Home Battery Sizing Calculator
Size a home battery storage system to cover a specific backup load for a chosen number of days without grid or solar input, accounting for depth of discharge, round-trip efficiency, and a safety reserve.
Last updated: June 2026
Battery capacity needed
29.63 kWh
20-40 kWh needs two stacked units. At ~29 kWh you are sizing for about 2 days of autonomy on a 10 kWh/day load; trimming autonomy by a day cuts capacity nearly in half.
This is the rated battery capacity needed to cover your backup load after depth-of-discharge, efficiency, and reserve losses; a single Tesla Powerwall is about 13.5 kWh usable.
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About this calculator
Battery capacity is rated in total kWh, but you cannot safely use all of it. Lithium iron phosphate (LFP) batteries are typically discharged to 80–90% depth of discharge (DoD), while lead-acid cells should stay above 50%. Additionally, energy is lost in the inverter and charging process, captured by round-trip efficiency. The required gross capacity formula is: capacity = (backupLoad × autonomyDays × (1 + reserveMargin / 100)) / ((depthOfDischarge / 100) × (systemEfficiency / 100)). The reserve margin adds headroom above your calculated need — typically 10–20% — to account for unexpected extra loads or battery aging. For example, a 10 kWh daily load over 2 days with 90% DoD, 95% efficiency, and 15% reserve requires a gross bank of about 27.5 kWh.
How to use
You want to back up 8 kWh/day of critical loads (fridge, lights, router) for 2 days without grid power. Your LFP battery has 90% DoD and 95% round-trip efficiency, and you add a 10% reserve margin. Numerator = 8 × 2 × (1 + 0.10) = 8 × 2 × 1.10 = 17.6 kWh. Denominator = (90/100) × (95/100) = 0.90 × 0.95 = 0.855. Required gross capacity = 17.6 / 0.855 = 20.6 kWh. You would therefore need at least 20.6 kWh of nameplate battery capacity — for example, two 10 kWh battery modules or three 8 kWh units.
Frequently asked questions
What depth of discharge should I use for a home battery backup system?
Depth of discharge depends on battery chemistry. Lithium iron phosphate (LFP) batteries, used in popular products like the Tesla Powerwall and Enphase IQ Battery, support 80–100% DoD with minimal cycle life impact. Standard lithium-ion NMC cells are typically limited to 80–90% DoD. Lead-acid and AGM batteries should not exceed 50% DoD without significantly shortening their lifespan. Always use the manufacturer's recommended DoD, not the theoretical maximum, to preserve warranty and longevity.
How many days of battery autonomy do I need for home backup power?
This depends on why you need backup power. For short grid outages due to storms, 1–2 days of autonomy covers the vast majority of events in most regions. Off-grid cabins or homes in areas with frequent multi-day outages should plan for 3–5 days, or pair storage with a generator. If the battery is being charged by solar, autonomy days represent the worst-case consecutive cloudy days your system must survive. Adding more autonomy days increases battery cost substantially, so balance that against your actual outage risk.
What is round-trip efficiency and why does it matter for battery sizing?
Round-trip efficiency (RTE) is the percentage of energy you get back out of a battery relative to what you put in. A battery with 95% RTE returns 95 kWh for every 100 kWh charged. Energy is lost as heat in the battery cells, the inverter, and the charge controller. Modern LFP systems achieve 90–97% RTE, while older lead-acid systems may only reach 70–85%. A lower RTE means you need a larger gross battery bank to deliver the same usable energy, so it directly inflates the capacity figure the calculator returns.