Inverters & equipment
Enter up to four loads with their running watts and surge multiplier. See your total running demand and worst-case combined surge, checked against your inverter's peak rating.
How it works
Step 1
Include everything realistically on at the same time, not just the motors. Steady loads like lights and refrigeration still add to the total running demand. The inverter sizing for motor loads calculator can help identify surge multipliers for common equipment types. The EV charger amperage vs panel output tool covers the EV side of the same question.
Step 2
Non-motor loads like lighting typically get a 1x multiplier, since they draw the same current the whole time. Motors and compressors need a higher multiplier for their startup demand.
Step 3
Add your inverter's peak rating and duration to see a direct pass or fail, or check the result manually using the inverter continuous vs peak rating calculator.
It might seem safest to size for every motor starting at the exact same instant, but that scenario essentially never happens in real household or cabin usage. The realistic worst case is every other load already running steadily in the background while one additional load, the one with the highest surge demand, kicks on. Sizing for a true simultaneous-everything scenario would push the required inverter size far beyond what's actually needed. Conductor and equipment ampacity references for whatever final wiring this feeds into trace to NEC Table 310.16.
It's tempting to assume the load with the highest running watts is automatically the worst-case surging load, but that's not always true. A load's actual surge demand is its running watts multiplied by its own surge multiplier, and a smaller motor with a steep multiplier can out-surge a larger one with a gentler startup curve. Comparing actual surge watts, not running watts alone, finds the true worst case, similar to the specific-load checks in the inverter sizing for well pump startup tool.
This model works well when loads cycle on independently based on their own thermostats, timers, or usage patterns. It works less well for loads that genuinely tend to start together, like a well pump that kicks on right as an air conditioner compressor cycles on during peak afternoon heat. If your actual usage pattern includes a realistic scenario where two loads could surge simultaneously, add their surge demands together manually rather than relying on the single-worst-load simplification. For a backup plan built around only your must-have circuits rather than every possible simultaneous load, the critical loads only sizing calculator takes that narrower, more conservative approach.
Total running load and worst-case surge demand are two of the core inputs to sizing an off grid or hybrid inverter properly. Once you have both figures from this tool, the off grid inverter sizing calculator and hybrid inverter sizing calculator can use them directly to recommend a full inverter size, not just check a single candidate.
If you expect to add equipment later, like a second refrigerator, a larger workshop tool, or air conditioning in a previously unconditioned space, it's worth running this calculation again with those additions included rather than assuming today's loads represent the system's full lifetime demand. EV charging is another increasingly common future addition sharing the same electrical panel, and the EV charger amperage vs panel output tool covers that specific capacity question separately. A little headroom now, along with the Solar Energy Industries Association's plain-language solar overview for the broader planning picture, avoids a costly inverter upgrade down the road.
Questions
Add up the running watts of every load except the one with the highest surge demand, then add that load's full surge watts on top. With a fridge, well pump, lights, and other loads running steadily, a well pump surging to 2,250W on top of 650W of other steady loads produces about 2,900W of total surge demand.
In most real households, motors don't start at the exact same instant by pure coincidence. The realistic worst case is every other load already running steadily while one additional load, typically whichever has the largest surge demand, kicks on at the same moment.
The load with the highest surge wattage, which is its running watts multiplied by its surge multiplier, not necessarily the load with the highest running watts alone. A smaller motor with a higher surge multiplier can produce more surge watts than a larger motor with a gentler startup.
If your actual usage pattern means two loads genuinely could start simultaneously, such as a well pump and an air conditioner both cycling on together, size for both of their surges added together rather than assuming only one surges at a time. This calculator's single-worst-load model is a common simplification, not a universal rule.
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