Home Battery Sizing Calculator
Size a home battery bank for backup from the daily load you want to protect, your desired days of autonomy, usable depth of discharge, round-trip efficiency, and a reserve margin.
Last updated: May 2026
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About this calculator
Nameplate battery capacity and usable capacity are not the same number, so sizing storage means inflating your raw energy need to cover three real-world losses. Start with the energy you actually need to deliver: the daily load you want to back up multiplied by the number of days of autonomy you want to ride through (a grid outage or a stretch of cloudy off-grid days). Add a reserve margin on top — headroom for cold-weather capacity loss, future load growth, and the simple fact that you don't want to routinely drain a battery to empty. Then divide by two efficiency factors. Depth of discharge (DoD) is the share of the battery you can safely use: lithium iron phosphate allows about 90–100%, while older lead-acid is limited to 50% to preserve cycle life, so a 50% DoD doubles the bank you must buy. Round-trip efficiency captures the energy lost converting DC to AC and back through the inverter and the cells themselves, typically 85–95%. Dividing the margin-inflated need by both fractions yields the nameplate kWh you must purchase. Because both denominators are fractions less than one, poor DoD or efficiency multiplies the required size — which is why chemistry choice dominates battery cost.
How to use
Worked example. You want to back up 10 kWh per day of essential loads (fridge, lights, internet, a few outlets) for 2 days of autonomy, using a lithium battery rated for 90% depth of discharge and a system that is 90% round-trip efficient, with a 20% reserve margin for cold weather and growth. Step 1 — raw energy need: 10 kWh × 2 days = 20 kWh. Step 2 — add the reserve margin: 20 × 1.20 = 24 kWh of deliverable energy. Step 3 — divide by usable depth of discharge: 24 ÷ 0.90 = 26.7 kWh. Step 4 — divide by round-trip efficiency: 26.7 ÷ 0.90 = 29.63 kWh of nameplate capacity. So you'd buy roughly three 10 kWh battery modules. Swap to a lead-acid bank at 50% DoD and the same need balloons to about 53 kWh — vivid proof that depth of discharge, not price per kWh alone, drives the real cost of backup.
Frequently asked questions
What size battery do I need to back up my house?
Size from the load you actually want to protect, not your whole house. Most owners back up essential circuits — refrigerator, lights, internet, a few outlets, maybe a well pump — totaling roughly 8–12 kWh per day. Multiply that by how many days you want to last without sun or grid, add a reserve margin, then divide by your battery's usable depth of discharge and round-trip efficiency to get nameplate capacity. A typical 10 kWh/day, 2-day backup needs around 30 kWh of lithium storage. Whole-home backup including HVAC and electric heat can require 40–80+ kWh.
Why can't I use 100% of my battery's rated capacity?
Two reasons, and the calculator accounts for both. First, depth of discharge: routinely draining a battery to zero shortens its life, so manufacturers rate usable capacity below nameplate — about 90% for lithium iron phosphate and only ~50% for lead-acid. Second, round-trip efficiency: converting stored DC energy through the inverter to AC and back loses 5–15% as heat. A '30 kWh' lead-acid bank at 50% DoD and 85% efficiency only delivers about 12.75 kWh of usable AC energy. That's why this tool divides your need by both fractions to find the true nameplate size you must buy.
How many days of battery autonomy should I plan for?
It depends on your goal. For grid-tied backup against occasional outages, 1–2 days is usually plenty, since the grid or sun returns quickly and you can ration loads. For off-grid living you typically plan 2–3 days of autonomy to ride through cloudy stretches, and in cloudy winter climates some designers use 3–5 days. More autonomy means a much larger and costlier bank, so most people balance autonomy against budget by backing up only essential loads and adding a reserve margin rather than stretching to a week.