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Enter the combined continuous wattage of the AC devices you'll run at once, plus the highest single-device surge/startup wattage among them, to see a recommended inverter size with margin.
How This Is Calculated
Recommended continuous rating = continuous watts × (1 + margin). Recommended surge rating = continuous watts + surge watts (the surging device starts while everything else keeps running). DC amps = continuous watts ÷ 12V ÷ 0.85 (inverter efficiency loss).
Continuous and peak ratings, efficiency and DC cable/fuse practice per Victron Energy's inverter datasheet and installation manual. The 85% efficiency figure is a conservative assumption below Victron's 87–92% maximums.
This is a sizing estimate from the wattage you enter, not a guarantee against every device combination. Start-up surge differs from one device to the next, so enter the figure from your own device's label or manual.
TL;DR
- Size two ratings: continuous (with margin above the load you run at once) and peak (the running load plus the biggest start-up surge).
- Check watts, not VA: Victron's Phoenix 12/800 is 800 VA but 650 W at 25°C and 560 W at 40°C, with a 1,500 W peak.
- Every 100 W of AC takes about 8.3 A from a 12 V battery before losses and nearly 10 A after them.
- An idle inverter still draws power: 6.5 W (about 13 Ah a day) for the 12/800, or about 1 W in ECO mode.
- Big inverters need big cables and fuses: Victron pairs its 12/1600 with 70 mm² cable, a 250 A fuse and a 300–800 Ah battery bank.
Reading an Inverter's Ratings: VA, Watts, Peak and Idle
Inverter spec sheets usually list several different numbers. Victron's Phoenix VE.Direct line shows them side by side for 12 V models:
| Victron Phoenix (12 V) | Continuous VA | Continuous watts at 25°C / 40°C | Peak | Max efficiency | Idle draw (ECO mode) |
|---|---|---|---|---|---|
| 12/250 | 250 VA | 200 / 175 W | 400 W | 87% | 4.2 W (0.8 W) |
| 12/375 | 375 VA | 300 / 260 W | 700 W | 89% | 5.6 W (0.9 W) |
| 12/500 | 500 VA | 400 / 350 W | 900 W | 90% | 6 W (1 W) |
| 12/800 | 800 VA | 650 / 560 W | 1,500 W | 90% | 6.5 W (1 W) |
| 12/1200 | 1,200 VA | 1,000 / 850 W | 2,200 W | 91% | 10 W (1 W) |
| 12/1600 | 1,600 VA | 1,450 / 1,200 W | 2,200 W | 92% | 12 W (1.8 W) |
Three lessons follow. The VA number in a model name is larger than the watts it delivers. Output falls as the inverter gets hot — Victron derates 1.25 percent per °C above 40°C — so compare your load with the 40°C figure if the inverter lives in a warm cab or bed box. And the idle draw is small in watts but adds up: 6.5 W left on for a day is about 13 Ah from a 12 V battery.
Worked Example With the Calculator's Defaults
A 150 W continuous load with a 20 percent margin calls for 180 W continuous. From the table, a 375 VA model (300 W at 25°C, 260 W at 40°C) covers it with room to spare. On the battery side, 150 W ÷ 12 V ÷ 0.85 is about 14.7 A, so four hours of use takes about 59 Ah — more than the whole daily budget of many camp setups. Check that against your battery with the 12V battery runtime calculator.
Appliance Power-Draw Reference Table
These manufacturer-published figures are for native 12 V devices. None of them needs an inverter; they show the scale of common overlanding loads in amps and watts.
| Device | Manufacturer-Rated Draw | Source |
|---|---|---|
| Dometic CFX3 55IM compressor fridge/freezer (native 12V DC — no inverter needed) | 8.9A at 12V DC ≈ 107W while running | Dometic CFX3 Operating Manual |
| ARB Twin Motor Onboard Air Compressor CKMTA12 (native 12V DC — no inverter needed) | Up to 68.6A at 12V DC ≈ 823W (maximum) | ARB USA product page |
| RIGID Industries E-Series PRO 20" LED light bar (native 12V DC — no inverter needed) | 214W / 15.29A at 14V | RIGID Industries product page |
Why Native-12V Devices Should Skip the Inverter
Running a 12 V fridge, compressor or light bar through an inverter converts DC to AC and then, inside the device's power supply, back again, losing energy at each step for no benefit. Wire 12 V devices straight to the battery through their own fuse, and keep the inverter for devices that have only a household plug.
Why Peak Rating Trips an Undersized Inverter
An inverter sized exactly to the running load can shut down the moment a device with a start-up surge switches on, even though it would run that device once started. Victron's datasheet names power converters for LED lamps, halogen lamps and electric tools as loads that need high start-up power. The surge size varies by device, so use the figure on your device's label or in its manual rather than a rule of thumb. The calculator adds that surge to the load already running, because the other devices don't stop while one starts.
The DC Side: Cable, Fuse and Battery Size
A 12 V inverter draws large currents from the battery, and Victron's Inverter Smart manual notes there is no safety fuse inside the inverter, so one must be fitted externally. Its table pairs each model with a cable size, fuse and battery capacity:
| Victron Inverter Smart (12 V) | Cable, 0–5 m | Cable, 5–10 m | Fuse | Battery capacity |
|---|---|---|---|---|
| 12/1600 | 70 mm² | Not recommended | 250 A | 300–800 Ah |
| 12/2000 | 70 mm² | Not recommended | 300 A | 350–1,000 Ah |
| 12/3000 | 90 mm² | 2 × 70 mm² | 400 A | 400–1,200 Ah |
Victron also says to mount the inverter as close to the batteries as possible, but never directly above them. Its Wiring Unlimited guide recommends keeping voltage drop to 2.5 percent, 0.3 V at 12 V, and shows the cost of missing it: a 2,400 W inverter drawing 200 A through two 1.5-meter, 16 mm² cables loses 0.64 V, about 5 percent of the energy, as heat.
Check the Battery's Current Limit Too
The battery has to deliver the current as well. Battle Born rates its 100 Ah BB10012 at 100 A continuous and 200 A for 30 seconds, and says two in parallel give 200 A continuous. A 1,600 W load at 12 V is about 133 A before inverter losses, so it needs at least two of those batteries. AGM batteries lose capacity at high current: Lifeline's discharge curves show about 60 percent of rated capacity at the 1-hour rate. For lithium banks, Victron adds that at least one fuse must have an interrupt rating equal to or greater than the bank's maximum short-circuit current.
How We Research This
Ratings, cable and fuse figures come from Victron Energy's inverter datasheet, Inverter Smart manual and Wiring Unlimited guide; battery limits come from Battle Born's and Lifeline's manuals; and the appliance figures come from each manufacturer's own product page or manual, cited above. See how we rank for the site's full research method.
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Sources
- Victron Energy — Inverter VE.Direct 250VA–1600VA datasheet — Continuous VA, continuous watts at 25°C/40°C, peak power, maximum efficiency, no-load and ECO-mode draw for the 12 V Phoenix 250–1,600 VA models; 1.25%/°C derating above 40°C; high start-up power for LED power converters, halogen lamps and electric tools.
- Victron Energy — Inverter Smart manual: Installation — No internal fuse; cable size, fuse rating and battery capacity for the 12/1600, 12/2000 and 12/3000; mount close to but never directly above the batteries.
- Victron Energy — Wiring Unlimited — 2.5% voltage-drop guideline, the 2,400 W / 200 A / 16 mm² cable-loss example, and the fuse interrupt-rating requirement for lithium banks.
- Battle Born Batteries — 100Ah 12V LiFePO4 battery (BB10012) manual — BB10012 100 A continuous and 200 A 30-second limits, and 200 A continuous for two in parallel.
- Lifeline Batteries (Concorde) — Technical Manual for Lifeline AGM batteries, Doc. 6-0101 Rev. F — AGM discharge curves showing about 60% of rated capacity at the 1-hour rate.
- Dometic — CFX3 Mobile Cooling Operating Manual (CFX3 25/35/45/55IM/75DZ/95DZ/100) — CFX3 55IM rated current at 12 V (8.9 A).
- ARB USA — Twin Motor Onboard 12V Air Compressor (CKMTA12) — CKMTA12 maximum current draw (68.6 A).
- RIGID Industries — E-Series PRO 20" Spot/Flood Combo LED Light Bar — 214 W / 15.29 A current draw at 14 V.