Inverter Size Calculator

Choosing the right inverter size is critical for any off-grid, RV, or backup power system. An undersized inverter will shut down or fail when you need it most, while an oversized unit wastes money and draws unnecessary standby power. Follow this systematic approach to determine your ideal inverter size.

Step 1: List your devices. Write down every electrical device you plan to power with your inverter. Include the wattage rating for each, typically found on the device label, in the manual, or by multiplying volts times amps if only those are listed.

Step 2: Identify simultaneous loads. Determine which devices will run at the same time. You do not need to size your inverter for every device you own, only those running concurrently. Group your loads into typical usage scenarios.

Step 3: Calculate surge requirements. For any device with a motor or compressor (refrigerators, air conditioners, pumps, power tools), multiply the running watts by the surge factor. Motors typically surge 2-3x at startup, while air conditioners can surge up to 6x.

Step 4: Add a safety margin. Once you have your total surge requirement, add 20-30% for safety margin. This accounts for efficiency losses, unexpected loads, and ensures your inverter runs at an efficient 70-80% capacity rather than stressed at 100%.

Step 5: Round up to standard sizes. Inverters come in standard sizes (300W, 500W, 1000W, 1500W, 2000W, 3000W, 5000W). Always round up to the next available size for reliable operation.

Every inverter has two power ratings: continuous (or running) watts and surge (or peak) watts. Understanding the difference is essential for proper inverter sizing and avoiding frustrating shutdowns.

Continuous power is the wattage an inverter can deliver indefinitely under normal operating conditions. This rating determines what loads you can run for extended periods. A 2000W continuous rated inverter can power 2000W worth of devices all day long without overheating.

Surge power is the peak wattage an inverter can deliver for a brief period, typically 1-3 seconds. Surge ratings are usually 2x the continuous rating. A 2000W continuous inverter might have a 4000W surge rating. This handles the startup spike from motors and compressors.

Why motors surge: Electric motors require a large inrush of current to overcome inertia and get the rotor spinning. Once at operating speed, current drops to the normal running level. Refrigerator compressors, air conditioner fans, well pumps, and power tools all exhibit this behavior. A refrigerator rated at 150W may draw 600W or more for those critical first few seconds.

Resistive loads are different. Heaters, incandescent lights, and toasters draw constant power without surge. A 1500W space heater draws 1500W instantly and continuously. These loads are easy to size for but can still overwhelm an inverter if combined with motor loads.

The type of power waveform your inverter produces significantly impacts which devices work properly and how efficiently they operate. This choice affects both initial cost and long-term reliability.

Pure sine wave inverters produce electricity that matches utility grid power exactly. The smooth, continuous wave is compatible with all devices and equipment. While more expensive, pure sine wave inverters are necessary for sensitive electronics, medical equipment, motors that run cooler and quieter, appliances with microprocessors, variable speed tools, and devices with timing circuits.

Modified sine wave inverters produce a stepped approximation of a sine wave. They cost 30-50% less than pure sine wave equivalents but have significant limitations. Modified sine wave works adequately for simple resistive loads like incandescent lights and basic heaters, but can cause problems with many modern devices.

Compatibility issues with modified sine wave: Motors run hotter and less efficiently, potentially shortening lifespan. Audio equipment produces a noticeable buzz or hum. Some laptop chargers and phone chargers may not work at all or charge slowly. Clocks and timers may run fast or erratically. Dimmer switches and speed controls malfunction. Medical devices like CPAP machines may not operate safely.

The bottom line: For most applications, pure sine wave inverters are worth the extra cost. They provide peace of mind, broader compatibility, and often better efficiency. Modified sine wave may be acceptable for very simple, temporary applications where budget is the primary concern.

Your inverter and battery bank must be properly matched for safe, efficient operation. An oversized inverter can overdraw batteries, while an undersized battery bank will experience excessive voltage sag and premature failure.

Current draw calculation: To find how many amps your inverter draws from batteries, divide inverter watts by battery voltage, then add 10-15% for inverter inefficiency. A 2000W inverter at 12V draws approximately 2000/12 = 167 amps, plus inefficiency = roughly 185-190 amps. This is a substantial current that requires appropriately sized batteries and cables.

Battery capacity requirements: Your battery bank should have enough amp-hour capacity to deliver the required current without excessive voltage sag. As a rule of thumb, lead-acid batteries should not be discharged faster than C/5 rate (capacity divided by 5 hours). A 200Ah lead-acid battery can safely deliver 40 amps continuously. LiFePO4 batteries can typically handle C/2 or even 1C discharge rates.

Voltage considerations: Higher voltage systems (24V or 48V) draw proportionally less current for the same wattage, allowing smaller cables and less stress on batteries. A 2000W load at 48V draws only 42 amps versus 167 amps at 12V. For inverters above 2000W, consider 24V or 48V systems.

Cable sizing: The cables connecting your inverter to batteries must handle high current without excessive voltage drop. Undersized cables cause power loss, voltage sag, and fire hazards. Use our wire gauge calculator to determine proper cable size based on current and distance.

Learning from others' mistakes can save you money and frustration. These are the most common errors made when selecting an inverter for off-grid and mobile power systems.

Mistake 1: Ignoring surge requirements. This is the most common error. People add up their running watts and buy that size inverter, then wonder why it keeps shutting down. Always account for surge loads, especially from refrigerators and air conditioners.

Mistake 2: Buying modified sine wave to save money. The savings disappear when devices malfunction, run inefficiently, or fail prematurely. Pure sine wave is almost always worth the investment for anything beyond the most basic temporary applications.

Mistake 3: Massively oversizing the inverter. While some headroom is good, a 5000W inverter for 500W of loads is wasteful. Large inverters have higher standby power consumption, often 20-50W just being on, which drains batteries even when nothing is running.

Mistake 4: Undersizing cables and fuses. The inverter is only one part of the system. Inadequate wiring causes voltage drop, inefficiency, and fire risk. Always size cables for peak current draw, not average.

Mistake 5: Not considering battery limitations. Your battery bank must supply the current your inverter demands. A small battery bank with a large inverter will experience severe voltage sag and shortened battery life.

Mistake 6: Forgetting about efficiency losses. Inverters are typically 85-95% efficient. A 1000W load actually draws 1050-1175W from your batteries. Factor this into both battery sizing and solar panel calculations.

Seeing how inverter sizing works in practice helps clarify the decision-making process. Here are typical configurations for common use cases.

Weekend camper van: Running LED lights (20W), phone chargers (15W), laptop (60W), and a 12V compressor fridge with separate circuit. Total AC loads: 95W continuous. A 300W pure sine wave inverter provides plenty of headroom for these light loads. The 12V fridge runs directly from the battery, bypassing the inverter entirely for efficiency. This setup pairs well with a 100-200Ah LiFePO4 battery.

Full-time RV living: LED lights (30W), entertainment center (150W), laptop (60W), 120V residential fridge (150W, 450W surge), occasional microwave (1000W), and phone chargers (20W). Maximum simultaneous load with microwave: 1410W continuous, 1710W surge. A 2000W inverter handles this comfortably. Without microwave running: 410W continuous. This setup needs at least 200Ah at 12V or 100Ah at 24V.

Off-grid cabin: LED lighting throughout (100W), refrigerator (200W, 600W surge), laptop and internet equipment (100W), well pump when needed (750W, 2250W surge), and occasional power tools. The well pump dominates sizing: 2850W surge when pump runs with other loads. A 3000W inverter is the minimum, but 4000-5000W provides headroom for future expansion. This system typically runs at 24V or 48V with 400-800Ah of battery capacity.

Off-grid home: All standard household loads including multiple refrigerators, washing machine (500W, 1500W surge), window AC units, well pump, and various electronics. Total peak simultaneous load can easily reach 5000-8000W. These systems typically require split-phase 5000W+ inverters at 48V, with battery banks of 10-20kWh or more. Professional system design is recommended for this scale.

Always size your inverter for peak surge loads, not just continuous loads. Motors, compressors, and pumps can draw 2-3x their rated wattage at startup. This momentary demand will shut down an undersized inverter.

Pure sine wave inverters are recommended for sensitive electronics, motors, and medical equipment. Modified sine wave inverters are cheaper but can cause issues with many modern devices including laptops, TVs, and anything with a microprocessor.

Efficiency matters — inverters are typically 85-95% efficient. A 1000W load actually draws about 1050-1175W from your batteries. Factor this into battery sizing and solar panel calculations for proper system design.

Consider standby draw. All inverters consume some power just being on, typically 10-50W depending on size. For systems where the inverter runs 24/7, this can add up to significant battery drain. Some users add a small secondary inverter for light loads to reduce standby losses.