How to Size an Inverter for Your RV: Wattage Guide & Calculator

Buying an undersized inverter is one of the most common and frustrating RV electrical mistakes. You install it, plug in your microwave, and the inverter shuts down under load. Getting the right size upfront takes 15 minutes of calculation — this guide walks through exactly that.

→ Use the Free RV Power Calculator to size your inverter automatically.

Two Numbers You Need: Continuous and Surge Wattage

Every inverter has two wattage ratings:

Continuous wattage: The power the inverter can deliver indefinitely. This is the number that matters for most appliances.

Surge (peak) wattage: The brief power spike the inverter can handle during motor startup — typically 2× the continuous rating, lasting 1–3 seconds. Motors in fridges, AC units, and power tools draw 2–6× their running wattage at startup.

Your inverter’s continuous wattage must exceed the total running wattage of all appliances you’ll run simultaneously. The surge rating must exceed the highest startup spike of any single motor in your system.

Step 1: List Every AC Appliance You’ll Run

Focus on what you’ll run at the same time — not everything in your RV at once.

Common RV Appliance Wattage

Step 2: Identify Your Simultaneous Load Scenario

Most RVers don’t run everything at once. Define your actual use case:

Scenario A — Basic boondocking: CPAP + laptop + phone charging + LED TV

• Total running: 60 + 65 + 15 + 60 = 200W
• Recommended inverter: 600W (with 3× headroom for comfort)

Scenario B — Morning routine: Coffee maker + toaster (sequential, not simultaneous)

• Peak running (one at a time): 1,200W max
• Recommended inverter: 1,500–2,000W

Scenario C — Full comfort boondocking: Microwave + TV + laptop + CPAP

• Total running: 1,100 + 60 + 65 + 60 = 1,285W
• Startup surge (microwave): 1,500W
• Recommended inverter: 2,000W continuous / 4,000W surge

Scenario D — Air conditioning: RV rooftop AC

• Running: 1,500–2,000W
• Startup surge: 3,000–6,000W
• Recommended inverter: 3,000W continuous / 6,000W surge minimum

Step 3: Apply the Sizing Formula

“
Minimum inverter size = (Total simultaneous running watts) × 1.25
“

The 1.25 multiplier provides safety headroom and accounts for inverter inefficiency (a 90% efficient inverter draws 10% more from your batteries than the appliance consumes).

Also verify: Your inverter’s surge rating exceeds the highest single-appliance startup surge. A 2,000W continuous inverter with 4,000W surge handles a microwave’s 1,500W startup easily.

Step 4: Match Inverter Size to Battery Bank

A larger inverter draws power from your batteries faster. Make sure your battery bank can support your inverter’s load duration.

Formula: Battery runtime (hours) = Usable battery Wh ÷ Inverter load (W)

Running a 1,200W microwave for 5 minutes draws: 1,200 × (5/60) = 100 Wh — a trivial battery draw. The concern is sustained high loads, not brief cooking cycles.

For a complete battery bank sizing guide, see our RV battery bank sizing article.

Recommended Inverter Sizes by RV Type

Pure Sine Wave vs Modified Sine Wave — One More Time

Every appliance list in this guide assumes a pure sine wave inverter. Modified sine wave inverters:

• Cannot safely power CPAP machines with humidifiers
• Cause overheating in variable-speed motors
• Create buzzing in audio equipment and some TVs
• May void warranties on sensitive electronics

For RV use, pure sine wave is the only acceptable choice. The price difference is $50–$100 — worth every penny.

Standalone Inverter or Inverter Charger?

If you’re adding battery charging capability (shore power or generator), an inverter charger is more cost-effective than separate inverter + converter units. See our full guide on RV converter vs inverter for when each makes sense, and our best RV inverter charger rankings for top picks.

Frequently Asked Questions

What size inverter for a CPAP machine?
A CPAP without humidifier draws 30–60W. A 300–600W inverter is more than sufficient. If using a humidifier, the draw increases to 60–100W — still easily handled by a 600W unit. Note: many CPAP machines are DC-compatible and can run directly from 12V, bypassing the inverter entirely.

Can I run two appliances at the same time on a 2,000W inverter?
Yes, as long as the combined running wattage stays below 2,000W. Laptop (65W) + TV (60W) + phone charging (15W) = 140W total — a 2,000W inverter handles this with enormous headroom.

What happens if I overload my inverter?
Quality inverters have overload protection — they shut down automatically rather than catching fire. After removing the excess load and letting the unit cool, you can restart. Repeated overloading degrades the inverter over time.

How much battery does an inverter use on standby?
Most inverters draw 10–30W at idle (no load). A 2,000W inverter idling for 8 hours draws 80–240 Wh — meaningful battery drain. Use a switched outlet or timer to turn the inverter off when not in use.

Key Takeaways

• Size to your simultaneous load scenario, not your RV’s total appliance wattage
• Multiply by 1.25 for safety headroom and efficiency losses
• Surge rating must exceed the highest single motor startup in your system
• Pure sine wave only — never modified sine wave for RV use
• 2,000W handles 95% of boondocking needs except air conditioning
• Use the Free RV Power Calculator for precise sizing

For the complete RV power system guide, read the RV Gear & Power Complete Guide.

Step 3: Account for Inverter Efficiency Loss

Inverters are not 100% efficient — they lose some energy converting DC to AC, typically 5–15% depending on load level and quality. A 2,000W inverter running at 1,000W might draw 1,100–1,150W from your batteries.

Why this matters for battery sizing: if your inverter draws 10% more than the appliance uses, your battery bank depletes 10% faster than your appliance watt-hour calculations suggest. Factor this into your battery sizing calculations:

• Quality pure sine wave inverters: 85–93% efficient at half load
• Budget modified sine wave: 75–85% efficient
• All inverters: draw 10–30W at idle even with no load connected — switch yours off when not needed

Practical rule: add 15% to your inverter wattage total when calculating how fast your batteries will drain. A 1,000W load through a 90% efficient inverter draws ~1,110W from your battery bank.

Step 4: Match Your Battery Bank to Your Inverter

An oversized inverter can damage an undersized battery bank. When an inverter tries to deliver more power than the battery bank can supply, voltage sags, the inverter triggers low-voltage shutoff, and you get unexpected shutdowns under load.

General matching guidelines:

• 1,000W inverter: minimum 100Ah LiFePO4 (or 200Ah AGM)
• 2,000W inverter: minimum 200Ah LiFePO4 (or 400Ah AGM)
• 3,000W inverter: minimum 300Ah LiFePO4 (or 600Ah AGM)

LiFePO4 batteries can deliver high current without voltage drop — a key advantage for inverter use. AGM batteries sag under heavy loads, which is why the AGM requirement is double the LiFePO4 equivalent.

Check your battery’s maximum continuous discharge rate (expressed as C-rating or amps). A 100Ah battery with a 1C discharge rating delivers 100A maximum — at 12V, that’s 1,200W. A 2,000W inverter on this battery will hit the limit immediately under heavy load.

Common Inverter Sizing Mistakes

• Sizing for peak, not average: A 3,000W inverter bought “just in case” wastes power at idle and costs more. Size for your realistic simultaneous load, not your theoretical maximum.
• Ignoring surge ratings: Undersizing surge capacity causes nuisance tripping every time a motor starts. The microwave is the most common culprit — its startup surge can be 1.5× its rated wattage.
• Installing near batteries without ventilation: Inverters generate heat. Install in a ventilated space with at least 3 inches of clearance on all sides.
• Using undersized cable: The short cables between your inverter and battery bank carry enormous current. A 2,000W inverter at 12V draws ~167A. Undersized cable creates heat, voltage drop, and fire risk. Use the cable gauge specified by the manufacturer — typically 2/0 AWG for runs under 3 feet at this power level.
• Skipping a proper fuse: Install a fuse or breaker rated for your cable within 18 inches of the battery. This is the single most important safety step in any inverter installation.

Frequently Asked Questions

Can I run my RV air conditioner on an inverter?
A standard 13,500 BTU rooftop AC draws 1,500–2,000W running and 3,000–6,000W at startup. You need a 3,000W+ pure sine wave inverter and a 400Ah+ LiFePO4 battery bank to run it for more than 1–2 hours. Possible, but expensive to set up correctly.

Is a 2,000W inverter enough for most RVers?
Yes — for the majority of boondockers who want to run a microwave, laptop, TV, CPAP, and phone charging, a 2,000W pure sine wave inverter covers all realistic simultaneous loads. It’s the most popular size for good reason.

Should I get a standalone inverter or an inverter-charger?
If you plan to use shore power at campgrounds and battery power when boondocking, an inverter-charger (single unit that does both) is worth the extra cost. It simplifies wiring, automatically switches between shore and battery power, and charges your batteries when plugged in. See our Best RV Inverter Charger guide for top-rated options.

What’s the difference between modified and pure sine wave?
Modified sine wave inverters are cheaper but produce choppy AC power that can damage CPAP machines, cause electronics to run hot, and create noise in audio equipment. Pure sine wave inverters produce clean power identical to shore power. For RV use, always choose pure sine wave.

Published on May 10, 2026

Mike Dowson

Mike Dowson is a 39-year-old van-life enthusiast and RV systems specialist. He writes practical, straightforward guides to help American travelers upgrade their campervans with reliable, eco-friendly gear. His work focuses on real testing, honest recommendations, and safe DIY conversions.