When to use: Answering "how many kWh of battery storage do I need" for a home or facility battery energy storage system (BESS). Sizing requires two independent numbers — energy (kWh), how much the battery can store and deliver, and power (kW), how fast it can deliver it. This tool covers both backup power sizing (critical load × duration) and solar self-consumption sizing (shifting daily excess solar into evening use).
Sizing a battery energy storage system (BESS) — whether for whole-home backup, critical-load backup, or solar self-consumption / time-shifting — requires answering two separate questions: how much energy (kWh) must the battery store and deliver, and how much power (kW) must it be able to deliver at once. This calculator walks through both, starting from a critical load and backup duration (or a daily excess solar figure), applying depth of discharge (DoD) to find the nameplate capacity needed, and sizing the inverter/power rating independently from the peak of the load being served.
A battery energy storage system has two independent ratings that both have to be sized correctly: energy capacity (kWh), the total amount of electricity the battery can store and discharge, and power rating (kW), the rate — inverter and battery C-rate limited — at which that energy can be delivered at any instant. A common mistake in DIY or back-of-envelope sizing is to calculate only the energy requirement (kWh needed for X hours of backup) and assume the inverter will simply handle whatever load shows up. It will not: if the critical load panel draws 8 kW at once (for example, a well pump and HVAC compressor starting simultaneously) but the inverter is only rated for 5 kW continuous, the system will trip or brown out even though there is plenty of stored energy left in the battery.
Energy sizing answers "how long." Power sizing answers "how much all at once." For backup power, energy sizing is Critical Load (kW) × Backup Duration (hrs); power sizing is the peak instantaneous draw of everything on the critical-load panel, usually with a margin for motor starting inrush (the "Peak Simultaneity Factor" in this calculator). For solar self-consumption, energy sizing is driven by how much daily excess solar production needs to be shifted into the evening, while power sizing is typically driven by the evening peak household or facility demand rather than the solar array's output.
Depth of discharge (DoD) is the percentage of a battery's rated (nameplate) capacity that can be safely cycled. A 100% DoD battery would deliver its full nameplate rating every cycle, but most battery chemistries reserve a margin at the top and/or bottom of the state-of-charge range to protect cell health and cycle life. Modern lithium iron phosphate (LFP) residential and commercial batteries commonly specify around 90% usable DoD, meaning a 13.5 kWh nameplate unit delivers roughly 12.15 kWh of usable energy per full cycle. Older lead-acid systems are far more conservative, often limited to 50% DoD to avoid rapid capacity fade.
Because DoD reduces how much of the nameplate rating is actually usable, sizing must work backward from the usable energy requirement to the nameplate capacity: Nameplate kWh = Usable kWh Needed ÷ DoD%. Round-trip efficiency (RTE) is a separate figure — it describes how much extra source energy (grid or solar) is consumed while charging the battery to make up for internal losses, not how much of the stored capacity is usable. RTE matters for sizing the solar array or charging source, not for sizing the battery's nameplate capacity itself.
Backup power sizing starts from the loads you want to keep running during a grid outage — typically a subset of circuits (refrigerator, well pump, some lighting and outlets, communications equipment) rather than the whole house or facility, since backing up everything requires a much larger and more expensive system. The two inputs are the combined critical load in kW and how many hours of autonomy are desired; the calculator applies DoD to find nameplate kWh and applies a peak simultaneity margin to find the required inverter kW.
Solar self-consumption (time-shifting) sizing instead starts from the daily production profile: how much solar energy is generated during the day beyond what is consumed in real time, which would otherwise be exported to the grid (often at a lower net-billing credit than the retail rate) or curtailed. Sizing the battery to capture that daily excess and discharge it during the evening peak maximizes the value of a solar array under net billing or time-of-use rate structures. The two sizing approaches can be combined — a battery sized primarily for solar self-consumption also happens to provide some backup capability, though it should be sized deliberately for backup if that is a primary goal, since a battery optimized purely for daily cycling may be at a low state of charge right when an outage begins in the evening.
Multiply your critical load in kW by the number of hours of backup you want, then divide by your battery's usable depth of discharge (DoD). For example, a 3 kW critical load backed up for 8 hours needs 24 kWh of usable energy; at 90% DoD, that requires about 26.7 kWh of nameplate battery capacity. You also need to separately confirm the inverter/power rating can handle the peak instantaneous draw of that critical load panel, including motor-starting inrush.
kWh (kilowatt-hours) is energy — the total amount of electricity the battery can store and deliver over time, which determines how long it can run your loads. kW (kilowatts) is power — the rate at which that energy can be delivered at any given instant, which determines how much load can be running simultaneously. A battery can have plenty of kWh remaining and still fail to serve a load if its kW (inverter) rating is exceeded.
For modern lithium iron phosphate (LFP) residential and commercial batteries, 90% usable DoD is a reasonable and commonly specified default — always check the specific manufacturer datasheet, since usable DoD varies by product and warranty terms. Older lead-acid battery banks should be sized much more conservatively, typically around 50% DoD, to avoid accelerated capacity fade and shortened cycle life.
Divide your required nameplate capacity by the unit size of a standard residential battery module — this calculator defaults to 13.5 kWh per unit as a generic reference size representative of common residential battery products, not tied to any specific brand, and rounds up to a whole number of units since batteries are sold as discrete modules. Adjust the unit size input to match whichever specific product you are evaluating.
Not directly — round-trip efficiency (RTE) affects how much extra source energy (from the grid or solar array) is needed to charge the battery to deliver a given amount of usable energy, not how much of the battery's nameplate capacity is usable. Depth of discharge (DoD), not RTE, is the figure that determines nameplate capacity needed for a given usable-energy target. RTE does matter when sizing the solar array or charging source for a self-consumption system, since some of the harvested energy is lost in the charge/discharge round trip.
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