Inverter Sizing Calculator
Find the right inverter for your solar or off-grid system. Enter your continuous AC load, motor surge factor, system voltage and inverter efficiency to get the required continuous rating, surge capacity, a suggested standard inverter size, and the battery-side DC current that drives your cable and fuse sizing.
Last updated: May 2026
Enter continuous load, surge factor, system voltage and inverter efficiency above.
Continuous rating = load × (1 + headroom) · surge capacity = load × surge factor · DC draw = load ÷ efficiency ÷ system V
How to size a solar inverter
An inverter is rated by two numbers, and you have to satisfy both. The continuous rating is the wattage it can supply indefinitely; the surge (or peak) rating is the much higher wattage it can supply for a few seconds while a motor starts. Sizing on the continuous figure alone is the most common mistake, because a fridge or pump that runs at 200 W can demand 600 to 1,000 W for the half-second its compressor spins up. Start by adding the running watts of everything that runs at the same time, add 20 to 25% headroom for future loads and hot-weather derating, then check that the surge rating clears your largest motor start.
Continuous rating versus surge rating
The continuous rating must cover the steady sum of your simultaneous loads with margin to spare. Running an inverter near 100% of its continuous rating for hours overheats it and trips the thermal cutout. The surge rating, usually quoted as a 3 to 5 second figure, is what actually starts motors: most quality inverters surge to roughly twice their continuous rating, so a 2,000 W inverter typically peaks around 4,000 W. This calculator applies your surge factor to the full continuous load, a deliberately conservative approach that guarantees a worst-case simultaneous start is covered. If only one motor surges, you can size more tightly by adding that motor's startup watts to the running watts of everything else.
Why efficiency only affects the DC side
Inverter efficiency, typically 85 to 95%, does not change the AC rating you need: a 1,500 W load still requires a 1,500 W (plus headroom) inverter. What efficiency changes is how much current the inverter pulls from your battery. At 90% efficiency a 1,500 W AC load draws 1,500 ÷ 0.9 = 1,667 W from the DC side, and on a 24 V bank that is 69 A. That DC current, not the AC wattage, sets your battery cable gauge and DC fuse. Higher system voltage cuts the current proportionally: the same load on a 48 V bank draws only 35 A, which is why larger systems move to 48 V.
Worked example
A cabin runs a 1,500 W continuous load and includes a large motor, so a surge factor of 3 is appropriate, on a 24 V bank at 90% efficiency with 20% headroom. Continuous rating needed = 1,500 × 1.2 = 1,800 W. Surge capacity = 1,500 × 3 = 4,500 W. The calculator suggests a 2,500 W inverter: its roughly 5,000 W surge clears the 4,500 W peak, whereas a 2,000 W unit's 4,000 W surge would fall short. Battery-side DC draw = 1,500 ÷ 0.9 ÷ 24 = 69 A, which is the figure you size the battery cable and fuse around.
Typical appliance running watts and surge factors
| Appliance | Running watts | Surge factor | Notes |
|---|---|---|---|
| LED lighting (whole room) | 50 to 150 W | 1× | Resistive, no inrush |
| Laptop / phone charging | 50 to 100 W | 1× | Switching supply |
| LED television | 50 to 200 W | 1.2× | Brief capacitor inrush |
| Microwave (800 W output) | 1,100 to 1,400 W | 1.5× | Draws more than its rated output |
| Refrigerator / freezer | 100 to 200 W | 3× | Compressor locked-rotor start |
| Water pump (1/2 hp) | 500 to 700 W | 3× | High starting torque |
| Washing machine | 500 to 1,000 W | 2 to 3× | Motor plus heater |
| Air conditioner (window) | 1,000 to 1,500 W | 3 to 5× | Compressor start surge |
| Power tools (circular saw) | 1,200 to 1,800 W | 2 to 3× | Universal motor inrush |
Frequently Asked Questions
How big an inverter do I need for an off-grid system?
Add the running watts of every appliance that can run at the same time, not the total of everything you own. That simultaneous sum is your continuous load. Multiply by 1.2 to 1.25 for headroom and you have the continuous inverter rating to shop for. Then identify your largest motor: its startup surge, often three times its running figure, must fall within the inverter's surge rating. A typical small off-grid cabin lands at a 2,000 to 3,000 W inverter; a full house with air conditioning and a well pump needs 5,000 W or more, frequently split across parallel units.
What is the difference between continuous and surge (peak) inverter power?
Continuous power is the wattage an inverter can deliver indefinitely without overheating. Surge or peak power is a much higher wattage it can hold for only a few seconds, specifically to start motors and compressors whose inrush current is several times their running draw. Most inverters surge to about twice their continuous rating, though some pure sine units reach three times. When a datasheet lists "2000 W / 4000 W," the first number is continuous and the second is the surge. You must satisfy both: continuous covers the steady load, surge covers the worst-case motor start.
Why does the inverter draw more current from the battery than the load suggests?
Two reasons. First, inverters are 85 to 95% efficient, so a 1,000 W AC load actually pulls 1,050 to 1,180 W from the battery, the difference lost as heat. Second, batteries are low voltage, so the current is large: 1,100 W on a 12 V bank is 92 A, on 24 V it is 46 A, on 48 V just 23 A. Always size battery cable and the DC fuse to the battery-side current at full load, including the efficiency loss, not to the AC wattage. The calculator's DC draw figure is exactly this number.
Can a solar inverter run a fridge, well pump or air conditioner?
Yes, but only if the surge rating covers the compressor start. A fridge that runs at 150 W can surge past 1,000 W for a fraction of a second; a 1/2 hp well pump running at 600 W can demand 5,000 to 7,000 W on startup. A pure sine wave inverter is strongly preferred for any motor or compressor: modified sine wave units run motors hot and can damage electronics. Size the surge rating to the locked-rotor demand of your single largest motor, then confirm the continuous rating still covers everything running together once that motor is up to speed.
Does a bigger inverter waste power when lightly loaded?
Somewhat. Every inverter has a no-load or idle draw, the power it consumes just being switched on, typically 10 to 40 W and roughly proportional to its size. A 5,000 W inverter idling can quietly consume 0.5 to 1 kWh per day doing nothing, which on a small battery bank is significant. Oversizing for surge headroom is sensible, but a 5,000 W inverter powering a 200 W night load is wasteful. Many inverters offer a power-save or standby mode that drops idle draw until a load is detected; enable it on lightly used circuits.