Understanding inverter pairing
Why battery capacity and inverter capacity are separate questions
Battery capacity in kWh determines how long your loads can run before the battery is depleted, while inverter capacity in watts determines how much power the system can deliver at any single instant, a distinction NREL's own Annual Technology Baseline treats as two separate sizing questions, power capacity and energy capacity. A large battery paired with an undersized inverter can hold plenty of stored energy and still fail to start a high-surge appliance, since the inverter simply can't push enough instantaneous power through. Both figures need to be checked independently when sizing a system, alongside the total capacity figure from the battery storage size calculator.
Why surge rating matters more than continuous rating for many households
Most inverters are rated with both a continuous power figure and a higher, short-duration surge figure, since motor-driven appliances like HVAC compressors, well pumps, and refrigerators all draw significantly more than their running wattage for a brief moment on startup. An inverter with plenty of continuous capacity but insufficient surge capacity will fail to start these appliances even though it can easily sustain them once running, a distinction explained in the Department of Energy's inverter basics guide. Checking both figures against your actual load list, not just the continuous total, prevents this common sizing mistake, especially for high-surge loads like the ones in the well pump backup tool.
How soft-start technology changes this calculation
Soft-start devices, either built into some newer battery and inverter systems or added as a separate component on the motor itself, reduce the current surge a motor draws on startup by ramping power up gradually instead of applying it all at once. This can allow a smaller inverter to reliably run appliances that would otherwise require significantly more surge capacity, which is worth discussing with an installer if your calculated surge requirement exceeds what your preferred inverter can deliver, a cost comparison the quick financing comparison tool in our Financing category can help frame against simply choosing a larger inverter instead.
What to do with this number next
If your calculated requirement exceeds your inverter's rating, the AC coupled vs DC coupled tool covers different inverter architecture options, and discussing soft-start technology or a higher-capacity inverter model with your installer are both worth exploring. For a deeper look at inverter surge and continuous ratings specifically, the inverter continuous vs peak rating calculator in our Inverters & Equipment category covers that distinction in more detail.