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How many batteries are needed to run a house off-grid? Calculate daily kWh, autonomy, usable depth of discharge and efficiency, then divide the required nominal energy by the kWh of one compatible battery and round up.
- Topic
- How Many Batteries Are Needed to Run a House Off-grid?
- Answer in brief
- How many batteries are needed to run a house off-grid? Calculate daily kWh, autonomy, usable depth of discharge and efficiency, then divide the required nominal energy by the kWh of one compatible battery and round up.
- Required project inputs
- Load list, daily runtime, peak power, backup target, location, installation environment and quantity
- Decision output
- PV power, usable battery energy, inverter power, controller range, protection configuration and purchasing checks
- Published
- 2026-07-12
- Last reviewed
- 2026-07-14
Sources: Santa Cruz County Off-Grid Solar Requirements; U.S. EPA Smart Energy Resources Guide
Use boundary: values support preliminary evaluation; confirm the final model, datasheet, certification, compatibility, price and delivery terms before purchase.
Direct Answer
How many batteries are needed to run a house off-grid? Calculate daily kWh, autonomy, usable depth of discharge and efficiency, then divide the required nominal energy by the kWh of one compatible battery and round up.
How many batteries are needed to run a house off-grid?
There is no correct battery count based only on house size or inverter watts. Calculate the home's critical-load energy in kWh/day, multiply by the required no-solar autonomy, divide by usable depth of discharge and delivery efficiency, then divide by the nominal kWh of one compatible battery. Round up and verify system voltage, BMS parallel limits, discharge current, surge loads and the PV array's ability to recharge the bank.
Battery quantity formula
Battery quantity = (daily critical-load kWh × autonomy days ÷ usable DoD ÷ delivery efficiency) ÷ nominal kWh per battery
The answer must be rounded up to whole batteries, but the electrical configuration may require a different number to meet series/parallel and voltage rules.
Worked example
For an illustrative 12 kWh/day critical-load profile, one day of autonomy, 80% usable depth of discharge and 90% delivery efficiency, required nominal energy is 16.7 kWh. A 5.12 kWh module gives 3.26 units, so the arithmetic result is four compatible modules. This is not a universal recommendation: use the selected manufacturer's permitted DoD, current, temperature and parallel data.
Checks that can change the answer
- Air conditioners, pumps, refrigerators and compressors can require surge power beyond their running watts.
- Cold or hot conditions, aging and high discharge current can reduce usable energy.
- A larger bank needs enough PV and charging current to recover after a low-solar period.
- Different battery brands, models, ages or capacities should not be mixed without an approved method.
- Critical-load separation can reduce the bank more effectively than trying to power every appliance.
Use the off-grid calculator or review the complete PV, battery and inverter sizing guide.
Official references
- Santa Cruz County uses winter daily consumption, autonomy and maximum battery discharge in its official sizing example.
- U.S. EPA guidance identifies autonomy, discharge rate and depth, life expectancy and environmental conditions as battery-sizing factors.
Verification Boundary
This article supports preliminary project selection. Confirm models, specifications, certifications, pricing, stock, protocol compatibility and lead time before purchase.
Frequently Asked Questions
Can a 5 kW inverter tell me how many batteries I need?
No. It indicates power capability, while battery quantity depends on daily kWh, autonomy, usable depth of discharge, efficiency, battery voltage and current limits.
How many 5.12 kWh batteries are needed for 12 kWh per day?
Using the illustrative assumptions of one day, 80% usable DoD and 90% efficiency gives 16.7 kWh nominal, or 3.26 modules, rounded to four. Replace every assumption with the selected product and site data.
Is a larger battery bank always better?
No. An oversized bank with insufficient PV may recover slowly, and excessive parallel strings can complicate current sharing, protection and maintenance.
Should the whole house remain powered?
Not necessarily. Separating refrigerator, lighting, communications and essential pumps from discretionary loads can reduce system size.
