If your inverter shuts down when the air conditioner starts, the most likely explanation is not that the inverter is too small. It is that the voltage at the inverter’s own terminals collapsed below its low-voltage cutoff for a few hundred milliseconds, and the inverter did exactly what it was designed to do.
This matters because the standard advice — buy a bigger inverter — fixes the problem perhaps a third of the time and costs money, weight and idle power the other two thirds. A 3000W inverter fed by the same undersized cable will trip on the same load, only now it will also draw 25W continuously doing nothing.
So the sizing question has two halves, and almost all published guidance only answers the first.
Half one: continuous rating
Start with the boring part. Add up everything that could plausibly run simultaneously through the inverter and take the total in watts. Not everything you own — everything that runs at once.
A realistic simultaneous set for most rigs looks like: microwave (1,000–1,500W) or induction hob (1,200–1,800W) or air conditioner, plus a baseline of laptop, lights and fridge (150–300W). You are rarely running the microwave and the hob and the AC together, and if you are, you have a generator problem, not an inverter problem.
That gives most people a continuous requirement between 1,200W and 2,000W. If you have not done the underlying load audit, do that first — the load calculation guide walks through it properly, and the numbers it produces feed everything else in your system.
Two adjustments to the raw total:
Power factor. Inverters are rated in watts, but they supply volt-amps. Motors, compressors and some electronics draw more VA than W — a 0.7 power factor load rated at 700W is asking the inverter for 1,000VA. Manufacturers vary in how honest they are about this. Add 20 percent headroom on motor-heavy loads and you have covered it.
Efficiency and heat. A quality inverter runs 88–93 percent efficient at moderate load and derates at high ambient temperature. Continuous ratings are usually quoted at 25°C. In a sealed compartment in Arizona in July, a “2000W continuous” inverter may be a 1,600W inverter.
Half two: surge, which is where sizing actually gets decided
Anything with a motor draws far more current at the instant of starting than it does while running. The rule of thumb is 3–7× running current, and the specific number that matters for compressors is locked rotor amps (LRA) — the current drawn at the moment the rotor is stationary and the motor is trying to break it free.
A 13,500 BTU RV air conditioner runs at roughly 12–13A at 120V. Its LRA is typically 50–60A. That is a momentary demand of 6,000–7,200VA from an inverter you sized at 2,000W.
Other common offenders:
| Load | Running | Typical surge |
|---|---|---|
| 13,500 BTU rooftop AC | 1,400–1,600W | 6,000–7,200VA |
| 15,000 BTU rooftop AC | 1,600–1,900W | 7,000–8,500VA |
| Compressor fridge (12V) | 45–60W | 150–250W |
| Residential fridge (120V) | 100–150W | 600–1,200W |
| Water pump | 60–100W | 200–350W |
| Microwave (1,000W rated output) | 1,400–1,700W draw | Minimal — resistive plus magnetron |
| Induction hob | 1,200–1,800W | Minimal |
| Corded power tools | 500–1,500W | 2–3× running |
| Air compressor | 800–1,500W | 3,000–5,000VA |
Note what is not on the surge list. Microwaves, induction hobs, kettles and heaters are effectively resistive. They draw a lot, continuously, and they are easy to size for. Motors are the entire problem.
Surge ratings are not comparable between manufacturers
Every inverter publishes a surge figure. Almost none of them publish it on the same terms.
The number you need is not the peak watts — it is peak watts for how long. A unit advertising “4000W surge” might sustain that for 20 milliseconds, or for 5 seconds. A compressor start takes roughly 0.5–2 seconds to get past its inrush. A 20ms surge rating is irrelevant to it.
Look for a surge specification stated with a duration. Reputable manufacturers publish curves or at least a figure like “2× continuous for 3 seconds”. Where the datasheet gives a bare peak number with no time, assume it is a marketing figure and discount it.
This is also where the low-frequency versus high-frequency distinction earns its keep. Low-frequency inverters use a heavy iron transformer and typically sustain 2–3× their continuous rating for several seconds. High-frequency inverters are lighter, cheaper and more efficient at part load, but surge capability is generally shorter and shallower. For motor starting, the heavy old-fashioned design still wins.
The part everyone gets wrong: it is usually the battery side
Here is the failure sequence in a real trip event.
The compressor calls for 60A at 120V. On the DC side at 12V, allowing for inverter efficiency, that is roughly 600A flowing from the battery for a second or so. That current passes through the battery cable, the main fuse, the terminals, the busbar and the shunt — twice, out and back.
If that path has 12 milliohms of total resistance, you lose 7.2V. The inverter’s low-voltage cutoff, typically around 10.5V, is reached long before the compressor gets moving. The inverter shuts down. You interpret this as “not enough inverter.”
Twelve milliohms sounds like a lot until you measure it. A 15-foot one-way run of 6 AWG is about 12.3 milliohms of copper alone, before terminals, fuse and crimps. That is not an exotic mistake — it is a common one, because 6 AWG is entirely adequate for the running load and only fails at surge.
The diagnostic is straightforward and takes five minutes:
- Put a meter directly on the inverter’s DC input terminals — not the battery, not the busbar.
- Have someone start the offending load.
- Watch the minimum voltage.
If the battery terminals hold 12.6V while the inverter terminals drop to 10.4V, your inverter is fine and your cable is not. If both sag together, the battery or its BMS is the constraint.
Three battery-side constraints produce the same symptom:
Cable resistance. Fixable, cheap in absolute terms, and the most common cause. Correct sizing by current and run length is tabulated in the DC wire gauge reference.
BMS current limit. Drop-in LiFePO4 batteries have a BMS with a maximum continuous discharge current, commonly 100A or 200A per battery. Two 100Ah batteries with 100A BMS limits give you 200A, and 200A at 12V is about 2,400W — before surge. The BMS will cut out, hard, and the inverter reports a fault it did not cause. Check the BMS spec before blaming anything else.
Bank capability. Lead-acid banks sag substantially under high current. A 200Ah flooded bank asked for 600A will drop well over a volt on internal resistance alone. This is one of the concrete advantages of lithium that rarely gets mentioned in the chemistry comparison: it holds voltage under load.
Soft starters change the sizing conversation entirely
A soft start device limits the inrush current to an air conditioning compressor, typically reducing LRA from 50–60A to around 20–25A. That takes the surge demand from roughly 7,000VA to roughly 2,800VA.
The consequence is direct: with a soft start fitted, a well-installed 2000W inverter on adequate cable can start a 13,500 BTU air conditioner. Without one, a 3000W inverter often cannot.
At $250–$350 installed, a soft starter is almost always cheaper than the inverter upgrade it replaces, and it reduces stress on the compressor as a side effect. If air conditioning is the reason you are considering a larger inverter, fit the soft start first and re-test. Whether running AC on battery makes sense at all — separate question, different arithmetic — is covered in running air conditioning on battery.
Pure sine versus modified sine, briefly and without drama
This question gets more attention than it deserves and the answer has not changed in a decade: buy pure sine wave.
A modified sine wave inverter produces a stepped square wave approximating mains AC. It is cheaper to build and slightly more efficient at part load. It also runs motors hotter and less efficiently, makes audible buzz in audio equipment, upsets some variable-speed appliances, and is rejected outright by certain electronics — CPAP machines with humidifiers, some microwaves’ control boards, induction hobs, and a number of modern battery chargers.
The price gap that once justified modified sine has largely closed. A reputable 2000W pure sine unit sits at $290–$700 against perhaps $180–$300 for modified sine at the same rating. Saving $150 to introduce a permanent compatibility question into a system you are building for a decade is not a good trade, and it is a worse one on a motor load, where modified sine both reduces starting torque and increases heating in the winding.
The only defensible case for modified sine is a dedicated inverter serving nothing but resistive loads — a kettle, a heater, a corded drill — where you know the full load list and it will not change. That is a narrower set of circumstances than most buyers think.
Where you should apply scepticism instead is the rating rather than the waveform. Budget inverters routinely advertise a continuous figure that the unit will hold for perhaps twenty minutes before thermal derating cuts it. If a 3000W inverter costs $180, the 3000W is a peak figure with an optimistic ambient assumption attached.
Sizing scenarios: what most rigs actually need
| Use pattern | Largest load | Recommended continuous | Notes |
|---|---|---|---|
| Laptops, phones, small electronics only | 200W | 300–600W | Consider DC outlets instead and skip the inverter |
| Above plus occasional coffee machine or blender | 1,000W | 1,000–1,500W | Resistive loads, minimal surge |
| Microwave and induction hob, no AC | 1,800W | 2,000W | Sizing driven by continuous, not surge |
| 13,500 BTU AC with soft start | 1,600W running, ~2,800VA surge | 2,000W | 2/0 cable, short run, verify BMS limit |
| 13,500 BTU AC without soft start | 1,600W running, ~7,000VA surge | 3,000W low-frequency | Fit the soft start instead — cheaper |
| Corded power tools, workshop use | 1,500W | 2,000–3,000W | Low-frequency preferred for repeated surge |
Why oversizing is not free
The reflex answer to any inverter question is “buy bigger.” There is a standing cost to that.
Inverters draw power to exist. No-load consumption scales roughly with size: a 1000W unit might idle at 6–10W, a 2000W unit at 12–20W, a 3000W unit at 18–35W. At 25W idle, an inverter left on continuously consumes 600Wh per day — that is a third of a typical 200Ah lithium bank, spent on nothing.
Most inverters have a search or standby mode that drops idle draw to 2–5W and wakes on load. It works well for larger appliances and unreliably for small ones — phone chargers and LED devices often fall below the wake threshold, so the inverter cycles or simply never wakes.
Efficiency also falls off at low load. A 3000W inverter running a 60W laptop charger is operating at 2 percent of rating, where conversion efficiency is poor. If most of your 120V use is small electronics, a smaller dedicated inverter plus DC outlets for everything that can run on DC is a better system than one large inverter doing everything.
The sizing recommendation
Working method, in order:
- Total your simultaneous continuous load. Add 20 percent for power factor and heat derating.
- Identify your largest motor load and find its LRA. It is on the nameplate or in the service manual. If it is a rooftop AC, assume 50–60A for 13.5k BTU and 60–70A for 15k unless you have better data.
- Decide on a soft start. If yes, divide the surge requirement by roughly 2.5.
- Choose an inverter whose surge rating exceeds the resulting figure at a stated duration of at least 2 seconds. Not a bare peak number.
- Size the DC cable for surge current, not running current. This is the step that gets skipped.
- Confirm the BMS discharge limit exceeds surge current on the DC side.
The inverter surge sizing tool runs this sequence and shows the working, including the DC-side current your cable has to carry.
For most rigs, the honest answer is a 2000W inverter, a soft start on the air conditioner, and 2/0 cable on a short run. That combination costs less than a 3000W inverter alone and works better.
One closing note on the paperwork: a hard-wired inverter installation is a permanent electrical modification to the vehicle. Insurers treat self-installed high-current electrical work differently from factory equipment, and the difference surfaces at claim time rather than at quote time. The guide to insuring a custom or converted vehicle covers what to declare and why agreed-value cover matters for a rig with significant owner-installed systems.