Battery bank size calculator

Enter the energy you use in a day, how many days you want to run without sun and your battery type. You get the bank size in amp-hours, how many batteries to buy, and a chart of the bank's charge through a day.

Inputs

Add up watts × hours for everything you run. A 100 W fridge running 5 hours a day is 500 Wh. Count AC loads at the watts they draw.

Days you want to run with no sun or charging. 1–2 is usual for a van or RV that recharges often; cabins use more.

Renogy rates its lithium for 80% use; Battle Born says 100%. Lead-acid lasts far longer if you stop near 50%.

Bigger systems use 24 V or 48 V to keep the current (and the cable) down.

Inverter losses: Victron’s inverters peak at 89–92%. Use 100 if everything runs on DC.

Extra capacity for cold weather, battery ageing and loads you forgot. Alchemy Industrial’s guide uses 15%.

Ah

The amp-hour rating of the 12 V battery you plan to buy.

Total panel watts. Set to 0 to see the bank run down with no sun at all.

4–5 is a typical baseline (Battle Born); 2.5–3 in winter.

Battery bank size

573Ah at 12.8 V

7.33 kWh of battery for 2,400 Wh a day over 2.0 days, using 80% of it.

The panels refill the bank each day. Overnight it falls to about 81%, which leaves the rest as your reserve for cloudy days.

Batteries to buy
6 × 100 Ah
Energy as bought
7.68 kWh
Days with no sun
2.3
Used overnight
18%
Battery charge over 24 hours
02040608010005101520Time of day (hour, 0 = midnight)State of charge (%)Low 81%
  • Your bank
  • Lowest charge to reach (20%)

Running 2,400 Wh a day for 2.0 days needs a bank of about 573 Ah at 12.8 V, which is 6 batteries of 100 Ah. The chart follows the bank through a day with 900 W of panels and 4.0 sun hours: it starts near full, falls through the evening and night to 81%, then refills from the sun. The panels refill the bank each day. Overnight it falls to about 81%, which leaves the rest as your reserve for cloudy days.

Safety first

A battery bank can deliver hundreds of amps into a short circuit, enough to melt cable, start a fire or cause serious burns.

  • Fuse every circuit, and put the fuse as close to the battery as you can. The fuse protects the wire, so it must not be bigger than the wire’s rating.
  • Follow the battery maker’s instructions for charging voltage, series and parallel connections and maximum current. Never mix old and new batteries or different types in one bank.
  • Follow your local electrical code and any marine or RV standard that applies to your vehicle or boat.
  • Have a qualified person check the installation before you switch it on, especially with a bank above 12 V or one that feeds an inverter.

How it works

bank energy (Wh)   = daily use ÷ usable share ÷ efficiency × days × (1 + margin)
bank size (Ah)     = bank energy ÷ bank voltage
  • Usable share is the depth of discharge you will allow: 50% for AGM and gel, 80% or 100% for lithium.
  • Efficiency is what the inverter loses. Victron’s Phoenix inverters peak at 89–92%, so the default is 90%. Use 100% if everything runs on DC.
  • Bank voltage is 12.8 V for a “12 V” lithium battery and 12 V for lead-acid; 24 V and 48 V banks put two or four batteries in series.
  • The chart runs the bank through repeated days, starting full, and draws the last one. Your load is spread evenly and the panels produce in a half-sine between 6 am and 6 pm holding watts × sun hours × 0.8 of energy. Battery charging losses are ignored.

Worked example

A van uses 2,400 Wh a day: a fridge, lights, laptops and a small inverter. You want two days without sun, and you choose lithium used to 80%.

  • 2,400 × 2 days = 4,800 Wh. Divide by 0.80 and by 0.90 for the inverter: 6,667 Wh. Add 10% margin: 7,333 Wh.
  • At 12.8 V that is 573 Ah. With 100 Ah batteries it is 6 batteries, 7.7 kWh as bought.
  • With 900 W of panels and 4 sun hours the panels make 2,880 Wh, more than the bank gives out, so the bank refills each day and the night-time low is about 80%.

Tips from the road

  • Count your real loads. A fridge rated 100 W runs maybe a third of the time. Measure a day with a battery monitor if you can, and use that number.
  • Do not size for the coldest day. Lithium cannot charge below freezing unless it is heated, so a cold climate calls for a heated battery or a warm place to put the bank.
  • Two small batteries are not the same as one big one. More parallel batteries share the current, which suits a large inverter, but each connection is another place for a fault.

FAQ

How big a battery bank do I need for off-grid solar?

Multiply your daily watt-hours by the days you want to run without sun, then divide by the share of the battery you will use and by your system efficiency. For 2,400 Wh a day, 2 days, lithium used to 80% and a 90% inverter that is about 6,700 Wh before margin, which is around 520 Ah at 12.8 V. The calculator does the sum and rounds up to whole batteries.

How many days of autonomy should I plan for?

One to two days is common for a van, RV or boat that recharges from the alternator or shore power often. A cabin that depends on solar alone in a cloudy climate may want three or more. Every extra day adds the same amount of battery again, so it is also the cheapest input to question.

Should I use 50%, 80% or 100% of the battery?

For lead-acid, about 50%; Victron's datasheet gives 600 cycles at 50% against 400 at 80% for its AGM. For lithium the makers disagree. Renogy rates 5,000 cycles at 80%, and Battle Born says its batteries can use 100% every time. The calculator defaults to the cautious 80%, which also leaves some margin for cold weather and age.

Why is the amp-hour answer smaller at 24 or 48 volts?

Energy is volts × amp-hours. Double the voltage and you need half the amp-hours for the same energy, so the cables and fuses are smaller too. Two 12 V batteries in series make a 24 V battery of the same amp-hours.

What does the chart show?

The bank's state of charge through a repeating day, with your load drawn evenly across 24 hours and the panels you enter producing between 6 am and 6 pm. Watch the low point before sunrise. It should stay well above the dashed line, which is the lowest charge this battery type should reach.

Sources

  1. How to Size a Deep Cycle Battery Bank, Battle Born Batteries. Watts × hours = watt-hours; watt-hours ÷ system voltage = amp-hours (2,400 Wh at 12 V is 200 Ah); two days off-grid doubles the capacity.
  2. How to Size a Victron Off-Grid Power System, Alchemy Industrial. 4.15 kWh a day × 2 days ÷ 0.80 depth of discharge ÷ 0.95 efficiency × 1.15 margin = 12.6 kWh.
  3. Renogy Core Mini 12.8 V 100 Ah lithium iron phosphate battery, Renogy. 12.8 V nominal; 5,000 cycles at 80% depth of discharge.
  4. BB10012 100 Ah 12 V LiFePO4 battery data sheet, Battle Born Batteries. Can be discharged to 100% of its rated capacity every time; 3,000–5,000 cycles.
  5. Victron GEL and AGM Deep Cycle batteries data sheet, Victron Energy. AGM Deep Cycle design life of 400 cycles at 80% discharge, 600 at 50% and 1,500 at 30%.

Formula and sources last checked September 30, 2026. How we test formulas.

  • Off-grid solar system builderList your loads and size the whole off-grid system at once, with battery amp-hours, solar watts, inverter size, main cable gauge and a 24-hour charge chart.
  • Solar panel size calculatorHow many watts of solar panels you need for an RV, van, boat or cabin from daily watt-hours and peak sun hours, with a chart of panel output against load.
  • Amp-hour and battery runtime calculatorHow long a battery runs a load. Enter amp-hours, voltage, battery type and watts. Covers LiFePO4 and AGM, inverter losses and lead-acid discharge-rate loss.
  • Inverter size calculatorWhat size inverter you need for a van, RV or cabin. Continuous watts for what runs together, surge watts for the biggest motor starting, and battery current.
  • Series and parallel wiring calculatorWire batteries or solar panels in series, parallel or both and see the voltage, capacity and current you get, with a wiring diagram that updates as you type.
  • Solar charge controller size calculatorWhat size solar charge controller you need, MPPT or PWM: its amp rating, the voltage it must take on a cold morning and the PV fuse, from your panels.

Safety note: Estimates only. Equipment ratings vary by manufacturer, so always follow the manufacturer’s instructions and ratings, and ask a qualified professional when safety is at stake.