Going off-grid is liberating until the lights go out on the second cloudy day. The battery bank is the heart of any off-grid system — it stores the sun you collected yesterday for the night you need it tonight — and sizing it wrong is the most expensive mistake in the whole setup. Too small, and you run dry, damage the batteries by over-discharging them, and live with a system you cannot trust. Too big, and you have spent thousands on capacity you never cycle. The off-grid battery bank size calculator finds the right size: your daily energy use, the number of cloudy days you want to ride through, your system voltage, and the safe depth of discharge for your battery chemistry, combined into one amp-hour figure you can shop for.
How to use this calculator
Battery sizing is energy arithmetic. Start with daily energy use in watt-hours — the sum of everything the system powers in a day. Multiply by days of autonomy, the number of sunless days the bank must cover alone. That gives the total stored energy needed. Divide by system voltage to convert watt-hours into amp-hours, the unit batteries are sold in. Finally divide by depth of discharge, because only part of a battery's capacity should be used regularly: lead-acid batteries should only be discharged to about 50%, lithium batteries to about 80%. The calculator performs all four steps and rounds to whole amp-hours.
- Add up daily energy use: list your loads, their watts, and hours used, then multiply and sum — or read it from an energy monitor.
- Choose days of autonomy: one day is minimal, two to three days is comfortable for most climates.
- Pick your system voltage — 12 volts for small cabins, 24 or 48 volts for larger homes.
- Choose depth of discharge for your battery type: 50% for lead-acid, 80% for lithium.
- Press Calculate and read your required amp-hours and the equivalent number of 100-amp-hour batteries.
Understanding your results
Amp-hours required. The headline figure — the battery bank capacity your system needs at your chosen voltage and depth of discharge.
100-amp-hour units. A practical translation, since many batteries are sold as 100-amp-hour blocks. Wiring them in series raises voltage; in parallel raises capacity.
Why depth of discharge matters. Sizing a lead-acid bank at 100% discharge would halve its life. The depth-of-discharge step is what makes the answer honest.
Zero results. Zero daily energy means zero battery needed. If your loads are real and you see zero, check the energy field.
Practical tips
- Size for winter, not summer — short days and long nights are when the bank earns its keep.
- Prefer lithium for daily cycling if the budget allows; the higher usable capacity and longer life usually win on lifetime cost.
- Keep batteries in a cool, ventilated, dry place — heat is the silent killer of battery life.
- Add a battery monitor so you always know the state of charge; guessing is how banks get damaged.
- Plan for growth: leave room and wiring capacity for extra batteries if your loads will increase.
- Re-check sizing whenever you add a major load, like a fridge, a pump or a workshop tool.
Common mistakes to avoid
- Sizing at 100% depth of discharge and destroying a lead-acid bank within a year or two.
- Counting only one cloudy day of autonomy in a climate where three-day storms are normal.
- Forgetting inverter losses — a 90% efficient inverter means the bank must supply about 11% more energy.
- Mixing old and new batteries in one bank, so the weakest battery limits the whole system.
- Choosing voltage by habit — a 12-volt system for a whole house means enormous, expensive cabling.
- Buying capacity without a monitor and discovering problems only when the lights go out.
Who benefits most
This calculator is for cabin owners, van-lifers and boat owners building off-grid power, homeowners adding battery backup to solar, and installers quoting systems who want a transparent number to show clients. It is also useful for anyone comparing quotes: two installers recommending different bank sizes can be judged against the same arithmetic. If your power comes from the sun and must last through the night, this calculation is the foundation everything else rests on.
Worked example
Take a small cabin using 1,000 watt-hours per day, wanting one day of autonomy, on a 12-volt system with lead-acid batteries at 50% depth of discharge: 1,000 × 1 ÷ (12 × 0.5) = 166.7, rounded to 167 amp-hours — two 100-amp-hour batteries. Now a weekend cabin using 2,400 watt-hours per day for two days on a 24-volt lithium system at 80%: 2,400 × 2 ÷ (24 × 0.8) = 250 amp-hours exactly. Same method, very different answers — because autonomy, voltage and chemistry all change the result, and guessing any one of them wrecks the sizing.
Checking and re-running your result
Check your result by comparing it with your installer's recommendation and by monitoring real consumption for a month — energy monitors often reveal loads you forgot. If the bank runs dry regularly, re-run the calculator with more autonomy days or a corrected daily figure rather than simply adding batteries blindly. Save each sizing to history, and revisit it whenever your loads change, because a bank sized for last year's cabin will not serve this year's workshop. Battery chemistry is the decision that shapes everything else about an off-grid bank, so compare chemistries on lifetime cost rather than sticker price. Flooded lead-acid batteries are cheap to buy but need watering, ventilation for hydrogen gas, and strict 50 percent discharge limits; sealed AGM lead-acid removes the watering but not the discharge limit. Lithium iron phosphate, or LiFePO4, costs two to three times more upfront yet delivers 80 percent usable capacity, thousands more cycles, and a decade or more of service, so it usually wins over the system's life. Temperature is the quiet killer: lead-acid capacity drops sharply below freezing, lithium should not be charged below zero without a heated battery, and every chemistry ages faster in a hot shed, so an insulated, ventilated battery room is part of the system, not an afterthought. Inverter sizing runs alongside battery sizing — the inverter must handle your largest simultaneous load plus motor surges, and a 90 percent efficient inverter means the bank supplies roughly eleven percent more energy than your loads consume. Solar pairing matters too: a bank sized for three days of autonomy still needs enough panel wattage to recharge it on an average winter day, or it will slowly walk down to empty across a cloudy week. Finally, budget for a battery monitor and a proper charge controller; the monitor is your fuel gauge, and guessing state of charge is how banks get ruined. Revisit the whole calculation annually, because loads creep upward and panels degrade. A quick reference for the systems people build most often: a 1,000-watt-hour-per-day cabin with one day of autonomy needs about 167 amp-hours at 12 volts on lead-acid; the same cabin on lithium needs about 104 amp-hours; a 2,400-watt-hour-per-day weekend cabin with two days of autonomy needs about 250 amp-hours at 24 volts on lithium; and a 5,000-watt-hour-per-day home with two days of autonomy needs about 260 amp-hours at 48 volts on lithium. Voltage and chemistry change the answer dramatically, so never copy a neighbour's bank size without running your own numbers.