-6x | 9,600-14,400W |
| Well Pump (1/2 HP) | 600W | 3x | 1,800W |
| Well Pump (1 HP) | 1,000W | 3x | 3,000W |
| Sump Pump | 400-800W | 3x | 1,200-2,400W |
| Circular Saw | 1,200W | 2-3x | 2,400-3,600W |
| Table Saw | 1,800W | 2-3x | 3,600-5,400W |
| Air Compressor | 1,500W | 3-4x | 4,500-6,000W |
Surge Management Strategies:
1. Soft Start Devices ($300-600) Reduce AC compressor starting current by 65-75%:
| Product | Application | Price | Current Reduction |
|---|---|---|---|
| MicroAir EasyStart 364 | RV AC 13-15k BTU | $350-420 | 65-70% |
| MicroAir EasyStart Breeze | RV AC (weatherproof) | $380-450 | 70-75% |
| MicroAir EasyStart Flex | Home AC to 6 tons | $500-650 | 70-75% |
2. Load Sequencing Start motors one at a time. Wait 3-5 seconds between each startup to let the previous motor reach running speed.
3. Generator Start Mode Many inverters have a "generator" or "motor load" mode that provides higher surge for 1-2 seconds. Enable this when running motors.
4. Larger Inverter Some inverters have 2x or 3x surge ratings (e.g., 5,000W continuous / 15,000W surge). Check surge duration specs carefully—some only sustain peak for 0.1 seconds.
Inverter Types and Features
Modern inverters come in several configurations. Understanding the differences helps you choose the right one:
Inverter Types:
| Type | Best For | Price Range | Key Features |
|---|---|---|---|
| Basic PSW Inverter | Simple loads | $80-300 | DC→AC only, no charging |
| Inverter-Charger | Off-grid, RV | $300-1,500 | Built-in battery charger, transfer switch |
| Split-Phase | Homes, 240V loads | $800-4,500 | 120V + 120V = 240V output |
| Hybrid Inverter | Solar systems | $1,000-4,000 | Built-in MPPT, grid-tie capable |
| All-in-One | Complete systems | $2,500-8,000 | Inverter + MPPT + charger + monitoring |
Key Features to Consider:
Transfer Switch:
- Manual: You flip a switch to change from grid to inverter ($0-50 extra)
- Automatic (ATS): Switches automatically in 10-20ms when grid fails ($100-300 extra)
- No-break: Switches in <16ms, suitable for computers ($200-500 extra)
Battery Charger:
- Multi-stage charging: 3-stage (bulk, absorption, float) minimum
- Equalization mode: Required for flooded lead-acid
- LiFePO4 profile: Essential if using lithium batteries
- Adjustable parameters: Set voltage, current, and timing
Monitoring and Control:
- LCD Display: Shows load, battery voltage, charging status
- Remote Panel: Wall-mount control from inside the home
- Wi-Fi/App: Monitor via smartphone, send alerts
- RS485/Modbus: For advanced energy management systems
2026 Hybrid Inverter Features: Modern hybrid inverters (Sol-Ark, EG4, Growatt) include:
- Built-in 100-200A MPPT solar charge controller
- Grid-tie capability with anti-islanding
- Time-of-use programming (charge from grid at night)
- Generator auto-start signals
- Smart load shedding
- Parallel stacking for larger systems
Inverter Efficiency and Operating Costs
Inverter efficiency directly impacts your battery capacity requirements and long-term operating costs.
Efficiency Curve Explained: Inverters are least efficient at very low and very high loads. Peak efficiency occurs at 40-75% of rated capacity.
Typical Efficiency by Load:
| Load % | Budget Inverters | Mid-Range | Premium |
|---|---|---|---|
| 10% | 70-80% | 80-85% | 85-90% |
| 25% | 82-88% | 88-92% | 92-95% |
| 50% | 86-92% | 92-95% | 95-97% |
| 75% | 88-93% | 93-96% | 96-98% |
| 100% | 85-90% | 90-94% | 93-96% |
No-Load Power Consumption: Even with zero load, inverters consume power:
| Inverter Size | Budget | Mid-Range | Premium |
|---|---|---|---|
| 1,000W | 10-20W | 5-12W | 3-8W |
| 2,000W | 15-30W | 8-18W | 5-12W |
| 3,000W | 20-40W | 12-25W | 8-15W |
| 5,000W | 30-60W | 18-35W | 12-22W |
| 10,000W | 50-100W | 30-55W | 20-35W |
Cost Comparison Example: A 5,000W inverter running 24/7 at average 30% load (1,500W):
| Inverter Type | Efficiency | Battery Draw | Annual Loss |
|---|---|---|---|
| Budget (88%) | 1,705W | 205W | 1,796 kWh |
| Mid-Range (93%) | 1,613W | 113W | 990 kWh |
| Premium (96%) | 1,563W | 63W | 552 kWh |
At $0.15/kWh equivalent battery cost, the annual efficiency savings from premium vs budget is (1,796 - 552) × $0.15 = $186/year. A $300-500 premium inverter can pay for itself in 2-3 years through efficiency alone.
Standby/Search Mode: Quality inverters have a "search" or "sleep" mode that pulses power briefly to detect loads, dropping standby consumption to 1-5W. This can save 50-100W continuously when the system is idle.
Installation Best Practices
Proper installation is critical for safety, efficiency, and inverter longevity.
Location Requirements:
- Ventilation: Allow 6+ inches clearance on all vented sides
- Temperature: Keep ambient below 95°F (35°C); derate capacity above this
- Moisture: Indoor/protected location; outdoor-rated enclosure if exposed
- Access: Easy access for maintenance and emergency disconnect
DC Side Wiring:
- Cable length: Keep battery-to-inverter cables as short as possible (3-6 feet ideal)
- Cable sizing: Use manufacturer specifications minimum; go larger for long runs
- Voltage drop: Maximum 3% drop at full load (use voltage drop calculator)
- Fusing: Install fuse within 18 inches of battery positive terminal
Fuse Sizing Guide:
| Inverter Size | 12V Fuse | 24V Fuse | 48V Fuse |
|---|---|---|---|
| 1,000W | 150A | 80A | 40A |
| 2,000W | 250A | 125A | 70A |
| 3,000W | 400A | 200A | 100A |
| 5,000W | 600A | 300A | 150A |
Grounding:
- Connect inverter chassis ground to system ground
- Follow NEC Article 690 for solar systems
- Use ground fault protection on DC circuits
- Bond neutral to ground at one point only (typically inverter or main panel)
AC Output Wiring:
- Use appropriately sized breakers for each circuit
- Install GFCI protection for outdoor and bathroom circuits
- Separate critical loads from high-inrush loads
- Consider sub-panel for generator/inverter backup circuits
Common Installation Mistakes:
- Undersized cables: Causes voltage drop, heat, and shutdowns
- Missing fuse: Fire hazard if short circuit occurs
- Poor ventilation: Leads to thermal shutdown and reduced lifespan
- Long cable runs: Increases losses and can prevent startup
- Wrong voltage: Connecting 12V batteries to 48V inverter = disaster
Sizing for Specific Applications
Different applications have different inverter requirements:
RV/Camper Systems:
| RV Type | Typical Load | Recommended | Voltage |
|---|---|---|---|
| Van/Small Trailer | 500-1,000W | 1,500W PSW | 12V |
| Travel Trailer | 1,000-2,000W | 2,500-3,000W PSW | 12V or 24V |
| 5th Wheel (no AC) | 1,500-2,500W | 3,000W PSW | 24V |
| 5th Wheel (with AC) | 3,000-5,000W | 5,000W + soft start | 24V or 48V |
| Class A RV | 4,000-8,000W | 6,000-10,000W | 48V |
Off-Grid Cabin:
| Cabin Size | Essential Loads | Full Comfort | Voltage |
|---|---|---|---|
| Small (400 sq ft) | 2,000W | 3,000W | 24V |
| Medium (800 sq ft) | 3,000W | 5,000W | 48V |
| Large (1,500+ sq ft) | 5,000W | 8,000-10,000W | 48V |
Home Backup (Essential Circuits):
| Coverage | Loads | Recommended | Notes |
|---|---|---|---|
| Critical Only | Fridge, lights, phone charging | 2,000-3,000W | Minimal battery needed |
| Partial Home | + Well pump, TV, computer | 5,000-6,000W | Add 10-20kWh battery |
| Most Loads | + Microwave, small AC | 8,000-10,000W | 20-40kWh battery |
| Whole Home | + Central AC, electric range | 15,000-25,000W | 50+ kWh battery, 48V |
Marine Applications:
| Boat Type | Typical Needs | Recommended |
|---|---|---|
| Small Sailboat | Lights, instruments, chargers | 600-1,000W @ 12V |
| Cruising Sailboat | + Refrigeration, electronics | 2,000-3,000W @ 12V/24V |
| Power Boat | + AC, watermaker, galley | 3,000-5,000W @ 24V |
| Large Yacht | Full amenities | 5,000-15,000W @ 48V |
Troubleshooting Common Issues
Knowing how to diagnose inverter problems saves time and money:
Inverter Shuts Down Under Load:
| Symptom | Likely Cause | Solution |
|---|---|---|
| Immediate shutdown | Surge overload | Check motor loads, add soft start |
| Shutdown after 5-30 sec | Continuous overload | Reduce load or upsize inverter |
| Random shutdowns | Low battery voltage | Check battery SOC and connections |
| Shutdown when hot | Thermal protection | Improve ventilation, reduce load |
Low Voltage Warnings:
| Battery Type | Warning Level | Shutdown Level | Healthy Voltage |
|---|---|---|---|
| 12V Lead-Acid | 11.5V | 10.5V | 12.4-12.8V |
| 12V LiFePO4 | 12.0V | 10.0V | 13.0-13.4V |
| 24V Lead-Acid | 23.0V | 21.0V | 24.8-25.6V |
| 24V LiFePO4 | 24.0V | 20.0V | 26.0-26.8V |
| 48V Lead-Acid | 46.0V | 42.0V | 49.6-51.2V |
| 48V LiFePO4 | 48.0V | 40.0V | 52.0-53.6V |
Output Voltage Issues:
| Problem | Possible Cause | Fix |
|---|---|---|
| Low AC voltage | Overload, long cables | Reduce load, shorten wiring |
| Fluctuating voltage | Varying load, weak battery | Check battery health |
| No output | Blown fuse, tripped breaker | Check protection devices |
| Wrong voltage (240V vs 120V) | Incorrect wiring or settings | Verify configuration |
Strange Noises:
| Noise | Source | Action |
|---|---|---|
| High-pitched whine | Normal transformer/inductor noise | Usually harmless |
| Buzzing/humming | Possible ground loop | Check grounding |
| Clicking | Search mode or fault retry | Normal unless continuous |
| Loud fan | Thermal management | Check ventilation, clean dust |
Pro Tips
- 💡Always size inverters 25-30% above your calculated continuous load for headroom and future expansion—undersizing is the most common and costly mistake.
- 💡Start motor loads one at a time with 3-5 seconds between each to avoid stacking startup surges that can trip even adequately-sized inverters.
- 💡Pure sine wave inverters are worth the modest extra cost for any system—modified sine wave savings rarely justify the risks to equipment and efficiency.
- 💡Use thick, short cables between battery and inverter (3-6 feet ideal). Voltage drop from undersized or long cables causes shutdowns under heavy load.
- 💡Consider inverter-charger combos if you have shore power or generator backup—they simplify the system and often cost less than separate components.
- 💡Match inverter voltage to expected system growth. If you might expand beyond 3,000W, start with 48V to avoid replacing everything later.
- 💡Install a soft-start device on any AC compressor before buying a larger inverter—$350-450 for soft-start vs $500-1,000+ for inverter upgrade.
- 💡Check the inverter surge duration, not just peak wattage. A 10,000W surge for 0.1 seconds is nearly useless; 5 seconds or more is needed for motor startup.
- 💡Keep the inverter cool—every 10°C (18°F) above optimal temperature roughly halves component lifespan. Ensure adequate ventilation.
- 💡Install DC fusing within 18 inches of the battery positive terminal, sized at 125-150% of the maximum inverter DC input current.
- 💡Use the search/sleep mode if available—it can reduce standby power consumption from 30-50W down to 1-5W when no loads are active.
- 💡For hybrid systems, consider inverters with built-in MPPT controllers—they reduce wiring complexity and often provide better solar/battery integration.
Frequently Asked Questions
Most homes need a 3,000-10,000W inverter for backup power. Calculate your essential loads: refrigerator (150-200W), lights (100-300W), well pump (750-1,500W), and electronics (500-1,000W). Add up running watts and multiply by 1.25 for your minimum size. For whole-home backup with central AC, you may need 10,000-15,000W or more. If including a 2-ton AC unit (needs 10,000-15,000W surge), consider a 10,000W+ inverter or install a soft-start device on the AC.
