What Size Inverter Do I Need? Calculator

Size an inverter by its continuous rating first, then confirm it can handle your surge load. Add up the running watts of everything that runs at once, divide by 0.85 to leave about 15% headroom, and round up to the next real inverter size. A 1,500 W running load becomes 1,500 ÷ 0.85 ≈ 1,765 W, which rounds up to a 2,000 W inverter. Then check surge: motors and compressors can draw 2–3× their running watts for a fraction of a second at start-up, so that same 1,500 W load may briefly need 3,000 W of surge capacity.

Every quality inverter lists two numbers: a continuous (running) rating it can sustain indefinitely and a higher surge (peak) rating it can supply for a few seconds. Undersize the continuous rating and the inverter overheats and shuts down under load; ignore surge and it trips the instant a fridge compressor or well pump kicks on. This calculator handles both so you buy once.

Two equations drive the recommendation above:

The ÷ 0.85 reserves roughly 15% continuous headroom. Running any inverter at 100% of its nameplate rating all day is a recipe for heat build-up, fan noise, reduced efficiency, and early failure. Leaving headroom keeps the unit in its efficient, cool operating band and covers small measurement errors in your load estimate.

The surge factor captures inrush current — the momentary spike when an inductive load energizes. A resistive load (incandescent bulb, heating element, kettle) has essentially no surge, so 1.5× is generous. A universal mix of electronics and small motors lands near 2×. A refrigerator, sump pump, air-conditioner compressor, or shop tool with a capacitor-start motor can spike to 3× or more. The inverter's surge rating must exceed this peak or start-up fails.

Once you know the load, size the storage behind it with the battery runtime calculator and pick the DC input current with the inverter size calculator.

Example 1 — RV essentials, 1,500 W running, 2× surge: Continuous need = 1,500 ÷ 0.85 = 1,765 W, rounded up to a 2,000 W inverter. Surge need = 1,500 × 2 = 3,000 W, which a 2,000 W unit with a 4,000 W surge rating handles easily.

Example 2 — chest freezer only, 200 W running, 3× surge: Continuous need = 200 ÷ 0.85 = 235 W, but you would never buy a 235 W inverter for a freezer because of surge. Surge need = 200 × 3 = 600 W. The tool rounds continuous up to the smallest sensible 1,000 W inverter (2,000 W surge), which starts the compressor with margin to spare.

Example 3 — small workshop, 2,600 W running, 1.5× surge: Continuous need = 2,600 ÷ 0.85 = 3,059 W → a 4,000 W inverter. Surge need = 2,600 × 1.5 = 3,900 W, well inside a 4,000 W unit's 8,000 W surge.

Example 4 — off-grid cabin, 3,400 W running, 2× surge: Continuous = 3,400 ÷ 0.85 = 4,000 W exactly → a 4,000 W inverter, but with zero headroom left, most installers step up to 5,000 W. Surge = 6,800 W. This is the point where a low-frequency inverter with a large surge reserve pays for itself.

The number that sells an inverter is usually the bigger one — the surge or "peak" watts printed in large type on the box. But the number that determines whether the inverter survives a hot afternoon is the smaller continuous rating. Read both.

• Continuous rating is the power the inverter delivers 24/7 without overheating. If your sustained load exceeds it, the inverter throttles, alarms, or shuts down on thermal protection.
• Surge (peak) rating is what the inverter supplies for a fraction of a second to a few seconds — long enough to spin up a motor. A typical high-frequency inverter surges to about 2× continuous; a heavier low-frequency (transformer-based) inverter can surge to 3× or more and hold it longer.

Inductive loads are the reason surge exists. When an electric motor starts, it has not yet begun spinning, so it looks almost like a short circuit for an instant and draws locked-rotor current several times its running current. Refrigerators, freezers, well pumps, sump pumps, air conditioners, and shop tools are the usual culprits. If several might start at once — two fridges, or a fridge while the microwave is running — add their surges. That combined worst-case peak is what your inverter's surge rating must beat.

Beyond wattage, inverters differ in the shape of the AC waveform they produce, and it changes what you can safely run.

• Pure sine wave inverters output a smooth waveform identical to (or cleaner than) grid power. They run anything: variable-speed motors, medical equipment (CPAP, oxygen concentrators), microwaves without buzzing, sensitive electronics, and modern appliances with switch-mode power supplies. They cost more but are the default recommendation for whole-home, RV, and off-grid use.
• Modified sine wave inverters approximate AC with a stepped, blocky waveform. They are cheaper and fine for simple resistive loads (incandescent lights, basic tools, phone chargers), but they can cause motors to run hotter and less efficiently, make audible hum in audio gear and microwaves, and outright damage or refuse to run some electronics and medical devices.

Because a modified sine inverter drives motors less efficiently, budget a slightly higher surge factor and expect a few percent more heat. For any load list that includes a variable-speed motor, a compressor, or medical equipment, choose pure sine. If you are running a house or cabin, pair this sizing with the generator sizing calculator so your backup generator and inverter are matched to the same load list.

1. List every load that runs at the same time. Add up their running watts. Use nameplate labels, a plug-in watt meter, or the power consumption calculator to fill gaps.
2. Enter total running watts. Do not include devices that are switched off; do include phantom/standby draws if they stay powered.
3. Choose a surge factor. Use 1.5× for mostly resistive loads, 2× for a normal mix, and 3× if a fridge, freezer, pump, or AC compressor is in the list.
4. Read the recommended continuous rating. The tool rounds up to a real inverter size and shows the surge watts the unit must supply.
5. Confirm the inverter's spec sheet. Its continuous rating should meet or beat the recommendation, and its surge rating should exceed your surge need.

Then size the battery bank behind it. A rough starting point (shown in the results) is amp-hours at 12V for one hour of runtime; the battery runtime calculator gives an exact figure for your target hours.

"Running a house" means different things depending on whether you want essentials or everything. Trying to power an entire unmanaged house — central air, electric range, dryer, water heater — can demand 10,000–15,000 W and a very large battery bank, which is rarely cost-effective. Most people instead build an essentials load list.

A typical whole-home essentials package — refrigerator, freezer, furnace blower or mini-split, lights, internet, and device charging — averages 800–2,000 W running, with surges to 3,000–5,000 W when the compressor and furnace start together. A 3,000–5,000 W pure sine inverter (or inverter/charger) covers this comfortably. Add a well pump or air conditioner and you move to 5,000–6,000 W or a stacked pair of inverters.

The trick is load management: stagger the microwave, kettle, and hair dryer rather than running them at once, and a modest inverter feels much larger. For a defensible number, total your essentials with the power consumption calculator, feed it into this tool, then size backup generation with the generator sizing calculator.

Use this reference to build a load list. Running watts are what the appliance draws in steady operation; surge watts are the momentary start-up spike used to check your inverter's peak rating. Only add surge for one or two of the largest motors likely to start at the same moment — they rarely all inrush simultaneously.

• Sizing to surge instead of continuous. A "3000 W" bargain inverter may only sustain 1,500 W continuously. Always match the continuous rating to your sustained load.
• Forgetting simultaneous loads. The fridge is not the only thing on; add everything that overlaps.
• Ignoring waveform. A modified sine inverter can hum, overheat motors, or damage medical and audio equipment. Choose pure sine when in doubt.
• No headroom. Running an inverter at 100% all day shortens its life; leave ~15% margin, which this tool builds in.
• Undersized battery or cable. A big inverter starves without enough amp-hours and adequately gauged DC cable — size both with the inverter size calculator.

The safest way to buy once is to size in this order: build the running-watts load list, divide by 0.85 for the continuous rating, identify the single largest motor and multiply its running watts by 3 for the surge check, then confirm the candidate inverter's spec sheet clears both numbers. Favor a genuine pure sine wave unit unless every load is simple and resistive, and prefer a low-frequency (transformer-based) inverter when heavy motor starting is routine, because its larger surge reserve and ability to hold that surge longer make the difference between a pump that starts and one that stalls. Leave room for growth — people almost always add loads after the install — and remember that the inverter is only one third of the system: the battery bank and the DC wiring must be sized to match, or the largest inverter available will still trip on low voltage the moment a compressor kicks on. Cross-check the battery side with the battery runtime calculator before you finalize the purchase.