What Size Inverter Do I Need?

An inverter is the least glamorous box in any backup power system and the one most often bought wrong. People shop it the way they shop a generator, by picking a watt number that sounds sufficient, and then discover that the number on the box was never the constraint. The constraint was on the other side of the box, in the cable and the battery, and nothing on the packaging mentioned it.

Sizing an inverter properly takes about fifteen minutes and two numbers you can measure yourself. Here is the whole method, including the direct current arithmetic that decides whether the system you are imagining is buildable at the voltage you were planning to build it at.

Quick Answer

Add up the running watts of everything you need on at the same time, then multiply by 1.25 for headroom. That is your continuous rating. Separately, find the single largest starting surge in that list, add it to the running total of everything else, and confirm the inverter's surge rating covers it. For most households running outage essentials, meaning a refrigerator, a freezer, lights, network gear, phone charging, and a furnace blower, that lands at a 1,500 to 2,000 watt continuous inverter. Adding a well pump, a sump pump, or a window air conditioner pushes it to 3,000 watts or more. Then check the direct current side before you buy: a 3,000 watt inverter on a 12 volt battery bank draws roughly 290 amps, which is a cable and fuse problem most people do not anticipate, and it is the reason systems above about 2,000 watts are built at 24 or 48 volts instead.

Key Takeaways

  • Every inverter has two ratings and only one of them is on the front of the box. Continuous watts is what it will deliver indefinitely. Surge watts is what it will deliver for a handful of seconds while a motor starts. Sizing to the first and ignoring the second gives you an inverter that shuts down every time the fridge compressor kicks in.
  • Motors draw far more to start than to run. A compressor, pump, or motor typically pulls several times its running wattage for a fraction of a second as it comes up to speed. This is the single most common reason a correctly sized inverter trips.
  • The DC current is the hidden constraint, and it scales with the inverse of battery voltage. Watts divided by battery volts divided by efficiency gives you amps. At 12 volts those amps get large enough to require very heavy cable and serious fusing well before you reach the wattage most people want.
  • Higher bank voltage is the fix, and it is a design decision, not an upgrade. The same 3,000 watts that draws about 290 amps at 12 volts draws about 73 amps at 48 volts. Cable, fuses, and losses all fall by the same factor. Choosing 24 or 48 volts at the start costs nothing and choosing it later means replacing everything.
  • Idle draw matters more than efficiency in a long outage. An inverter left on consumes power doing nothing, often ten to thirty watts. Over a full day that is a few hundred watt hours of a battery bank you were counting on, spent on an empty room.
  • Pure sine wave is the default answer now. Modified sine wave is cheaper and will run resistive loads like a kettle or an incandescent bulb. It runs motors hotter, upsets some electronics, and is a bad match for anything with a microprocessor or a variable speed motor. The price gap has narrowed enough that the compromise is rarely worth making.
  • Look for UL 458 or UL 1741 listing, or an ETL or CSA equivalent. UL 458 covers mobile and vehicle-mounted inverters, UL 1741 covers stationary and grid-interactive equipment. An unlisted inverter is a device converting a battery bank into household voltage with nobody having checked its failure behaviour.

Step One: The Running Total

List what has to be on at the same time, not everything you own. This distinction does most of the work, because the honest simultaneous list is usually about a third of the list people start with.

Typical running figures, useful for planning and no substitute for the label on your own equipment:

Load Running watts Starting surge Notes
Refrigerator, modern 100 to 200 800 to 1,200 Cycles on and off, surges every cycle
Chest freezer 80 to 150 600 to 1,000 Holds cold for a long time unpowered
Gas furnace blower 300 to 600 1,000 to 1,800 The heat is gas, the fan is not
Well pump, 1/2 hp 750 to 1,000 2,000 to 3,000 Usually the largest surge in a house
Sump pump, 1/3 hp 500 to 800 1,500 to 2,500 Non-negotiable in a wet basement
LED lighting, whole house 60 to 120 None Essentially free on a modern system
Router, modem, and a laptop 60 to 150 None Small, and the first thing people want
CPAP, no humidifier 30 to 60 None Humidifier can triple it, turn it off
Microwave, 1,000 W rated 1,400 to 1,700 Minimal The rating is cooking power, not draw
Coffee maker or kettle 1,000 to 1,500 None Resistive, brief, but genuinely large

Two traps in that table. The microwave line is the one that catches people: a microwave labeled 1,000 watts delivers 1,000 watts of cooking power and draws considerably more from the wall. And the coffee maker line is a reminder that anything that makes heat is enormous compared with anything that makes light or moves data. Heating with an inverter is almost always the wrong plan.

Step Two: The Surge Check

Take the running total, remove the single largest surging item, and add that item's surge figure back in. That is the realistic worst moment: the well pump starting while everything else is already running.

Worked example, a fairly typical outage-essentials list:

  • Refrigerator, 150 W running, 1,000 W surge
  • Chest freezer, 100 W running, 800 W surge
  • LED lighting, 90 W
  • Router, modem, laptop, 120 W
  • Phone charging, 40 W
  • Gas furnace blower, 500 W running, 1,500 W surge

Running total: 1,000 watts. Multiply by 1.25 and you need at least 1,250 watts continuous, so a 1,500 watt inverter.

Worst surge moment: the furnace blower is the biggest surge at 1,500 watts. Everything else running is 500 watts. So the peak instantaneous demand is about 2,000 watts. A 1,500 watt inverter typically surges to 3,000 watts for several seconds, which covers it comfortably.

Add a half horsepower well pump to that list and the arithmetic changes character entirely. Running total goes to about 1,900 watts, so you need 2,400 watts continuous, and the surge moment becomes roughly 1,150 watts of other loads plus a 3,000 watt pump start, which is over 4,000 watts. That is a 3,000 watt inverter with a 6,000 watt surge, and it is a completely different machine and a completely different battery bank.

Step Three: The Direct Current Side, Which Is Where People Get Caught

This is the part the box does not tell you and the part that decides whether your plan is buildable.

An inverter draws its power from the battery at battery voltage. Power is volts times amps, so the lower the battery voltage, the more amps it takes to make the same watts. Efficiency, typically eighty-five to ninety-two percent for a decent unit, makes it slightly worse again.

The arithmetic is one line:

DC amps = AC watts / battery volts / efficiency

Run it on a 3,000 watt inverter at eighty-five percent efficiency and the answer depends enormously on the bank voltage you chose:

Bank voltage DC amps at 3,000 W What that means in practice
12 V About 294 A Very heavy cable, 300 A class fusing, real losses in any run over a metre
24 V About 147 A Manageable with ordinary heavy battery cable
48 V About 73 A Straightforward, and the standard for anything house-scale

Three hundred amps is not a theoretical inconvenience. It requires very large conductors, it makes every crimp and every loose terminal a heat source, and it means a short circuit anywhere in that cable is delivering the full fault current of the bank. This is why a fuse or breaker rated for the inverter's maximum draw, mounted as close to the battery as physically possible, is not optional equipment.

The rule that follows: 12 volts is fine up to roughly 1,000 to 1,500 watts. Between 1,500 and 3,000 watts, build at 24 volts. Above 3,000 watts, build at 48 volts. Choose this at the start, because the bank voltage determines the inverter, the charge controller, and the batteries, and changing your mind later means changing all three.

Step Four: The Battery Bank Behind It

An inverter is a tap. It does not hold anything. Sizing it correctly tells you what you can run, and says nothing at all about how long you can run it.

Runtime is capacity divided by draw. A 100 amp hour lithium iron phosphate battery at 12 volts holds about 1,280 watt hours and, unlike lead acid, most of that is genuinely usable. Pulling 1,000 watts through the inverter, accounting for its losses, empties it in roughly an hour. Pulling the 200 watts that a fridge and some lights actually average over a day gets you most of a day.

That gap between peak draw and average draw is the whole game. Size the inverter for the peak, size the battery for the average, and be honest that the average includes the inverter's own idle consumption. A unit drawing 20 watts at idle burns 480 watt hours over 24 hours, which is more than a third of that 100 amp hour battery spent on nothing. Switch the inverter off when you are not using it, or choose one with a search or standby mode that wakes on load.

If solar is charging that bank, How Many Solar Panels Do I Need? works through the generation side of the same system, and Generator vs. Solar: What Actually Keeps the Lights On covers the strategic choice between storing energy and making it on demand.

Pure Sine Versus Modified Sine

The grid delivers a smooth sine wave. A pure sine wave inverter reproduces it. A modified sine wave inverter approximates it with a stepped, blocky waveform that is cheaper to generate.

The Department of Energy's description of how inverters work is useful here: they switch a direct current input back and forth very rapidly, and the quality of the output depends on how faithfully that switching reconstructs the shape the load expects. Resistive loads do not care about shape, because a heating element or an incandescent filament simply converts whatever it gets into heat. Everything else cares.

Motors run hotter and less efficiently on a modified wave, because the harmonic content shows up as heat rather than rotation. Devices with microprocessors, variable speed drives, or timing circuits can behave unpredictably. Some medical equipment specifies pure sine explicitly. Certain older audio equipment will hum audibly.

Given that the price difference is now modest, the sensible default is pure sine for anything running household equipment, and modified sine only for a dedicated resistive job where you know exactly what is plugged in.

Wiring It Into a House, Which Is Its Own Problem

⚠️ SAFETY: Never connect an inverter or generator to house wiring through a cord plugged into a wall outlet. That practice, known as backfeeding, energises your home's circuits and can push voltage back through the utility transformer onto the service drop, which can kill a line worker who has every reason to believe the line is dead. It also bypasses your main breaker's overcurrent protection entirely. Any permanent connection to house circuits requires a properly installed transfer switch or interlock, installed to the National Electrical Code (NFPA 70) and inspected under your local jurisdiction's rules. Anything grid-interactive must additionally meet IEEE 1547 and the anti-islanding requirements that go with it. This is licensed electrician work in most jurisdictions, and it is one of the few places in home preparedness where doing it yourself puts someone other than you at risk.

The two honest options are these. Run extension cords from the inverter directly to the specific appliances you want powered, which is inelegant, entirely legal, and what most households actually do. Or have an electrician install a manual transfer switch feeding a small subpanel with your genuinely essential circuits on it, which is the version that works at three in the morning without anyone dragging cords through a doorway.

If the inverter is intended to work alongside solar and the grid rather than as an island, it moves into a different regulatory category. Grid-interactive inverters must disconnect when the grid goes down, so that they do not energise a line that utility crews believe is dead. That behaviour, called anti-islanding, is what IEEE 1547 and UL 1741 exist to enforce, and it is why a grid-tied solar array without a battery and a compatible inverter produces nothing during an outage, which surprises a great many new solar owners.

Matching the Inverter to the Rest of the Plan

An inverter only makes sense as part of a system, and the sizing decision reaches in both directions.

Upstream, it constrains the battery bank voltage and therefore the charge controller and the batteries themselves. Downstream, it caps what you can run, which is the same list you would work through when sizing a generator. The two exercises use identical load arithmetic and reach different conclusions, because a generator makes power continuously as long as it has fuel, while an inverter spends a fixed store. A generator's constraint is how long its fuel stays good. An inverter's constraint is how many watt hours you bought.

The combination most people converge on is a modest inverter and battery bank for the quiet loads, meaning lights, network, phones, and the refrigerator's duty cycle, plus a generator for the heavy intermittent jobs like a well pump or a laundry cycle. That arrangement runs silently most of the time, which matters more during a multi-day outage than any spec sheet suggests.

FAQ

What size inverter do I need to run a refrigerator?

A modern refrigerator runs on 100 to 200 watts but surges to somewhere between 800 and 1,200 watts each time the compressor starts. A 1,000 watt inverter with a 2,000 watt surge rating will generally handle a fridge on its own. If the same inverter has to carry lights and a router alongside it, step up to 1,500 watts so the surge headroom survives the other loads being present.

Can I run a 3,000 watt inverter on a 12 volt battery?

Electrically yes, practically it is a poor design. At eighty-five percent efficiency it draws close to 290 amps, which requires very large conductors, a fuse in the 300 amp class mounted at the battery, and careful attention to every connection because a loose terminal at that current becomes a heat source quickly. Build a 3,000 watt system at 24 or preferably 48 volts and the same power moves at 147 or 73 amps, with ordinary cable and far lower losses.

How many watts do I actually need for a power outage?

For a genuinely essentials-only list of refrigerator, freezer, lights, network gear, phone charging, and a gas furnace blower, a 1,500 watt continuous inverter with 3,000 watt surge covers it. Adding a well pump, a sump pump, or a window air conditioner roughly doubles the requirement. Adding electric cooking or electric heat takes it out of inverter territory altogether, because resistive heat is measured in kilowatts and there is no clever way around that.

Is a bigger inverter always better?

No, and there are two costs to oversizing. A larger inverter usually has a higher idle draw, so it wastes more of a limited battery bank simply being on. And it demands heavier cable and larger fusing whether or not you ever use the extra capacity. Size to the load you actually have with a sensible margin, rather than to the load you might one day imagine.

What is inverter surge rating and how long does it last?

Surge, sometimes called peak power, is the output an inverter can sustain briefly beyond its continuous rating to cover motor starting. Depending on the design and how far above continuous the demand sits, that window typically runs from a few seconds to around half a minute. It exists specifically because motors draw several times their running current for a fraction of a second as they come up to speed, and it is not a rating you should plan to use continuously.

Do I need a pure sine wave inverter?

For anything with a motor, a microprocessor, a variable speed drive, or a timing circuit, yes. That covers most modern refrigerators, furnace blowers, medical devices, laptops, and any appliance with a digital control board. Modified sine wave still works for purely resistive loads such as a kettle or an incandescent lamp, but the price gap has narrowed to the point where the compromise rarely pays for itself.

Why does my inverter shut off when the fridge starts?

Almost always a surge problem rather than a continuous power problem. The compressor's inrush current briefly exceeds what the inverter can supply, so it protects itself and shuts down. Confirm the inverter's surge rating against the appliance's starting draw, remember to add whatever else was already running, and check the DC cable and connections, because voltage sag across undersized cable during the surge produces exactly this symptom on an inverter that is nominally large enough.

Can I plug an inverter into a wall outlet to power my house?

No. That is backfeeding, and it can energise the utility line outside your home and injure or kill a line worker, in addition to bypassing your main breaker's protection. The legal and safe routes are running cords from the inverter directly to appliances, or having an electrician install a transfer switch or interlock to the National Electrical Code. This is the one part of a backup power build where the risk lands on someone other than you.

The Bottom Line

Sizing an inverter is two arithmetic problems, not one. The first is the familiar one: add the running watts, add twenty-five percent, check the biggest surge against the surge rating. The second is the one that is missing from most buying guides, and it is the one that decides whether the system works: divide those watts by the battery voltage and look honestly at the number of amps that comes out.

If that number is uncomfortable, the answer is almost never heavier cable. It is a higher bank voltage, decided before anything is bought. Twelve volts up to about 1,500 watts, 24 volts through 3,000, 48 volts above that.

Then size the battery for the day rather than the moment, turn the inverter off when nothing needs it, buy a listed unit, and never plug it into a wall socket. That is the entire discipline, and it is the difference between a system that carries you through a three day outage and a very expensive box that trips every time the refrigerator wakes up.

Go read the labels on the three appliances you would miss most. The numbers are on them. Be prepared. Stay savage.

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