Start with the simultaneous load, then check the bank and installation. A 2,000W inverter may suit a microwave-sized load, while higher combined loads may justify 3,000W. Neither choice is established by counting 100Ah batteries. Check continuous current, voltage under load, startup demand and runtime.
Three numbers, in the right order
The pages ranking for this query hand you a formula — add up your appliance watts and multiply by 1.4 — and stop. That produces an inverter that starts your loads and then discovers, on its first weekend out, that the battery bank disconnects whenever the microwave runs.
Sizing that works needs three numbers, checked in order:
- Your biggest simultaneous load. Not the sum of every appliance you own — the most you will genuinely run at once. Watts, from the labels.
- The startup surge of the worst load in that set. Compressors and motors pull far more for a moment than they run at.
- What your battery bank will pass. The BMS continuous rating, in amps, which caps everything above it no matter what the inverter is rated for.
The first two size the inverter. The third decides whether you are allowed to buy it yet.
Step one: what actually runs at once
Typical draws for the appliances people buy inverters for. These are representative ranges — the number that matters is on your appliance’s own label, so check it before buying anything. To have the peak, the surge and the BMS check done from your own list, the RV battery calculator sizes the inverter and the bank together.
| Load | Typical running draw | Startup behaviour |
|---|---|---|
| LED lighting, phone charging | 5–30W | — |
| Laptop | 60–100W | — |
| TV | 60–150W | — |
| CPAP (no humidifier) | 30–60W | — |
| Residential 12 cu ft fridge | 100–200W | Compressor start, several × running |
| Coffee maker | 900–1,400W | — |
| Microwave (1,000W cooking power) | 1,400–1,600W from the outlet | Brief start spike |
| Toaster | 800–1,500W | — |
| Induction burner | 1,500–1,800W | — |
| Hair dryer | 1,200–1,875W | — |
| 13,500 BTU air conditioner | 1,300–1,700W | Large start surge — see below |
Two things stand out. Anything with a heating element lives in the 1,000–1,800W band, which is why 2,000W is the workhorse size. And a microwave’s cooking power is not its draw — a “1,000W” microwave takes roughly 1,500W from the outlet, a distinction that has undersized a lot of inverters. Loads with motors or transformers also care what shape of power they get, which is the pure sine question — settle it alongside the wattage.
Step two: the surge, and how long it is allowed to last
Motors and compressors pull two to three times their running draw at startup. Inverters carry a surge rating for exactly this, but the ratings are shorter than the shopping pages imply, and the honest manufacturers publish the window: Renogy’s 4,000W and 6,000W surge figures are for one second; Xantrex publishes 4,000W for five seconds on the Freedom XC 2000; Go Power states no duration at all for the ISW series.
One second starts a fridge compressor. It does not run an air conditioner through a hard start — a 13,500 BTU unit’s compressor is the surge that defeats otherwise sensible systems, which is why the workable answer there is a soft starter plus the 3,000W class, and why most owners simply leave the air conditioner on shore power. The full arithmetic, parts list included, is in running an RV air conditioner on batteries.
Step three: the ceiling your battery sets
Use the actual battery’s continuous-discharge limit. A BMS cutoff is protection, not a normal load-control strategy, and trip timing varies. A 100A rating at 12.8V corresponds to 1,280W DC; at 85% conversion that is about 1,088W AC before other DC loads. At 12V it is only 1,020W.
1 × 100Ah lithium battery 100A BMS
Example assumes a 100A continuous limit. 1,280W is DC at 12.8V, before inverter losses and other loads.
2 × 100Ah in parallel 200A BMS
Assumes two approved, balanced 100A paths. Check voltage under load and other DC consumption.
3 × 100Ah in parallel 300A BMS
300A is an illustrative total, not a guarantee of 3,000W across the operating range.
Battle Born GC3, single battery 300A BMS
Published 300A continuous rating; account for voltage, losses and other loads before choosing AC output.
The markers compare DC draw with DC bank power at the stated nominal voltage. They do not certify a finished system. Check the lowest intended voltage under load, other DC demand, battery sharing and startup limits. A lower battery voltage raises the current needed for the same AC load. The Renogy review’s current-limit diagram shows the 12V and 11V cases explicitly.
The DC side: what those watts look like in copper
Watts feel abstract until they become amps at 12V, which is what your cables, fuse and battery terminals experience:
| Inverter at full output | Draw from a 12V bank | Manufacturer’s fuse & cable call |
|---|---|---|
| 1,000W | ~92A | — |
| 2,000W | ~184A | Renogy 2000W: 250A fuse, 1/0 AWG (included) |
| 2,200W | ~202A | GIANDEL: not stated — size as for 2,000W+ |
| 3,000W | ~276A | Renogy 3000W: 400A fuse, 4/0 AWG (not included) |
| MultiPlus 12/3000 | ~215A at its 2,400W | Victron: 400A fuse, 2 × 50mm² per terminal |
Amps are computed at 12.8V nominal and 85% full-load efficiency; fuse and cable figures are from each manufacturer’s manual. The practical point: a 3,000W install is welding-cable territory, and the cabling, lugs and Class T fuse are a real second purchase — budget them with the inverter, not after it.
Those same amps decide one more component, if you are fitting a battery monitor: the shunt goes in the battery negative, so the inverter’s draw is the largest current it will ever see. Xantrex publishes 192A at full load and up to 400A on starting surges for its 2000W unit, which is why an inverter rig wants a 500A shunt where a rig without one does not — what size shunt do you need works through both of its ratings.
One caveat on that last column, because the manuals are quoted more often than they are read: each of those cable calls assumes a short run. Renogy’s 1/0 is the three-foot pair in the box. Mount the same 2,000W inverter ten feet from the bank and 1/0 loses about 4% of your 12 volts, which is past the target and two sizes short of what the distance needs. Put your own run length into the DC voltage drop calculator before ordering cable to a figure from a manual.
Why “just buy the big one” backfires
An inverter consumes power by being on. The published idle figures on the units we recommend run from under 10W (Krieger) through 24–30W (GIANDEL, Renogy) to 20W with an 8W search mode (Victron MultiPlus). Take a middle-of-the-road 25W and leave it on around the clock: that is 600Wh a day — roughly 50Ah, half of a 100Ah battery, spent powering nothing.
An oversized inverter idles higher, sits further below its efficient operating range on your actual loads, and cost more to cable. Size to your loads and bank, and put the remote switch where you will actually flip it.
Three worked examples
Light-load example, one battery with a verified 100A limit. A 900W AC load at 12V and 85% efficiency needs about 88A before other DC loads. A 1,000W inverter may suit that load; verify remaining current margin, startup requirements and runtime rather than assuming the Ah label proves it fits.
Two-battery bank, microwave rig. Microwave (1,500W from the outlet) plus a TV and charging, ~1,700W peak. At 12V and 85% efficiency that is about 167A before other DC loads. An approved bank with 200A continuous capacity may suit it after checking the remaining margins. Compare the Renogy 2000W with its included cables, or the GIANDEL 2200 where budget leads.
Family fifth wheel, induction cooking. Induction burner and microwave staggered, residential fridge always on — 2,200–2,600W peaks. This is the 3,000W class, with the bank sized for the actual load, lowest intended terminal voltage and other DC consumption. Price in the Renogy 3000W and its complete cable and protection installation (how it stacks against Victron’s alternative is in Victron vs Renogy). On a rig like this, also read inverter vs inverter/charger — replacing the converter and inverter with one box starts making sense at this scale, and the charger that refills a bank this size is a serious pedestal load in its own right (30 amp vs 50 amp covers what fits).
The picks, by size class
The 2,000W class
Renogy 2000W P2
Check actual load and permitted bank current. The package includes battery cables, but installation requirements still apply.
2,000W class on a budget
GIANDEL 2200W
2,200W pure sine with a published spec table and a 3-year warranty.
The 3,000W class — verify the bank current
Renogy 3000W P2
The documented, buyable 3,000W unit — plan on 4/0 cable and a 400A fuse.
If the converter is being replaced anyway
Xantrex Freedom XC 2000
2,000W inverter, 80A lithium charger and a 30A transfer switch in one box.
Full reviews of all seven units we compared, including the dealer-network and inverter/charger options, are in the best RV inverters — and the 3,000W class pick has a dedicated review covering bank-current requirements and the documentation differences.
Related guides
- Best RV inverters — the seven units compared on the figures this page sizes against.
- Best RV lithium batteries — BMS ratings, charge limits and warranty terms for the bank side of the equation.
- What size lithium battery do I need? — this page’s twin, for the battery bay.
- Lithium vs AGM — why chemistry decides whether these load numbers work at all.
- Do I need a DC-DC charger? — refilling the bank from the tow vehicle between hookups.
- 30 amp vs 50 amp RV — the shore-power half of the arithmetic.
- The electrical section collects everything published so far.
For how products on this site are selected and what we do not claim about them, see the editorial policy.
Questions worth answering
How many batteries do I need for a 2,000 watt inverter?
At 85% assumed efficiency, 2,000W needs about 196A at 12V or 214A at 11V at the inverter. Check the actual bank’s current rating, other DC loads and surge demand. Two approved 100A batteries are an example, not a guarantee across the whole operating range.
How many batteries do I need for a 3,000 watt inverter?
At 85% assumed efficiency, 3,000W needs about 294A at 12V or 321A at 11V. Ah alone does not establish discharge current. Size the whole installation for voltage under load, other DC loads and startup demand; a nominal 300A bank does not automatically cover every condition.
What size inverter runs a residential fridge?
Almost any of them, most of the time — a residential RV fridge runs at roughly 100 to 200W. The catch is the compressor start, which can pull several times that for a moment, so 1,000W is a sensible floor. The real cost of a residential fridge is capacity, not inverter size: it cycles all day and can consume 70 to 110Ah of a 12V bank per day.
What size cable and fuse does an inverter need?
Whatever the manual says, and the manuals are specific. Renogy calls for a 250A fuse and 1/0 AWG cable on its 2000W unit, and a 400A fuse with 4/0 AWG on the 3000W. Victron specifies a 400A fuse and two 50mm² cables per terminal on the MultiPlus 12/3000. Undersized cable on a big inverter is a fire risk, not a performance problem.
Is it bad to buy a bigger inverter than I need?
It costs you twice. The purchase price is the visible part; the idle draw is the part that hurts off-grid. Big inverters burn more power doing nothing — around 25W of idle consumption is 50Ah a day from a 12V bank, half of a 100Ah battery spent powering nothing. Buy the size your loads and your bank justify, and wire the remote switch somewhere you will use it.
Why does my inverter draw more watts than the appliance uses?
Conversion losses. An inverter turning 12V DC into 120V AC loses some energy as heat — Renogy publishes at least 85% efficiency at full load — so a 1,500W microwave costs the bank roughly 1,750W. Divide by the battery voltage and that is about 138A of current, which is also why inverter cables are as thick as they are. How thick depends on how far the inverter sits from the battery: the [12V wire size calculator](/electrical/12v-wire-size-calculator/) turns that current and distance into a gauge.