Inverter size calculator: what size inverter do I need?
List what could run at the same time, with running watts and — for anything with a motor or compressor — starting watts. The calculator gives the continuous and surge ratings to look for and the battery current at that load.
Enter your measurements
More options: Headroom · Inverter eff.
- Running watts, all together
- 1,460 W
- Continuous rating with headroom
- 1,825 W
- Surge (peak) rating needed
- 2,460 W
- Battery current at your load
- 135.2 A
- Battery current at the inverter’s full rating
- 185.2 A
Running: 1,460 W × (1 + 25%) = 1,825 W → 2,000 W inverter
Surge: 1,460 W running + biggest start-up extra = 2,460 W
Battery current: 1,460 W ÷ 90% ÷ 12 V = 135.2 A (185.2 A at 2,000 W)
- The biggest motor start adds 1,000 W for a moment. Check the inverter’s surge or peak rating on its spec sheet — it must cover 2,460 W, even if the continuous rating is smaller.
- At 12 V this inverter can pull well over 150 A. Many larger systems use 24 V or 48 V to halve or quarter that current.
- For motors, compressors, CPAP machines and electronics, choose a pure sine wave inverter.
Quick answers for common sizes
| Example | Answer |
|---|---|
| What size inverter to run a microwave | 1,500 W pure sine (for a 1,200 W microwave) |
| What size inverter for a refrigerator | 600 W or more, with a 1,200 W surge rating |
| What size inverter to run a CPAP | 150 W pure sine (300 W with a heated humidifier) |
| How many amps a 1000 watt inverter draws | 92.6 A |
| How many amps a 2000 watt inverter draws | 185.2 A |
Battery current at other battery voltages
| Battery voltage | Battery current at your load | Battery current at the inverter’s full rating |
|---|---|---|
| 12 V | 135.2 A | 185.2 A |
| 24 V | 67.6 A | 92.6 A |
| 48 V | 33.8 A | 46.3 A |
Worked examples: what size inverter to run a microwave, what size inverter for a refrigerator, what size inverter to run a CPAP, how many amps a 1000 watt inverter draws and how many amps a 2000 watt inverter draws.
Two ratings: continuous and surge
continuous ≥ (sum of running watts) × (1 + headroom) surge ≥ sum of running watts + the largest (starting − running) of any one motor battery amps = watts ÷ inverter efficiency ÷ battery volts
An inverter has a continuous rating — what it can supply for hours — and a surge (or peak) rating it can hold for a moment. Heaters, microwaves, kettles and electronics only need continuous watts. Anything with a motor or compressor needs a brief burst to start: Champion’s chart lists a household fridge at 150–400 running watts but 800–1,200 starting watts, and a 1/2 hp sump pump at 800–1,050 running and 1,300–2,150 starting. Only the biggest start counts, on the assumption that two motors don’t start in the same instant.
Typical running and starting watts
| Appliance | Running W | Starting W |
|---|---|---|
| Microwave | 600–1,200 | no extra |
| Refrigerator | 150–400 | 800–1,200 |
| Mini fridge / cooler | 50–100 | 400–600 |
| CPAP, no heat | 30–60 | no extra |
| Coffee maker | 900–1,200 | no extra |
| TV, 55 in (ENERGY STAR certified) | 64–114 | no extra |
| Circular saw, 7-1/4 in | 1,200–1,800 | 2,400–3,600 |
| RV air conditioner, 13,500 BTU | 1,500–2,000 | 2,800–3,500 |
Full list: appliance wattage chart.
The battery side
An inverter is only as good as the battery behind it. At 12 V, every 1,000 W of AC takes about 93 A from the battery through a 90% inverter. Victron recommends at most 100 A continuous from its 12.8 V 100 Ah LiFePO4 battery, so a 2,000 W inverter at full load needs at least two in parallel for current alone. Lead-acid gives up capacity quickly at these currents. The battery bank calculator checks both energy and current.
Inverters in an RV
- Turn the converter off when you run on the inverter. An RV’s converter (its battery charger) is usually powered from the 120 V side. If the inverter feeds that side, the converter tries to charge the battery from the battery itself — a loop that just burns energy. Switch the converter off, or put it on a circuit the inverter doesn’t feed.
- Use a transfer switch or an inverter-charger. To run the RV’s outlets from the inverter, connect it through an automatic transfer switch, or use an inverter-charger that switches between shore power and inverter and replaces the converter. Have it installed by a qualified RV technician or electrician.
- Plug-in is the simple option. A small inverter with its own outlets, wired only to the battery, runs a CPAP, TV or laptop without touching the RV’s wiring.
Pure sine or modified sine?
Modified (square-ish) sine inverters are cheaper, but motors run hotter and noisier on them, and some chargers, CPAP machines and electronics misbehave. For a system that will run a fridge, a CPAP or anything you care about, pure sine is the sensible default.
What you’ll need
- Pure sine wave inverter, 1000 WMid-range price
Enough for a CPAP, laptop, TV and small appliances. Pure sine is the safe choice for anything with a motor or sensitive electronics.
See options on Amazon (opens in a new tab) - Pure sine wave inverter, 2000–3000 WPremium price
For microwaves, coffee makers and power tools. At 12 V these draw hundreds of amps, so the battery and cabling matter as much as the inverter.
See options on Amazon (opens in a new tab) - DC clamp meterMid-range price
Clamps around one cable to read DC amps without breaking the circuit — checks what an inverter or fridge actually draws from the battery. Make sure it says DC; many cheap clamps read AC only.
See options on Amazon (opens in a new tab)
As an Amazon Associate we earn from qualifying purchases. Links to products are affiliate links — they cost you nothing extra and help keep these calculators free.
Frequently asked questions
What size inverter do I need to run a microwave?
Size it on the microwave’s input watts on the label, not its cooking power: for a microwave drawing 1,200 W, about 1,500 W continuous with headroom. Champion’s chart lists microwaves at 600–1,200 W with no extra starting watts. See what size inverter to run a microwave.
How many amps does a 1000 watt inverter draw?
At full load from 12 V, about 92.6 A through a 90% inverter; about 46 A at 24 V. See how many amps a 1000 watt inverter draws.
What size inverter do I need for a refrigerator?
A fridge runs on 150–400 W but needs 800–1,200 W for a moment to start (Champion). Look for a pure sine inverter with a surge rating of at least the fridge’s starting watts — often a 1,000 W-class unit or larger. See what size inverter for a refrigerator.
Can I use a bigger inverter than I need?
Yes, but a bigger inverter usually has a higher idle draw, which matters if it stays on for small loads. Its full-load current also tells you how big the battery and cabling must be.
What size inverter do I need to run a house?
Add up everything that could run at the same time and the biggest motor start. Whole-house inverters are commonly 48 V units of several kilowatts, and they connect to house wiring only through equipment installed by a licensed electrician.
Related guides
Sources
- Champion Power Equipment — Generator Wattage Chart (opens in a new tab) — Typical running and starting watts for household, RV and job-site appliances (a manufacturer’s planning chart — your nameplate wins).
- Virginia Cooperative Extension, BSE-301: Estimating Appliance and Home Electronic Energy Use (2020) (opens in a new tab) — Wattage × hours ÷ 1,000 = kWh per day; watts = amps × volts when only amps are listed; a refrigerator runs at full wattage about a third of the time; typical nameplate wattages.
- Victron Energy — Energy Unlimited (book) (opens in a new tab) — Lead-acid charge efficiency 80–95% (85% suggested as a starting value); battery energy efficiency about 75% for full cycles, about 89% in partial state of charge; inverter efficiencies above 90%.
- Victron Energy — Lithium Battery Smart manual, technical data (opens in a new tab) — LiFePO4 12.8 V: cycle life 2,500 at 80% DoD, 3,000 at 70%, 5,000 at 50% (to 80% of capacity); 92% round-trip efficiency; 12.8 V 100 Ah: recommended continuous discharge ≤100 A (1C), maximum 200 A; capacity about 20% lower at 0 °C; charge only between +5 °C and +50 °C.
- ENERGY STAR — Certified Televisions data set (on-mode power) (opens in a new tab) — Certified models, on-mode watts, retrieved October 2026: 32 in about 27–36 W; 43 in about 45–78 W; 50 in about 61–90 W; 55 in about 64–114 W; 65 in about 85–135 W; 75 in about 120–176 W.