Buying guide
Inverter sizing: continuous watts and surge
Why start-up surge often decides the inverter, not running watts.
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Size an inverter so its continuous rating covers the total watts of everything running at once, with headroom (often 20–25%), and its surge rating covers the start-up spike of your biggest motor load. Motors in fridges, pumps, compressors and power tools can briefly draw several times their running watts. Use a pure sine wave inverter for most modern electronics and motors. The battery and cabling must also be able to supply the inverter's current. At 12 V, a 2,000 W inverter can draw well over 150 A.
- Continuous rating must exceed everything that runs at the same time.
- Surge rating must cover the biggest motor start.
- Pure sine wave output suits most modern electronics.
Continuous vs surge ratings
Every inverter lists at least two power ratings. The continuous rating is what it can supply steadily, often specified at a certain temperature. The surge or peak rating is a short burst it can handle, typically for motors starting up.
Motors and compressors need extra current to get moving. A refrigerator that runs at about 150 W might briefly need several times that when the compressor kicks on. Honda's published wattage guide shows how big the gap can be: an Energy Star refrigerator or freezer at about 1,200 W starting vs 132–192 W running, a 1/2 hp sump pump at 2,150 W vs 1,050 W, a 10,000 BTU window air conditioner at 2,200 W vs 1,500 W, and a 13,500 BTU RV air conditioner at 2,800 W vs 1,800 W (Honda Power Equipment, approximate starting and running watts, checked Nov 2, 2026). These are approximate figures; check your appliance's label or manual for its own. If the inverter's surge rating can't cover that spike, it will shut down or the appliance won't start.
Step-by-step sizing
- List loads that might run at the same time. Not everything you own, only what runs together.
- Add their running watts. That's your continuous load.
- Add headroom. Many planners add 20–25% so the inverter isn't at its limit.
- Find the largest surge. Usually the biggest motor. Add its surge watts to the running watts of everything else.
- Choose an inverter whose continuous rating covers step 3 and surge rating covers step 4.
| Example load | Running W | Surge W (estimate) |
|---|---|---|
| Refrigerator | 150 | 450–1,200 |
| Lights and router | 60 | 60 |
| Laptop | 60 | 60 |
| Microwave | 1,200 input | 1,200+ |
| Well pump (1/2 hp) | 700–1,000 | 2,000+ |
These figures are rough examples. Check labels and manuals.
Worked examples
Example 1: fridge and essentials
Running: fridge 150 + lights and router 60 + laptop 60 = 270 W. With 25% headroom, that's about 340 W continuous. Surge: fridge start of about 1,200 W plus 120 W of other loads makes about 1,320 W. A 1,000 W continuous inverter with 2,000 W surge covers this with room to grow.
Example 2: adding a microwave
The microwave draws about 1,200 W from the outlet. Running together with the fridge makes about 1,470 W. With headroom, about 1,850 W continuous. A 2,000 W continuous inverter is a reasonable fit, if the battery can supply the current.
Example 3: a well pump
Well pumps can have large start-up surges. Many installers size inverters with generous surge capacity or use soft-start devices. This is a job for a qualified installer.
Battery and cable current
An inverter's AC output comes from DC current from the battery. Lower system voltage means higher current for the same power.
| Inverter load | Approx. DC current at 12 V | At 24 V | At 48 V |
|---|---|---|---|
| 500 W | ~45 A | ~23 A | ~11 A |
| 1,000 W | ~90 A | ~45 A | ~23 A |
| 2,000 W | ~180 A | ~90 A | ~45 A |
| 3,000 W | ~270 A | ~136 A | ~68 A |
These include 92% inverter efficiency, the inverter derate in the NREL PVWatts manual (a grid-tie figure, so treat it as an estimate for battery inverters). High current needs thick cables, proper fusing and a battery rated for that discharge. That's one reason larger systems use 24 V or 48 V. See system voltage.
Waveform and features
Most modern systems use pure sine wave inverters. Features to compare include:
- Idle draw: important for small batteries.
- Built-in charger (inverter-charger): charges batteries from a generator or grid.
- Transfer switch: switches loads between sources automatically.
- Low-voltage cutoff: protects batteries.
- Cooling and noise: fans can be loud under load.
Common mistakes
- Sizing for running watts only. Motors need surge.
- Using a modified sine inverter for sensitive equipment.
- Undersized cables and missing fuses.
- A battery that can't supply the current.
- Ignoring idle draw.
Why motors surge
An electric motor at rest has very low resistance in its windings. When you first apply power, a large current flows until the motor spins up and starts generating back-voltage, which limits the current. In compressors like refrigerators and air conditioners, the motor also has to start against the pressure of the refrigerant, which adds to the load. That startup spike is why a fridge running at 150 W can briefly ask for many times more.
Different loads behave differently. Resistive loads like kettles, toasters and incandescent bulbs draw about the same power from start to finish. Electronics with power supplies, like computers and TVs, can have a short inrush when first plugged in but generally run steadily. Motors and compressors have the largest and longest surges. Some appliances, like inverter-drive refrigerators and mini-split air conditioners, use variable-speed motors that start gently and have much smaller surges.
If you're unsure about a particular appliance, the most reliable approach is to check its manual or ask the manufacturer for the locked-rotor current or startup watts. Soft-start kits are available for some air conditioners and pumps to lower the surge.
Efficiency and runtime
Inverters lose some energy as heat when converting DC to AC. Many datasheets quote peak efficiency in the low to mid 90s percent, but efficiency is usually lower at very light loads. A large inverter powering only a phone charger wastes a bigger share of energy than one running near its sweet spot.
To estimate runtime, divide the battery's usable watt-hours by the AC load, then multiply by inverter efficiency. A 1,280 Wh LiFePO4 battery with 80% usable depth (Victron LiFePO4 data), powering 200 W at 92% efficiency (NREL PVWatts manual), gives roughly 1,280 × 0.80 × 0.92 ÷ 200 ≈ 4.7 hours. Idle draw adds up over a day, so subtract it for long runs. The power planner handles this math.
Safety notes
- Install inverters in dry, ventilated spaces, as the manual specifies.
- Use correctly rated fuses or breakers near the battery.
- Never backfeed home wiring through an outlet. Connections to home wiring need a code-compliant transfer switch installed by a licensed electrician, following the NEC and local codes.
- See solar and battery safety and the power planner.
FAQ
What is the difference between pure sine and modified sine?
Pure sine inverters produce a smooth waveform like grid power. Modified sine inverters produce a stepped waveform that's cheaper but can make some motors, chargers and electronics run hot, buzz or fail.
How long can an inverter run at surge?
Surge ratings usually last only for a short time, often fractions of a second to a few seconds. Check the datasheet.
Does the inverter use power when idle?
Yes. Idle or no-load draw varies by model and can matter for small battery banks. Turn the inverter off when not needed if possible.
Can I measure surge myself?
Some clamp meters have an inrush current function. Otherwise, use the appliance maker's data or a general multiple as an estimate.
Where do I go next?
Try the power planner to put numbers to your own situation, or see the product picks on the deals page.
Terms used here
- Watt (W)
- Rate of power use or production.
- Watt-hour (Wh)
- Energy: one watt for one hour.
- LiFePO4 (LFP)
- Lithium iron phosphate battery chemistry.
- Inverter
- Converts DC battery power to AC household power.
- Pure sine wave
- Inverter output that closely matches grid power.
- Surge rating
- Short-term peak power an inverter can supply.
- Derate
- Factor for real-world losses in solar output.
- Transfer switch
- Device that isolates home circuits from the grid during backup.
- Backfeed
- Power flowing into utility lines from a home source; dangerous.
Sources
This guide is informational and based on published specifications and general references, not hands-on testing. Check the manufacturer's current specs before you buy.