Select your critical loads, choose your backup duration, and pick your battery chemistry. This calculator sizes your battery bank, tells you how many batteries you need, recommends an inverter, and gives you a realistic cost range — all in one step.
| System Voltage | Total Ah Needed | Est. Battery Count | Notes |
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Sizing a battery backup system comes down to three numbers: how many watts your critical loads draw at once, how many hours you need to run them, and how deeply you can discharge the battery without damaging it. Get those three things right and the math is straightforward — everything else is packaging and cost.
These are the two battery chemistries most homeowners and homesteaders will encounter. AGM (Absorbent Glass Mat) lead-acid batteries are cheaper upfront — typically $100–$150 per usable kilowatt-hour — but they have a critical limitation: you should never discharge them below 50% of their rated capacity. Doing so repeatedly destroys their lifespan, which is already limited to 300–500 deep cycles. That means a 200 Ah / 12 V AGM battery (2.4 kWh nameplate) gives you only 1.2 kWh of truly usable energy.
Lithium iron phosphate (LiFePO4) batteries cost more — roughly $200–$300 per kilowatt-hour — but they tolerate 80% depth of discharge (DoD) routinely, deliver 2,000–5,000+ cycles, and maintain a flat voltage curve through most of their discharge. That flat curve means your inverter runs more efficiently and your appliances see steadier power. Over a 10-year horizon, LiFePO4 almost always wins on cost-per-cycle for whole-home or critical-load backup systems.
Depth of discharge is the percentage of a battery's rated capacity you actually use before recharging. Exceeding the recommended DoD doesn't just mean the battery is empty — it chemically degrades the plates (in lead-acid) or stresses the cells (in lithium), cutting total lifespan sharply. This calculator applies a 50% DoD limit for AGM and an 80% DoD limit for LiFePO4. The result is your required nameplate capacity — always buy at least that much, not just the usable amount.
A critical load is anything that must keep running during a power outage: the refrigerator, a well pump for water access, medical equipment, or a sump pump protecting your basement. Non-critical loads — air conditioning, electric water heaters, clothes dryers, ovens — draw far too many watts to run economically on battery. The goal of a backup system is not to replace the grid; it is to keep the essential appliances alive long enough to weather the outage safely.
A UPS (Uninterruptible Power Supply) is designed for short bridge power — 5 to 30 minutes — to protect sensitive electronics during brief blinks or to allow a graceful shutdown. Whole-home or whole-homestead battery systems (like the ones this calculator sizes) are designed to run critical loads for hours or days. They use full-size inverter-chargers rather than the sealed internal inverters in consumer UPS units, and they connect to a transfer switch or subpanel rather than individual outlets.
Grid-tied battery systems stay connected to the utility and charge primarily from the grid or solar panels when rates are low. Off-grid systems are your only source of power and must be sized for worst-case — multiple cloudy days, peak seasonal loads, and full backup duration without a recharge. This calculator sizes for a fixed backup window, which is appropriate for grid-tied backup, a generator-paired system, or off-grid scenarios where the generator or solar will eventually recharge the bank.
The inverter converts DC battery power to AC household power. Its continuous watt rating must exceed your peak simultaneous load. This calculator applies a 1.25 safety factor — meaning if your total load is 1,000 W, it recommends at least a 1,250 W continuous inverter. Well pumps, sump pumps, and refrigerators all have motor starting surges that can be 2–3 times their running watts; your inverter must handle those surge loads as well. Check both the continuous and peak surge ratings on any inverter you purchase.