Inverter Sizing Guide - VA Rating Calculator | SolarRatio
Calculate the right inverter size for your solar system. Enter total load wattage and usage patterns to find recommended inverter VA rating.
Inverter sizing matches the converter to your actual AC load profile — both continuous wattage and motor surge — without overbuilding. Pick too small and the inverter shuts down when the AC, well pump, or microwave kicks on. Pick too large and you pay for capacity you never use while suffering elevated idle losses that quietly drain the battery 24/7. This tool computes a VA rating that accounts for power factor, surge multiplier, simultaneity factor, and altitude derate. For US grid-tied or hybrid systems, choose a UL 1741-SB listed inverter to satisfy NEC Article 705 interconnection requirements and qualify for utility anti-islanding compliance — a prerequisite in most US states for net metering approval.
How it Works
Sum the running wattage of every device that might run simultaneously, then apply a simultaneity factor (typically 0.6–0.8 for residential, 1.0 for off-grid critical loads). Identify the single largest motor load and apply a 3–7x surge multiplier — this surge determines the inverter's peak rating, not the continuous. Divide by power factor (typically 0.8 for mixed loads) to convert from W to VA. Account for altitude (derate 1% per 300 m above 1,500 m) and ambient temperature (derate 0.5%/°C above 40°C). For pure-sine vs modified-sine, always specify pure-sine for inductive motors, medical equipment, and modern electronics. The tool reports both continuous VA and surge VA, plus recommended DC input voltage and cable size.
Usage Scenarios
Off-grid home installers spec a 6,000 W continuous / 12,000 W surge 48V pure-sine inverter to start a 1.5 HP well pump alongside a running refrigerator and HVAC blower. RV and van builders pick a 2,000–3,000 W inverter sized to run an induction cooktop and Starlink without browning out USB-PD laptop chargers. Mobile workshop owners size a 5,000 W inverter to run a 110V table saw, dust collector, and battery chargers simultaneously from a 24V LiFePO4 bank. Emergency backup designers pair a 3,000 W transfer-switch inverter with a 10 kWh battery to run furnace, fridge, lights, and modem during multi-day outages. A Florida homeowner pairing a 5 kW UL 1741-SB inverter with a 10 kWh LiFePO4 ESS can maintain Florida's 1:1 retail net metering credit while having whole-home backup through hurricane outages. Marine installations spec dual stacked 24V inverters with 240V split-phase output for galley appliances and a tankless water heater.
Frequently Asked Questions
What is the difference between pure sine wave and modified sine wave inverters?
Pure sine wave inverters produce clean AC power identical to grid power, safe for all electronics. Modified sine wave inverters are cheaper but can damage sensitive electronics, motors, and medical equipment. Always use pure sine wave for computers, TVs, and appliances with variable speed motors.
Why do motors need surge power at startup?
Electric motors (refrigerators, pumps, AC units) draw 3–7× their running wattage for 1–3 seconds at startup. A 500W refrigerator may need 2,000–3,500W surge capacity. Your inverter must handle this peak without shutting down.
How much headroom should I add to my inverter size?
Add 20–25% headroom above your calculated peak load. If your calculation shows 2,000W needed, choose a 2,500W inverter. This prevents overheating and extends inverter lifespan.
Can I run an air conditioner on an off-grid inverter?
Yes, but AC units are power-hungry. A 1-ton (12,000 BTU) AC draws about 1,200W running and 3,600W+ at startup. You'll need a 3,000–4,000W inverter and a large battery bank to run AC off-grid.
What is inverter efficiency and how does it affect my system?
Inverter efficiency (typically 90–95%) means some power is lost as heat during conversion. A 95% efficient inverter wastes 5% of battery power. Factor this into your daily consumption calculations.
How to Use the Inverter Sizing Guide
Enter total load wattage and simultaneous usage rate. For motor loads (refrigerators, AC, pumps), enter the largest motor load — these require 3-7x surge current at startup.
The recommended VA rating is based on the higher of continuous power need and surge power. Add 20-25% headroom to the calculated value for safety.
Use pure sine wave inverters for sensitive electronics (computers, medical devices, audio equipment). Modified sine wave inverters work for simple resistive loads like heaters and incandescent lights.