RV Inverter Sizing Guide: Choosing the Right Inverter for Your Rig
You’re parked at a gorgeous off-grid campsite. No hookups, no pedestal, just you and the mountains. Your batteries are charged, the sun is out, and all you want is a cup of coffee. You flip on the coffee maker and… nothing. Or worse, something pops.
The problem isn’t your batteries — they have plenty of 12-volt DC power. The problem is that your coffee maker runs on 120-volt AC, the same kind of power that comes out of the outlets in your house. To bridge that gap, you need an inverter. But not just any inverter — the right size inverter, matched to the appliances you actually want to run and the battery bank behind it. Get it wrong in either direction and you’ll either trip the overload protection or drain your batteries before lunch.
What an Inverter Does (and Doesn’t Do)
An inverter takes 12V DC power from your batteries and converts it into 120V AC household power — the same kind of electricity you’d get from a wall outlet at home. That conversion is what lets you run a microwave, charge a laptop, or brew coffee when you’re miles from the nearest power pedestal.
One important clarification: an inverter is not a converter. A converter does the opposite — it takes 120V AC shore power and steps it down to 12V DC to charge your batteries. The names are confusingly similar, but they do opposite jobs. Also worth knowing: no inverter is 100% efficient. Most modern units operate at 85-95% efficiency, meaning 5-15% of the energy pulled from your batteries is lost as heat during conversion.
Pure Sine Wave vs Modified Sine Wave
Inverters produce AC power in one of two waveforms. A pure sine wave inverter creates a smooth, continuous wave that’s identical to grid power. A modified sine wave inverter produces a blocky, stepped approximation — close enough for some appliances, but not all.
The distinction matters because many modern electronics expect clean power. Here’s the breakdown:
Appliances that require pure sine wave:
- CPAP machines and medical devices (will not function on modified)
- Modern LED/LCD televisions
- Laptops and computers
- Microwave ovens (run hot and inefficiently on modified)
- Variable-speed motor appliances (newer fridges, coffee makers)
- Smart battery chargers with microprocessor controls
Appliances that tolerate modified sine wave:
- Incandescent lighting
- Simple power tools (drills, saws)
- Resistive heating elements (space heaters, hair dryers)
- Basic fans and pumps
Modified sine wave also runs AC motors about 20% less efficiently and generates more heat, which shortens motor lifespan. You’ll hear buzzing from audio equipment, fans, and fluorescent lights.
How to Size Your Inverter
Inverter sizing comes down to two numbers: continuous (running) watts and surge (startup) watts. Continuous watts are the steady power the inverter can deliver indefinitely — this is the primary rating to match. Surge watts are the short burst of extra power (lasting 1-5 seconds) needed to start motor-driven appliances, typically 2-3x the running watts.
Here’s the step-by-step process:
- List every appliance you plan to run at the same time
- Add up the running wattage of all those appliances
- Identify the highest-surge appliance in your list
- Add a 20-30% safety margin to account for inverter efficiency losses
- Choose an inverter with a continuous rating at or above your total and a surge rating that covers the biggest startup demand
Worked Example
Say you want to run a microwave, a TV, and a laptop charger at the same time:
| Appliance | Running Watts | Surge Watts |
|---|---|---|
| Microwave | 1,200 | 1,800 |
| TV (LED) | 200 | 200 |
| Laptop charger | 75 | 75 |
| Total | 1,475 | — |
Total running watts: 1,475W. Add a 25% safety margin: 1,844W. The microwave surges to 1,800W on startup. A 2,000W inverter covers both the running load with margin and the microwave’s surge demand.
Common Appliance Wattage Reference
| Appliance | Running Watts | Surge Watts |
|---|---|---|
| Air conditioner (13,500 BTU) | 1,400-2,000 | Up to 3,500 |
| Microwave oven | 1,000-1,500 | 1,500-1,800 |
| Coffee maker | 550-1,200 | Same (resistive) |
| Hair dryer | 500-1,500 | Same (resistive) |
| Toaster | 1,200 | Same (resistive) |
| Residential refrigerator | 600-1,000 | 1,200-2,000 |
| Television (LED/LCD) | 100-400 | Same |
| Laptop computer | 50-75 | Same |
| Electric water heater (6 gal) | 1,000-1,500 | Same (resistive) |
| Crock pot | 230 | Same (resistive) |
| Food processor | 720 | 1,000+ |
| Phone charger | 5-25 | Same |
| LED lights (total RV) | 5-50 | Same |
Want to calculate your exact load? The Arvee AmpSmart calculator lets you select your specific appliances and see total power draw instantly.
What Each Inverter Size Can Run
1,000W — The Basics. Laptops, phone chargers, LED TVs, lighting, fans, and small blenders. Good for weekenders or RVers who mostly use shore power and just want to run electronics while boondocking. Cannot run a microwave, coffee maker, hair dryer, or AC.
2,000W — The Sweet Spot. Everything above plus a microwave, coffee maker, toaster, or small induction cooktop — one high-draw appliance at a time. This is the most popular size for full-time RVers who don’t need to run air conditioning off battery power.
3,000W+ — Full Power. Everything above plus an RV air conditioner (1,400-2,000W running, up to 3,500W surge), a residential refrigerator, or an electric water heater. Required for running AC off batteries, but demands a substantial battery bank — 400+ Ah of lithium minimum for meaningful runtime.
Inverter/Charger Combos
An inverter/charger combo unit does two jobs in one box: it converts 12V DC to 120V AC when you’re off-grid (inverter mode), and it converts 120V AC shore power to 12V DC to charge your batteries when you plug in (charger mode). Most also include a built-in automatic transfer switch that detects shore power and seamlessly switches between sources — no manual intervention needed.
The transfer switch uses a “break-before-make” design: it completely disconnects the inverter before connecting shore power, preventing dangerous backfeed. Some higher-end units (Victron, Go Power) also offer a power assist feature — when plugged into limited shore power, the unit monitors draw and supplements with battery power if your RV’s demand exceeds what the pedestal can provide. This prevents tripped breakers on a 20-amp connection.
Battery Bank Sizing for Your Inverter
An inverter is only as useful as the battery bank behind it. Here’s the formula:
Battery capacity (Ah) = Inverter power (W) x Runtime (hours) / Battery voltage (V)
Example: a 1,500W load running for 2 hours on a 12V system needs 1,500 x 2 / 12 = 250 Ah.
But that’s the raw number. You need to adjust for depth of discharge (DoD) — how deeply you can safely drain your batteries before shortening their lifespan:
- Lead-acid/AGM: Only discharge to 50%. Double the calculated Ah. That 250 Ah becomes 500 Ah of AGM.
- Lithium (LiFePO4): Can discharge to 80-90%. Multiply by 1.2. That 250 Ah becomes 300 Ah of lithium.
A handy rule of thumb: on a 12V system, every 100W of inverter load draws approximately 10 amps from your battery bank.
Minimum Battery Bank by Inverter Size
| Inverter Size | Minimum Lithium Battery Bank | Minimum AGM Battery Bank |
|---|---|---|
| 1,000W | 100-200 Ah | 200-400 Ah |
| 2,000W | 200-400 Ah | 400-800 Ah |
| 3,000W | 400+ Ah | 800+ Ah |
Lithium batteries have a clear advantage for inverter use: they weigh half as much as AGM (25-30 lbs vs 65-70 lbs per 100Ah), charge 4x faster, handle high current loads far better, and deliver more usable capacity from the same rated Ah. The upfront cost is higher, but over 10 years lithium saves $3,600-$4,400 compared to repeatedly replacing AGM batteries.
The Hidden Cost: Parasitic Draw
Even with nothing plugged in, your inverter draws power just to stay on. This “parasitic” or “idle” draw ranges from 10-20W for a 1,000W inverter to 20-40W for a 2,000W unit. That doesn’t sound like much, but over 24 hours at 25W idle draw, you’re burning through 600Wh — that’s 50Ah at 12V, gone with zero actual use.
Inverters are also most efficient at moderate load (40-80% of capacity). Running a 3,000W inverter to charge a phone is wildly inefficient. Inverter design matters here too: high-frequency pure sine wave units have lower idle draw than older low-frequency transformer-based designs — something to consider if you leave your inverter on for long stretches.
Most quality inverters offer a power-saving or search mode that reduces idle draw by up to 80%. The inverter periodically pulses to check for connected loads, only fully powering on when it detects demand.
The simplest solution: turn off the inverter when you’re not using it. A remote on/off switch makes this easy — flip it off when you go to bed, flip it on when you need AC power.
Installation Essentials
Inverter installation involves high-current DC wiring — the kind that can start a fire if done wrong. Here’s what you need to know.
Wire Gauge and Fuse Sizing (12V Systems)
| Inverter Size | Wire Gauge (AWG) | Fuse Size | Max DC Amps |
|---|---|---|---|
| 1,000W | 2/0 AWG | 200A | 134A |
| 1,500W | 4/0 AWG | 300A | 200A |
| 2,000-2,500W | 4/0 AWG | 400A | 320-334A |
| 3,000W | 4/0 AWG | 400A | 400A |
Battery-to-inverter cable runs should not exceed 10-12 feet. Every extra foot of cable increases voltage drop and reduces efficiency.
Mistakes That Cause Problems
- Undersized wiring. Cable too small for the current draw causes voltage drop, heat buildup, and potential fire. Use the table above — don’t guess.
- No fuse between battery and inverter. A short circuit without a fuse can cause a catastrophic fire. Always fuse at the battery terminal.
- Too far from batteries. Long cable runs increase resistance and voltage drop. Mount the inverter as close to the battery bank as practical.
- Poor ventilation. Inverters generate significant heat under load. Follow the manufacturer’s clearance guidelines and ensure airflow around the unit.
- No transfer switch. Without one, connecting to shore power while the inverter is active can backfeed into the grid, damaging equipment or endangering utility workers.
- Reversed polarity. Swapping positive and negative connections can destroy the inverter instantly. Double-check every connection before powering on.
- Co-locating with flooded lead-acid batteries. Flooded batteries vent hydrogen gas, which creates a fire and explosion risk near the heat and electrical sparks of an inverter. Sealed AGM or lithium batteries can safely share a compartment.
Popular Inverter Brands
A quick lay of the land if you’re shopping:
- Victron Energy — Top-tier ecosystem with web-based monitoring, power assist, and excellent integration with lithium battery systems. Higher price, but the monitoring and configurability are unmatched.
- Xantrex (Freedom series) — Popular in high-end motorhomes. Excellent quality at a slightly lower price point than Victron. Recommended replacement for the now-discontinued Magnum Energy units found in many existing rigs.
- Go Power — Good mid-range value with power assist features similar to Victron. Popular with DIY installers.
- Renogy — Budget-friendly and gaining popularity in the DIY community. Good support and documentation.
- AIMS Power, Giandel, VEVOR — Budget options available on Amazon. Lower reliability and support, but accessible price points for basic setups.
When choosing, look beyond wattage: consider warranty length, customer support quality, and whether the brand’s ecosystem (monitoring, app control) matters to you.
Right-Size Your System
The best inverter isn’t the biggest one — it’s the one that matches your actual usage. Oversizing wastes money, increases idle draw, and demands a bigger battery bank. Undersizing trips overload protection and leaves you unable to run the appliances you bought it for.
Start by listing what you actually need to run off-grid. Add up the watts. Apply the safety margin. Match that to a battery bank that can sustain the load for the runtime you want. If that means a 2,000W inverter with 200Ah of lithium, great — you don’t need 3,000W “just in case.”
Use the Arvee AmpSmart calculator to build your specific appliance list and see exactly what your power system needs to support. And if you’re building out a complete off-grid electrical system, these guides cover the rest of the picture:
- RV Shore Power Explained: 30 Amp vs 50 Amp — understanding your AC power source
- RV Battery Types Compared — choosing the right battery chemistry
- RV Solar Sizing Guide — keeping your batteries charged off-grid
- Boondocking Power Management — putting it all together for dry camping