What size inverter do you need?
By Sithum Imarsha Last reviewed
Size an inverter for the watts that run at once, plus headroom, then check the start-up peak and the battery current. A 2,000 W heater or kettle with a fridge running needs about 2,700 W continuous, and pulls about 199 A from a 12 V lithium battery near empty.
Your result
Inverter size, continuous
2,700W
2,150 W running + 25% headroom, rounded up = 2,700 W
- Running
- 2,150W
- Peak at start-up
- 2,450W
- Battery current
- 187A
- Near empty
- 199A
- Peak from
- Fridge (240 V) starting (our estimate)
- Smallest listed model for the running watts
- Victron MultiPlus-II 3000 VA (2,400 W)
- With headroom
- Victron MultiPlus-II 5000 VA (4,000 W)
Near empty the battery supplies about 199 A, above the 100 A limit Renogy and Enerdrive give their 100 Ah LiFePO4 batteries. Use a bigger battery bank, a higher voltage, or run fewer loads at once.
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Current limit
How long will it run?Work out the battery runtime.
How we calculated this
Running watts
Add the watts of everything that runs at the same time. Use the input watts on the rating plate. A microwave's headline watts may be its cooking output, not what it draws (we have no source for the gap), so read the input watts.
2,150 W
Peak at start-up
A motor (fridge compressor, pump, washing machine) draws several times its running watts for a moment as it starts. We assume one starts at a time: running watts + the biggest extra start. The starting factors come from one generator maker's US chart, so treat them as an estimate, not a pass or fail.
2,450 W (Fridge (240 V) starting)
Inverter size
We pick the smallest listed model whose continuous rating at 25 °C covers the running watts, and show a second pick with headroom (25%, our choice, not a rule), rounded up to 100 W. The start-up peak is only a warning, because its factors are estimates.
2,700 W continuous, 2,450 W peak
Current from the battery
Amps = running watts ÷ (inverter efficiency × battery volts). We show it at nominal volts and near empty (12.0 V for LiFePO4, 11.5 V for lead-acid on 12 V, our estimate), where it is highest.
187 A at 12.8 V, 199 A at 12 V, limit: 100. At the start-up peak: about 227 A (our estimate)
| Row | Running W | Starting W |
|---|---|---|
| Appliance 1 | 2,000 | 2,000 |
| Appliance 2 | 150 | 450 |
| Appliance 3 | — | — |
| Appliance 4 | — | — |
| Appliance 5 | — | — |
| Appliance | Running | Starting factor | Where it comes from |
|---|---|---|---|
| Kettle | Rating plate | 1× (Not yet checked) | Champion Power Equipment generator wattage chart (US, 120 V): resistive and electronic loads need no extra starting watts |
| Microwave | Rating plate | 1× (Not yet checked) | Champion Power Equipment generator wattage chart (US, 120 V): no extra starting watts |
| Fridge (240 V) | 150 W (Estimate) | 3× (Estimate) | Our pick at the low end of 150–400 W running (Champion Power Equipment generator wattage chart (US, 120 V)). Our pick: Champion Power Equipment generator wattage chart (US, 120 V) starting 800–1,200 W against running 150–400 W, about 3.6× |
| Washing machine | 500 W (Estimate) | 2× (Estimate) | Our pick at the low end of 500–1,000 W running (Champion Power Equipment generator wattage chart (US, 120 V)). Our pick: Champion Power Equipment generator wattage chart (US, 120 V) starting 1,000–2,300 W, about 2.2× |
| Water pump (1/3 HP) | 500 W (Estimate) | 2× (Estimate) | Our pick at the low end of 500–800 W running (Champion Power Equipment generator wattage chart (US, 120 V)). Our pick: Champion Power Equipment generator wattage chart (US, 120 V) starting 1,000–1,600 W, about 2× |
| TV (55 inch LED) | 80 W (Estimate) | 1× (Not yet checked) | Appliance running cost preset: Our pick near the low end of two ranges for a 55 inch TV: Home Upkeep 70 to 110 W, EcoFlow AU 70 to 130 W (LED or LCD). Champion Power Equipment generator wattage chart (US, 120 V): resistive and electronic loads need no extra starting watts |
| Wi-Fi modem or router | 15 W (Estimate) | 1× (Not yet checked) | Our pick in the middle of 10–20 W (Champion Power Equipment generator wattage chart (US, 120 V)). Champion Power Equipment generator wattage chart (US, 120 V): resistive and electronic loads need no extra starting watts |
| Fan heater | 2,000 W (Dated third-party source) | 1× (Not yet checked) | Appliance running cost preset: CHOICE electric heater lab tests, updated 27 May 2026. Champion Power Equipment generator wattage chart (US, 120 V): resistive and electronic loads need no extra starting watts |
| Oil column heater | 2,400 W (Dated third-party source) | 1× (Not yet checked) | Appliance running cost preset: CHOICE electric heater lab tests, updated 27 May 2026. Champion Power Equipment generator wattage chart (US, 120 V): resistive and electronic loads need no extra starting watts |
| Panel heater | 2,200 W (Dated third-party source) | 1× (Not yet checked) | Appliance running cost preset: CHOICE electric heater lab tests, updated 27 May 2026. Champion Power Equipment generator wattage chart (US, 120 V): resistive and electronic loads need no extra starting watts |
| Electric wall oven | 1,000 W (Estimate) | 1× (Not yet checked) | Appliance running cost preset: Our pick inside 0.66 to 1.39 kWh an hour. That range is our maths from CHOICE (27 Jul 2022): $31 to $65 a year for one hour at 170 °C, three times a week, at 30c per kWh. Champion Power Equipment generator wattage chart (US, 120 V): resistive and electronic loads need no extra starting watts |
| Electric blanket | 100 W (Dated third-party source) | 1× (Not yet checked) | Appliance running cost preset: CHOICE (10 Dec 2025): most draw 100–150 W. Champion Power Equipment generator wattage chart (US, 120 V): resistive and electronic loads need no extra starting watts |
| Pool pump (single speed) | 1,100 W (Estimate) | 2× (Estimate) | Appliance running cost preset: Our pick inside the 600–1,700 W range of single-speed pumps (Energy Rating scope, Canstar Blue). Our pick for a pump: Champion Power Equipment generator wattage chart (US, 120 V) gives about 2× for pumps |
Kettle and microwave have no default watts: we did not find a source for a typical Australian one. Read the rating plate. Inverter (variable-speed) fridges and air conditioners start softly; we found no figure, so lower the factor if yours is one.
| Inverter | Continuous 25 °C | Continuous 40 °C | Peak | Battery |
|---|---|---|---|---|
| Victron Phoenix 250 VA (200 W) | 200 W | 175 W | 400 W | 12 / 24 / 48 V |
| Victron Phoenix 375 VA (300 W) | 300 W | 260 W | 700 W | 12 / 24 / 48 V |
| Victron Phoenix 500 VA (400 W) | 400 W | 350 W | 900 W | 12 / 24 / 48 V |
| Victron Phoenix 800 VA (650 W) | 650 W | 560 W | 1,500 W | 12 / 24 / 48 V |
| Victron Phoenix 1200 VA (1,000 W) | 1,000 W | 850 W | 2,200 W | 12 / 24 / 48 V |
| Victron Phoenix 1600 VA (1,450 W, 12 V) | 1,450 W | 1,200 W | 2,200 W | 12 V |
| Victron MultiPlus-II 3000 VA (2,400 W) | 2,400 W | 2,200 W | 5,500 W | 12 / 24 / 48 V |
| Victron MultiPlus-II 5000 VA (4,000 W) | 4,000 W | 3,700 W | 9,000 W | 12 / 24 / 48 V |
| Victron MultiPlus-II 8000 VA (6,400 W, 48 V) | 6,400 W | 5,500 W | 15,000 W | 48 V |
| Victron MultiPlus-II 10000 VA (8,000 W, 48 V) | 8,000 W | 7,000 W | 18,000 W | 48 V |
| Victron MultiPlus-II 15000 VA (12,000 W, 48 V) | 12,000 W | 10,000 W | 27,000 W | 48 V |
- VA is not W. Victron's "1200 VA" Phoenix gives 1,000 W. Compare watts with watts.
- Continuous ratings fall with heat: the Phoenix 1200 VA gives 1,000 W at 25 °C and 850 W at 40 °C. A caravan cupboard in summer is not 25 °C.
- The datasheets give a peak rating but not how long it lasts. Peak is 1.5 to 2.3 times the continuous rating on these models.
- We do not size cables or fuses. Use the inverter and battery maker's installation manual, and a licensed electrician for anything on 230 V.
“Official source” means read in full on the maker’s own datasheet, manual or spec page, or in an official dataset (BOM). “Dated third-party source” means read on a dated third-party page, such as a product test. “Not yet checked” means one non-official source. “Estimate” means our own pick or reasoning: type your own figure.
Sources
- Victron: LiFePO4 Smart battery datasheet
- Victron: Phoenix inverters VE.Direct datasheet
- Victron: MultiPlus-II datasheet
- Renogy: 12.8 V 100 Ah LiFePO4 battery
- Enerdrive: B-TEC 12 V 100 Ah G2 lithium battery
- Champion Power Equipment: generator wattage chart
- CHOICE: which electric heater type is best
- Canstar Blue: electric heaters
- Home Upkeep: TV electricity use
- Canstar Blue: pool pump running costs
- Home Upkeep: pool pump running costs
- CHOICE: cheapest cooking appliances to use
- CHOICE: electric blanket buying guide
Three numbers
- Running watts. Add the input watts of everything that runs at the same time. The inverter’s continuous rating must cover this.
- Start-up peak. Motors (fridge compressors, pumps, washing machines) draw several times their running watts for a moment. The inverter’s peak rating must cover the running watts plus the biggest start.
- Battery current. Amps = watts ÷ (inverter efficiency × battery volts). The battery must be able to supply it.
Why the battery is often the limit
A 2,000 W kettle through an inverter at 90% efficiency pulls about 174 A from a 12.8 V LiFePO4 battery, and about 185 A near empty. Renogy and Enerdrive limit their 100 Ah LiFePO4 batteries to 100 A (Enerdrive for 30 minutes). Victron allows 200 A on its 100 Ah battery but recommends 1C (100 A) or less.
So a big inverter on one 100 Ah battery can ask for more current than the battery allows. A higher voltage helps: the same kettle on 48 V pulls about a quarter of the current.
Start-up surge is an estimate
We found only one source for starting watts: a generator maker’s chart for US appliances. It gives 800 to 1,200 W starting for a fridge that runs at 150 to 400 W, and about twice the running watts for pumps and washing machines. Kettles, heaters, TVs and chargers need no extra. We use these as estimates and let you change them. Treat the peak figure as a guide, not a pass or fail.
Read the datasheet carefully
- VA is not W. Victron sells a “1200 VA” Phoenix that gives 1,000 W.
- Heat lowers the rating. The same Phoenix gives 850 W at 40 °C.
- Peak time is not stated. Victron’s datasheets give a peak watts figure but not how long it lasts.
What we do not do
We do not size cables or fuses. The current in these systems is high. Use the inverter and battery maker’s installation manual, and a licensed electrician for anything on 230 V.
Questions
What size inverter do I need to run a kettle?
Read the kettle’s rating plate. For a 2,000 W kettle, at least 2,000 W continuous, plus headroom: 2,500 W with our 25%. Then check the battery: that kettle pulls about 185 A from a 12 V lithium battery near empty.
What size inverter for a 100Ah lithium battery?
Many 100 Ah LiFePO4 batteries are limited to 100 A (Renogy, and Enerdrive for 30 minutes). At 12.8 V and 90% efficiency that is about 1,150 W, or about 1,080 W near empty. A bigger inverter can pull more than the battery allows.
Why does a fridge need a bigger inverter than its watts?
Its compressor motor draws several times its running watts for a moment as it starts. One generator maker’s chart gives 800 to 1,200 W starting for a fridge running at 150 to 400 W. We use 3× as an estimate.
Is a 1200 VA inverter 1,200 W?
No. Victron’s Phoenix 1200 VA gives 1,000 W continuous at 25 °C and 850 W at 40 °C. Compare watts with watts.
Can I run a microwave on an inverter?
Yes, if the inverter covers its input watts, which are on the rating plate. The headline watts may be its cooking output instead. A microwave has no big start-up surge.