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 |
Sizing the inverter from the loads, not the bank
Unlike every other component in this chain, the inverter spec is driven entirely by what the AC loads demand, not by how large the array or bank turned out to be. The bank voltage is already settled by step 4, but the wattage rating comes straight back to step 1. Follow the chain in order and that number is waiting for you:
- Catalogue the AC loads before anything else. Running watts and duty cycles for every appliance that could be on simultaneously are the raw material the inverter spec is built from. The off-grid cabin sizing guide covers the full appliance-by-appliance process, including how to handle seasonal and occasional loads.
- Look up the gloomiest-month sun figure for your location. A system designed against the January number in your region won't run short during the weeks when recharging is hardest. The peak sun hours reference lists seasonal and annual values side by side so you can see the gap.
- Confirm the array covers the load on a short recharge day. Real panel output runs 75 to 85% of nameplate once temperature, wiring and angle losses are counted. The solar panel output calculator applies those factors and shows whether the panels can put back what the loads took overnight.
- Bank sizing follows from the daily load and autonomy target. The battery bank calculator converts those inputs into the Ah and kWh required at your system voltage, with a depth-of-discharge limit applied.
- Fit the charge controller between array and bank. Voltage and current from the array must match what the bank can accept. The charge controller calculator works out the required current rating, including a cold-weather boost allowance.
- You are here: derive the inverter rating from the loads, not the bank. Sum the running watts of everything that could run at once, identify the largest motor start and multiply by its surge factor, then add 20 to 25% headroom. The resulting continuous and surge figures are independent of bank capacity; a larger bank does not change what inverter you need.
- Run the economics once the full component list is fixed. Array, bank and inverter costs together are the input the solar payback calculator needs to show whether the system pencils out or needs rethinking.
The surge column is where inverter sizing diverges from the rest of the chain. A fridge compressor or pump motor draws three to five times its running watts for a fraction of a second on startup, and it is that brief peak, not the continuous wattage, that trips an undersized unit. I treat the locked-rotor demand of the single largest motor as the hard floor for the surge rating, with the continuous load and headroom sitting comfortably below it.
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.
My inverter keeps tripping when the fridge or pump starts. What size do I actually need?
The trip is almost certainly a surge overload, not a continuous overload. A fridge compressor or pump motor draws three to five times its running watts for the fraction of a second it starts, and that peak must fall within the inverter's surge rating, not just its continuous rating. Check your inverter's datasheet for the surge figure, usually listed as a 3 to 5 second peak: a unit rated "2000 W continuous / 4000 W surge" can handle a 4,000 W startup spike. If the motor's locked-rotor demand exceeds the surge rating, you need the next size up regardless of how comfortable the continuous load looks. Use this calculator's surge capacity output as the floor for the surge rating you shop for.
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.
Methodology and sources
This tool sizes an inverter from its two limits at once: the continuous rating it can supply indefinitely and the short surge it can deliver to start motors. It also returns the battery-side DC current, which is what actually sets your cable gauge and DC fuse.
- Method: Continuous rating = load × (1 + headroom). Surge capacity = load × surge factor. Battery-side DC draw = load / efficiency / system voltage (from input power = output power / efficiency, and current = power / voltage). The suggested size is the smallest standard inverter whose continuous rating clears the load-with-headroom and whose surge (taken as 2 × continuous) clears the surge capacity.
- Standards and sources: Standard electrical physics, not a wiring standard: conservation of power (input = output / efficiency) and Ohm's-law power relation P = V × I. The surge multiple and the running-watt / surge-factor reference figures are conventional values for residential off-grid loads, not values fixed by any code.
- Assumptions and limits: Assumes a default 20% headroom unless you change it, the surge factor is applied to the full continuous load (a conservative worst-case where every load could start at once), and the inverter's surge ceiling is approximated as twice continuous. Real inverter surge ratings, duration windows and idle draw vary by model; pure sine wave is preferred for motors and compressors. The DC current shown is at full load and ignores cable and connection losses.
Reviewed and maintained by Rick Oosterling, who builds and wires 12 V, solar and EV systems hands-on. Last reviewed: June 2026. This is a planning aid, not a substitute for a qualified professional or your local wiring and building code; have battery cable, fusing and any fixed wiring verified against the rules that apply where you are.