Battery & storage
Enter your battery size and available excess solar power. See how long charging takes, and whether it fits your daily sun hours.
How it works
Step 1
This is production beyond what your home consumes at that moment, not your total system size, similar to the approach in the self consumption calculator. If this number changes your budget, the quick financing comparison tool shows what the payments look like.
Step 2
This is the peak production window in a day, usually shorter than total daylight hours.
Step 3
If charge time exceeds your sun hours, the battery may not fully charge on an average day. The charge time from grid tool shows how a top-up would help.
Only solar production beyond what your home is actively consuming at any given moment is available to charge the battery, since home loads draw from solar production first before any surplus flows to the battery. A system producing 8 kW while the home uses 3 kW only has 5 kW of excess available for charging, not the full 8 kW, which is an easy distinction to overlook when estimating charge time.
Solar production varies throughout the day based on sun angle, and fluctuates day to day based on cloud cover and weather, making available charging power a moving target rather than the steady rate a grid charger provides, consistent with NREL's solar resource variability data. This is why comparing calculated charge time against typical strong sun hours matters: if the math shows charging takes longer than your area's typical peak production window, the battery may only partially charge on an average day. The charge time from grid calculator covers that more predictable alternative.
On days with less available sun, either from weather or higher home energy use eating into the excess, the battery may only partially charge, simply picking up where it left off charging the following day rather than losing that progress. Over a typical week this pattern usually balances out on its own, though an extended run of cloudy weather can leave a battery persistently undercharged, which is where the charge time from grid tool becomes useful as a backup, a distinction the Department of Energy's inverter basics guide covers as part of how grid-tied systems handle both charging sources.
If solar charge time regularly exceeds your available sun hours, the self consumption battery size tool shows how quickly a grid top-up could fill the remaining gap, and the quick financing comparison tool in our Financing category helps weigh whether a larger solar array would pay for itself against the cost of ongoing grid top-ups. The inverter's own charging behavior also matters here; the hybrid inverter sizing calculator in our Inverters & Equipment category covers that hardware side.
Questions
A 13.5 kWh battery at 90% depth of discharge charging from 4 kW of excess solar power takes about 3.2 hours, comfortably within a typical 5 hour window of strong daily sun.
Excess solar is whatever your panels produce beyond what your home is actively consuming at that moment. If your system produces 8 kW and your home is using 3 kW, only the remaining 5 kW is available to charge the battery, not the full 8 kW production figure.
On days with less sun, cloud cover, or higher home energy use eating into available excess, the battery may only partially charge, picking up where it left off the following day. Over the course of a typical week this usually balances out, though a run of poor weather can leave the battery persistently undercharged.
A bigger array increases total production, but only the portion beyond home consumption charges the battery, so a larger array mainly helps once home loads are already covered. Beyond that point, more solar production directly speeds up available charging power for the battery.
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