What Size Inverter Do I Need? Continuous and Surge Watts
Updated 2026-09-15 · 8 min read
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Size an inverter on two numbers, not one. First, the continuous watts of everything running at the same time. Second, the single largest starting surge among your motor loads, checked against the inverter's peak rating. Add 20–25% headroom to the continuous figure and buy above that.
Most people size on continuous watts alone, and it is the reason inverters get returned. A 2,000 W inverter running a 200 W fridge has enormous continuous headroom and will still shut down the instant that fridge's compressor asks for 900 W more than the inverter's peak can deliver.
Continuous rating vs peak rating
Every inverter carries two numbers:
- Continuous (rated) watts — what it can deliver indefinitely.
- Peak (surge) watts — what it can deliver for a few seconds, typically about 2× continuous, though this varies by model and some units hold it for longer than others.
Motors need that peak. Anything with a compressor or a motor pulls 2 to 3 times its running watts at the instant it starts: refrigerators, freezers, well pumps, sump pumps, power tools, air conditioners. Resistive loads — heaters, kettles, incandescent bulbs — have no surge at all and draw the same starting as running.
Typical loads in continuous and surge watts
| Load | Continuous | Starting surge |
|---|---|---|
| LED lights (a few rooms) | 20–60 W | none |
| Laptop | 45–90 W | none |
| Phone and tablet charging | 10–30 W | none |
| Wi-Fi router and modem | 10–20 W | none |
| TV (50 in LED) | 60–120 W | none |
| Refrigerator | 150–400 W | 400–1,000 W |
| Chest or upright freezer | 100–300 W | 400–900 W |
| CPAP (no humidifier) | 30–60 W | none |
| CPAP (with heated humidifier) | 60–90 W | none |
| Microwave | 600–1,200 W | 600–1,200 W |
| Coffee maker | 800–1,200 W | none |
| Toaster | 800–1,500 W | none |
| Electric kettle | 1,200–1,500 W | none |
| Space heater | 1,000–1,500 W | none |
| Circular saw / power tools | 1,200–1,800 W | 2,400–4,500 W |
| Sump pump (1/2 HP) | 800–1,050 W | 1,300–2,900 W |
| Well pump (1 HP, 240 V) | 1,000–2,000 W | 2,000–4,000 W |
| RV air conditioner (13,500 BTU) | 1,200–1,700 W | 2,800–4,000 W |
Read the two columns separately. The kettle has the bigger continuous number; the well pump has the bigger surge. Which one sizes your inverter depends entirely on which loads you actually have.
Work out your number
- List what runs at the same time. Not everything you own — everything simultaneously on.
- Sum the continuous watts.
- Add 20–25% headroom. That is your minimum continuous rating.
- Find the single largest surge on the list and check it fits the inverter's peak rating. Only the largest — motors do not all start together.
- Take the higher of the two requirements.
Worked example — a modest off-grid or RV setup:
- Refrigerator: 200 W continuous, 800 W surge
- LED lighting: 50 W
- Router and laptop: 100 W
- TV: 100 W
- Occasional microwave: 1,000 W
Continuous total with the microwave running: 1,450 W. Add 25% → ~1,800 W. Largest surge: the fridge at 800 W, comfortably inside a 2,000 W inverter's ~4,000 W peak. A 2,000 W pure sine wave inverter fits.
Now change one thing — add a 1/2 HP sump pump at 2,900 W surge. Continuous barely moves, but the surge requirement nearly doubles. That setup needs 3,000 W continuous or more, sized entirely by a load that runs a few minutes a day.
Do not oversize either
An inverter draws idle or no-load current simply by being switched on — commonly 10–30 W on a large unit. That runs 24 hours a day against your battery whether you are drawing power or not, and on a small bank it can quietly consume a real share of your storage.
Many inverters offer a power-save or standby mode that drops idle draw until a load appears. Worth having if the inverter sits energised most of the day.
Pure sine wave or modified sine wave?
Pure sine wave for almost every real setup. Modified sine wave is a stepped approximation of AC. It is cheaper, and it is fine for simple resistive loads, but:
- Motors and compressors run hotter and less efficiently, shortening their life.
- Some electronics, variable-speed motors and medical devices — CPAP machines in particular — will not run on it, or will run with audible buzzing.
- Some battery chargers and switching supplies misbehave.
The price gap has closed enough that the saving rarely justifies the constraints.
Match the inverter to the battery and wiring
The inverter is one part of a system, and the other parts have to carry it:
- Battery bank. The inverter sets peak power draw; the bank sets how long you can sustain it. A 3,000 W inverter pulling full load from a 12 V bank draws roughly 250 A — more than many small banks will deliver safely.
- System voltage. At the same wattage, 24 V halves the current of 12 V and 48 V quarters it. Above about 2,000 W, 24 V or 48 V is the sane choice, because 12 V at high wattage demands enormous cable.
- Cable and fusing. Inverter cable is sized on current, not watts, and must be fused for the inverter's peak draw. This is where DIY installs most often go wrong.
Sizing storage is the companion question: see what size power station do you need for home backup for the battery side of the same calculation.
The bottom line
Continuous watts plus 20–25% headroom, and a peak rating that clears your single largest surge. Whichever of those two demands more is your inverter size. Choose pure sine wave, avoid oversizing because idle draw is constant, and confirm your battery voltage and cable can actually deliver the current the inverter is rated to pull.
Next: string inverters vs microinverters for grid-tied solar, hybrid inverters explained if you want solar and battery backup in one unit, or browse all solar and storage guides.
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