An inverter calculator is an online tool that determines the correct inverter size, battery capacity, and expected runtime for your appliances. You enter your total running wattage, surge wattage, battery voltage, and desired backup time, and the calculator applies standard electrical formulas to recommend a system size. This removes the guesswork from shopping for an inverter and helps you avoid both underpowered and oversized systems. Most calculators, including ours, also factor in inverter efficiency, since real-world inverters lose some power as heat during DC-to-AC conversion. The result is a practical starting point you can use when comparing inverter models and battery banks before making a purchase.

Add up the running watts of every appliance you plan to use simultaneously, then separately note the highest surge wattage among motor-driven appliances like refrigerators or pumps. Add roughly 20% margin to your running wattage total, and compare that number to your surge wattage figure. Whichever number is larger becomes your minimum inverter size, rounded up to the next commercially available size. Our calculator above automates this entire process and also estimates the matching battery capacity you'll need for your desired runtime.

It depends on your battery voltage and how long you want to run the load. Using the formula Battery Ah = (Watts × Hours) ÷ (Voltage × Efficiency), a 1000W load running for 4 hours on a 12V battery at 90% efficiency needs roughly 370Ah. On a 24V battery, that drops to about 185Ah, and on 48V it's around 93Ah. Higher battery voltage significantly reduces the amp-hour requirement for the same energy output, which is why larger loads typically use 24V or 48V systems.

Running watts are the continuous power an appliance draws during normal operation. Surge watts, also called starting watts, are the momentary spike of power — often two to three times higher than running watts — needed to start a motor, compressor, or pump. Your inverter must be sized to handle both figures: the running watts continuously, and the surge watts for a brief moment during startup. Ignoring surge watts is one of the most common reasons an inverter shuts down unexpectedly when a refrigerator or pump kicks on.

Runtime depends on your battery's usable capacity, your battery voltage, your inverter's efficiency, and how much power your appliances are drawing. You can estimate it using Energy (Wh) = Watts × Hours, then solving for hours based on your battery's available watt-hours. Our calculator does this automatically — enter your load and battery details, and it will estimate runtime for you. Keep in mind that real-world runtime is often shorter than the theoretical number due to battery aging, temperature, and discharge-rate effects.

Pure sine wave inverters produce clean power that closely matches utility grid electricity, making them safe for sensitive electronics, motors, and medical devices. Modified sine wave inverters are cheaper but produce a choppier waveform that can cause humming, reduced efficiency, or even damage in some devices, particularly those with motors or sensitive circuitry. If your load includes a refrigerator, laptop, CPAP machine, or audio equipment, a pure sine wave inverter is the safer and more efficient long-term choice, even at a higher upfront cost.

A standard residential refrigerator typically draws around 150W of running power but can surge to 1,000–1,200W when the compressor starts. This means you need an inverter rated for at least 1,200–1,500W of surge capacity, even though the continuous load is much lower. Chest freezers and larger refrigerators can surge even higher, so it's worth checking your specific unit's nameplate rating or using our appliance table above as a reference point before sizing your system.

A 2000W inverter can typically handle a combination like a window air conditioner, a few lights, and small electronics, or a refrigerator plus microwave and lighting circuits, as long as the combined running wattage stays under roughly 1,600–1,800W to leave a safety margin. The exact number of appliances depends heavily on which specific devices you're running and whether any of them have high surge requirements. Use the appliance table on this page to add up your specific combination before finalizing your purchase.

The standard formula is Battery Ah = (Watts × Hours) ÷ (Battery Voltage × Efficiency). Multiply your load's wattage by the number of hours you need it to run to get total watt-hours, then divide by your battery voltage and your inverter's efficiency (expressed as a decimal, such as 0.9 for 90%). This gives you the theoretical amp-hour capacity needed; most professionals then add a 20% margin to account for battery aging and depth-of-discharge limits.

No. Oversizing an inverter wastes money upfront and can slightly reduce efficiency at low partial loads, since inverters are typically most efficient near 50–80% of their rated capacity. A significantly oversized inverter also draws more idle "standby" power continuously, which shortens battery runtime for the loads you actually use. The best approach is to size your inverter to your actual running and surge wattage plus a reasonable 20–30% margin for future growth, not to buy the largest unit available "just in case."

Choose your battery voltage based on your total load. Loads under 1,000W work fine on 12V systems, which use widely available and affordable components. Loads between 1,000W and 3,000W are better suited to 24V, which reduces current draw and allows thinner cabling. Loads above 3,000W should generally use 48V systems, since the lower current at higher voltage keeps cable sizes, heat, and voltage drop manageable even for large whole-home backup setups.

Inverter efficiency directly affects how much energy you draw from your battery to deliver a given amount of usable AC power. A 90%-efficient inverter needs about 11% more energy from the battery than the appliance actually uses, while an 80%-efficient inverter needs 25% more. Over a long backup period, this difference adds up significantly, which is why efficiency is a required input in accurate battery sizing calculations rather than an afterthought.

Yes, but it requires a substantial inverter, typically in the 5,000W–7,000W range or higher, because central air conditioning compressors have very high surge wattage requirements, often 2–3 times their running wattage. You'll also need a correspondingly large battery bank, usually on a 48V system, to support the load for any meaningful runtime. Many homeowners choose to pair central A/C backup with a whole-house generator instead, using the inverter system for smaller, more essential circuits.

A safety margin is extra capacity added above your calculated running wattage, typically 20%, to account for real-world variability like voltage fluctuations, appliance wear over time, and slightly underestimated loads. Without this margin, an inverter sized exactly to your calculated load may struggle or shut down under normal operating conditions. The safety margin also gives you some room to add small additional loads later without immediately overloading the system.

Current draw in amps is calculated using the formula Current (A) = Power (W) ÷ Voltage (V). For example, a 1,000W load on a 12V battery draws about 83.3 amps, while the same 1,000W load on a 24V battery draws only about 41.7 amps. This is why higher battery voltages are preferred for larger loads — the lower current requirement means thinner, less expensive cabling and reduced resistive losses.

Most CPAP machines draw between 30–60W of running power, so even a small 300W pure sine wave inverter is more than sufficient for the device itself. However, if your CPAP includes a heated humidifier, power draw can increase significantly, so checking the device's nameplate rating is important. Because CPAP machines are medical devices, a pure sine wave inverter is strongly recommended over a modified sine wave unit to avoid any risk of electrical interference or malfunction.

Both inverters and batteries lose some capacity in extreme temperatures. Very cold conditions reduce a battery's ability to deliver current efficiently, while very hot conditions can cause inverters to derate their output to avoid overheating and can accelerate battery degradation over time. For this reason, it's best to install both components in a moderate, ventilated environment, and to check manufacturer specifications for temperature-based derating if your installation location experiences temperature extremes.

A UPS (uninterruptible power supply) is typically a smaller, integrated unit designed to provide near-instant, short-duration backup power for sensitive electronics like computers, protecting against brief outages and power quality issues. A home backup inverter system is generally larger, designed for longer backup durations, and often paired with a separate battery bank sized for hours rather than minutes of runtime. Both use similar underlying DC-to-AC conversion principles, but their sizing, switching speed, and intended use cases differ significantly.

Standard off-grid inverters typically require power to come from a battery bank rather than directly from solar panels; you need a solar charge controller, ideally an MPPT controller, between the panels and the battery to regulate voltage and prevent overcharging. Hybrid inverters are designed to accept both solar input and battery storage in an integrated unit, simplifying the wiring. If you're building a solar-plus-battery system, check whether your chosen inverter is rated as "solar-ready" or hybrid before assuming direct panel connection is supported.

Nameplate wattage is often a maximum or worst-case rating rather than typical running wattage, and it can sometimes list amperage instead of watts, requiring you to multiply by voltage to get a watt figure. For the most accurate reading, use a plug-in watt meter to measure your appliance's actual running and starting draw under real conditions. This is especially useful for older appliances or motors where nameplate data may be outdated or difficult to read.

An undersized inverter will typically trigger an overload protection shutdown when demand exceeds its rated capacity, cutting power to connected appliances until it's reset. In some cases, especially with lower-quality units, repeated overload conditions can cause overheating and shorten the inverter's lifespan. If you consistently find your inverter shutting down under normal use, it's a strong sign you need to recalculate your load and move up to a larger unit rather than continuing to reset it.

If you're connecting your inverter system to your home's existing wiring rather than running extension cords to individual appliances, you need a properly installed transfer switch to prevent backfeeding power into utility lines, which is both a serious safety hazard for utility workers and illegal in most jurisdictions. A licensed electrician can install a manual or automatic transfer switch that isolates your backup circuits from the grid. This is a required step for any whole-home or partial-home backup wiring project.

Costs vary widely based on inverter size, sine wave type, and battery capacity, ranging from under $100 for a small 300W portable unit to several thousand dollars for a whole-home 7,000–10,000W system with a large lithium battery bank. Pure sine wave inverters and lithium batteries cost more upfront than modified sine wave or lead-acid alternatives but often provide better long-term value through efficiency and lifespan. Getting your sizing right before purchasing is one of the best ways to avoid paying for unnecessary capacity or having to upgrade shortly after installation.

Standard automotive starting batteries are designed for short bursts of high current, not the sustained, repeated discharge cycles that backup power systems require, and using one this way typically shortens its lifespan significantly. Deep-cycle batteries, whether AGM, gel, flooded lead-acid, or lithium, are purpose-built for repeated deep discharge and are the appropriate choice for inverter backup systems. Using the wrong battery type is a common and costly mistake among first-time backup power buyers.

Continuous rating is the wattage an inverter can sustain indefinitely under normal operating conditions, while peak (or surge) rating is the higher wattage it can handle for a brief period, often just a few seconds, to accommodate motor startup spikes. When shopping, check both numbers against your calculated running and surge wattage separately, since a unit with an adequate continuous rating but insufficient surge rating will still shut down when a motor-driven appliance starts.

Most professionals recommend testing your inverter system at least once every one to three months, running it under a real or simulated load for at least 15–30 minutes to confirm proper operation, battery charge retention, and automatic switching if applicable. Regular testing catches battery degradation, loose connections, or inverter faults before an actual outage, when you can least afford a system failure. Keep a simple log of test dates and observed runtime to track performance over time.

Standard inverter and battery sizing calculators, including the one on this page, calculate based on a battery's rated capacity when new. Batteries lose usable capacity over their lifespan due to charge cycles, age, and temperature exposure, typically retaining 70–80% of original capacity by the time replacement is recommended. Because of this, many professionals size their battery bank with additional margin beyond the calculated minimum, specifically to maintain adequate runtime as the battery ages over several years of use.