How do I know what size inverter I need?

Total your running AC loads in watts, multiply by 1.2 for headroom, and round up to the next available inverter size, which gives the continuous rating you need. Separately, check the largest single motor startup among those loads, such as a compressor or a microwave's magnetron transformer, against the inverter's surge rating, since that number decides whether the inverter can actually start the load rather than just run it.

Why does a 2,000W inverter pull close to 200A from the battery?

Power in watts equals voltage times current, so at a fixed 12V, watts and amps scale together. A 2,000W continuous inverter running near its rated load divides that wattage by roughly 0.85 for efficiency losses and by 12V, which lands close to 196A. That current is why 12V inverter installs above about 1,500W need substantial battery cable and a battery bank with a matching BMS continuous rating.

Does inverter efficiency really matter for sizing?

Yes, though the effect is moderate rather than dramatic. Using 0.85 as a planning convention instead of assuming 100 percent efficient conversion adds about 18 percent to the DC current figure, which can be the difference between a battery cable and BMS rating that is adequate and one that is marginally undersized. Check your specific inverter's published efficiency figure when it is available and use that instead of the convention.

What happens if the battery BMS continuous rating is lower than the inverter's DC draw?

The BMS will limit or cut off current to protect the battery, which can shut the inverter down under load even though the inverter itself is rated to handle it. This is a common mismatch when a 2,000W or 3,000W inverter is paired with a battery whose BMS is only rated for 100A continuous. Always check the battery's published continuous BMS amperage against the inverter's expected DC draw before buying either one.

Why does the appliance table show a small inverter for a refrigerator with 800W startup watts?

The continuous requirement, 180W with headroom, is what decides the buy column, since that is the sustained load the inverter carries most of the time. The 800W startup figure is the surge requirement, and many inverters, including 600W-class units, publish a surge rating well above their continuous rating specifically to survive a brief compressor start. Check the specific inverter's published surge rating against the appliance's startup watts before assuming any given size will work.

Should I size an inverter for every appliance running at once, or one at a time?

Almost nobody runs every appliance on the appliance table simultaneously, so sizing for that worst case usually means paying for far more inverter than daily use requires. A more realistic approach adds up the running watts of whatever plausibly runs together, such as a residential fridge alongside general electronics, then separately checks the single largest startup surge among those loads against the inverter's surge rating, which is exactly the method behind the continuous and surge columns in the table above.

Is the 1.2 headroom figure a published standard?

No, it is a planning convention used consistently across this site's calculators and charts, not a figure from NEC, ABYC or a manufacturer standard. It exists to leave margin above a load's nameplate running watts for real-world variation. Manufacturers of individual inverters publish their own continuous and surge ratings, and those specific numbers should be the final check before a purchase.

Reference

RV Inverter Size Chart by Running Load and DC Current

Researched from published standards and manufacturer specifications. Updated 2026-09-17.

Quick answer

Inverter continuous rating is running watts x a 1.2 headroom convention. A 1,500W running load needs about 1,800W continuous, which rounds up to a 2,000W inverter. At 12V that load pulls roughly 147A from the battery, using a labeled 0.85 inverter efficiency convention; at 24V the same load pulls about half that current. A 2,000W inverter can pull close to 200A at 12V, and the battery BMS continuous rating has to cover it.

Inverter sizing has two separate numbers that get confused constantly: the continuous rating, which has to cover the steady running load with headroom, and the surge rating, which has to cover the largest single motor startup on top of whatever else is running. This site applies a 1.2 headroom convention to running watts for the continuous figure. The DC current an inverter pulls from the bank is running watts divided by a labeled 0.85 efficiency convention, divided by system voltage, and that current is the number that actually decides both the battery cable gauge and the battery's own BMS continuous amperage rating.

The tables below run that math across a wide range of running loads at 12V, show the same loads at 24V for comparison, and then apply it to a list of common RV appliances using their published running and startup watts.

Inverter size and DC current by running load, 12V

Continuous watts is running watts x 1.2. Inverter to buy rounds up to the next size this site's calculator recognizes. DC amps and minimum BMS rating both assume a 12V bank and the 0.85 efficiency convention.

Inverter size and 12V DC current by running AC load
Running load	Continuous needed	Inverter to buy	DC amps at 12V	Minimum BMS rating
100W	120W	600W	9.8A	25A
200W	240W	600W	19.6A	25A
300W	360W	600W	29.4A	50A
500W	600W	600W	49.0A	50A
600W	720W	1,000W	58.8A	75A
750W	900W	1,000W	73.5A	75A
1,000W	1,200W	1,500W	98.0A	100A
1,200W	1,440W	1,500W	117.6A	125A
1,500W	1,800W	2,000W	147.1A	150A
1,800W	2,160W	3,000W	176.5A	200A
2,000W	2,400W	3,000W	196.1A	200A
2,500W	3,000W	3,000W	245.1A	250A
3,000W	3,600W	4,000W	294.1A	300A

Convention Source: 1.2 headroom on continuous watts and 0.85 inverter efficiency for DC current are both planning conventions used by this site's inverter sizing calculator, not published specifications; substitute your own inverter's published efficiency when you have it.

The same load at 24V pulls about half the current

Doubling system voltage halves current for the same power, which is why a 24V inverter installation can use noticeably lighter battery cable than the 12V equivalent for the identical running watts.

DC current by running load, 12V versus 24V
Running load	DC amps at 12V	DC amps at 24V
500W	49.0A	24.5A
1,000W	98.0A	49.0A
1,500W	147.1A	73.5A
2,000W	196.1A	98.0A
3,000W	294.1A	147.1A

Convention Source: AC watts / 0.85 efficiency convention / system volts, at 12V and 24V. See the battery cable size chart for the conductor gauge each current level actually needs at a given run length.

Inverter sizing for common appliances, using published running and startup watts

Surge needed is the larger of the continuous figure or the appliance's own published startup watts, since a motor-driven appliance's inrush can exceed its steady running load by a wide margin.

Inverter continuous and surge requirements by appliance
Appliance	Running watts	Startup watts (published)	Continuous needed	Surge needed	Inverter to buy
Residential refrigerator	150W	800W	180W	800W	600W
Microwave, 1,000W output	1,500W	1,500W	1,800W	1,800W	2,000W
Electric kettle	1,500W	1,500W	1,800W	1,800W	2,000W
Induction cooktop, single burner	1,800W	1,800W	2,160W	2,160W	3,000W
Space heater, high setting	1,500W	1,500W	1,800W	1,800W	2,000W
Hair dryer	1,500W	1,500W	1,800W	1,800W	2,000W
Residential washer/dryer combo	1,200W	2,000W	1,440W	2,000W	1,500W
RV air conditioner, 13,500 BTU	1,400W	3,000W	1,680W	3,000W	2,000W
RV air conditioner, 15,000 BTU	1,700W	3,500W	2,040W	3,500W	3,000W

Published standard Source: Running and startup watts are typical published figures for each appliance class; the continuous, surge and buy columns apply this site's 1.2 headroom convention to those figures. An inverter's own surge rating on its spec sheet still has to be checked against the surge needed column; not every inverter model surges well above its continuous rating.

Why the 1.2 headroom convention exists at all

An inverter run flat out at its exact continuous rating for hours at a time runs hotter than one with a margin to spare, and heat is what shortens an inverter's working life and triggers thermal shutdown at the worst moment, mid-load. The 1.2 figure is not a number any single manufacturer publishes as a requirement; it is a planning convention this site applies consistently so that a 1,500W running load lands on a 2,000W inverter rather than one rated right at 1,500W with no margin. A rig that runs its inverter near its rated capacity for extended periods, such as a residential fridge and other loads together for most of a day, benefits from erring toward the higher end of headroom, while an inverter that only sees occasional short bursts of its rated load can run closer to the line.

Inverters sized to this chart

Intermediate Small loads

Renogy

Renogy P2 1000W Pure Sine Inverter

$175.99

Right-sized for running loads up to roughly 830W continuous, such as a laptop, a residential fridge, and general electronics together.

Best for: Running a residential fridge or a microwave on a modest bank.

Check price on Amazon

Expert Large single loads

Renogy

Renogy P2 3000W Pure Sine Inverter

$414.99

Needed for a 2,500W-class continuous running load, the bracket an induction cooktop or a 15,000 BTU air conditioner on inverter power falls into.

Best for: A large lithium bank where air conditioning on inverter is the goal.

Check price on Amazon

Frequently asked questions

How do I know what size inverter I need?: Total your running AC loads in watts, multiply by 1.2 for headroom, and round up to the next available inverter size, which gives the continuous rating you need. Separately, check the largest single motor startup among those loads, such as a compressor or a microwave's magnetron transformer, against the inverter's surge rating, since that number decides whether the inverter can actually start the load rather than just run it.
Why does a 2,000W inverter pull close to 200A from the battery?: Power in watts equals voltage times current, so at a fixed 12V, watts and amps scale together. A 2,000W continuous inverter running near its rated load divides that wattage by roughly 0.85 for efficiency losses and by 12V, which lands close to 196A. That current is why 12V inverter installs above about 1,500W need substantial battery cable and a battery bank with a matching BMS continuous rating.
Does inverter efficiency really matter for sizing?: Yes, though the effect is moderate rather than dramatic. Using 0.85 as a planning convention instead of assuming 100 percent efficient conversion adds about 18 percent to the DC current figure, which can be the difference between a battery cable and BMS rating that is adequate and one that is marginally undersized. Check your specific inverter's published efficiency figure when it is available and use that instead of the convention.
What happens if the battery BMS continuous rating is lower than the inverter's DC draw?: The BMS will limit or cut off current to protect the battery, which can shut the inverter down under load even though the inverter itself is rated to handle it. This is a common mismatch when a 2,000W or 3,000W inverter is paired with a battery whose BMS is only rated for 100A continuous. Always check the battery's published continuous BMS amperage against the inverter's expected DC draw before buying either one.
Why does the appliance table show a small inverter for a refrigerator with 800W startup watts?: The continuous requirement, 180W with headroom, is what decides the buy column, since that is the sustained load the inverter carries most of the time. The 800W startup figure is the surge requirement, and many inverters, including 600W-class units, publish a surge rating well above their continuous rating specifically to survive a brief compressor start. Check the specific inverter's published surge rating against the appliance's startup watts before assuming any given size will work.
Should I size an inverter for every appliance running at once, or one at a time?: Almost nobody runs every appliance on the appliance table simultaneously, so sizing for that worst case usually means paying for far more inverter than daily use requires. A more realistic approach adds up the running watts of whatever plausibly runs together, such as a residential fridge alongside general electronics, then separately checks the single largest startup surge among those loads against the inverter's surge rating, which is exactly the method behind the continuous and surge columns in the table above.
Is the 1.2 headroom figure a published standard?: No, it is a planning convention used consistently across this site's calculators and charts, not a figure from NEC, ABYC or a manufacturer standard. It exists to leave margin above a load's nameplate running watts for real-world variation. Manufacturers of individual inverters publish their own continuous and surge ratings, and those specific numbers should be the final check before a purchase.

Buy the inverter for the continuous load and check the surge separately. Then take the DC current row for your inverter size straight into the battery cable size chart to confirm the wiring and the BMS rating both match what the inverter will actually pull.