How Many Batteries Do I Need for Solar?

Q: Is three days of autonomy better than two?

Only with no backup. The third day of an 8 kWh cabin is about 10 kWh of extra lithium after derating, usually more expensive than a small generator.

Q: Why divide by depth of discharge?

You work backwards from usable energy to nameplate. Using only 80% means 10 kWh lives inside 12.5 kWh of nameplate.

Q: Can I mix old and new batteries?

Not in parallel. Replace a bank as a set, or keep ages and chemistries on separate controllers.

Published 17 September 2026

The battery bank calculator will take daily kilowatt-hours, days of autonomy, depth of discharge and efficiency and return amp-hours. This page is the part the form cannot decide: how many days you actually need, which chemistry those days imply, and when a generator is cheaper than a fifth day of lithium. Two worked systems, a van and a cabin, using the same formula as the tool.

The short version. Usable energy = daily kWh × days of autonomy. Bank kWh = usable ÷ (DoD × round-trip efficiency × temperature derate). A van that moves is one day on lithium at 12 V or 24 V. A cabin with no backup is two to three days, and three to five only if you have already priced a generator and still want the silence. Lead-acid at 50% DoD roughly doubles the bank. This is a planning aid, not a system design.

The formula, and what the form hides

Usable kWh = Daily kWh × Days of autonomy
Bank kWh = Usable kWh ÷ (DoD × RTE × Temp derate)
Bank Ah = Bank kWh × 1,000 ÷ System volts

Those are the same three lines as the battery bank size calculator. Daily kWh is not a guess from the grid bill. Add the appliances on the off-grid load calculator, including inverter idle, and use a winter day. Summer kWh will undersize the bank for the season that actually stresses it.

DoD is 0.8 for LFP, 0.5 for AGM, gel and flooded if you want the cycle life the brochure printed. Round-trip efficiency is about 0.95 LFP, 0.85 AGM, 0.80 flooded. Temperature derate is 1.0 for LFP above freezing and for any chemistry in a heated space; it is 0.80 at 0 °C for lead-acid. Charging LFP below 0 °C is a prohibition, not a derate. The tool will let you type 1.0 at -10 °C on lithium; the cells will not.

How many days of autonomy

Days of autonomy is how long the bank covers the load with no useful charging. It is a risk choice. It is not "how many cloudy days your climate has in December", because that number is often five to ten and the bank that covers it sits full for the rest of the year.

Van, boat, or anything with an alternator. One day. You can motor or drive. A second day of lithium is insurance against a blown fuse, not against weather.
Cabin with a generator or a rare grid connection. Two days. The third day is almost always cheaper as 40 litres of fuel and a 2 kVA set than as another string of batteries that ages at float.
Cabin with no backup, winter-critical loads (lights, well pump, radio). Three days, and split the load: the bank covers the critical circuits, not the kettle and the space heater. Five days is for a site you cannot reach in a storm, and you should still own a generator.

If the calculator's output shocks you, the days figure is the lever, not the kettle. Halving autonomy halves the bank. Changing chemistry from flooded to LFP roughly halves it again. Changing daily kWh by 10% barely moves it.

Worked example: a van, 2 kWh a day

Fridge, lights, laptop, diesel night heater, inverter idle. One day of autonomy, LFP, 12 V, DoD 0.8, RTE 0.95, indoor (derate 1.0):

Usable = 2 × 1 = 2.0 kWh
Bank = 2.0 ÷ 0.76 = 2.63 kWh
At 12 V = 2,630 ÷ 12 = 219 Ah
Buy: one 230 to 280 Ah 12 V LFP, or two 100 Ah in parallel. One large battery beats two small ones sharing current.

Same van on AGM at 0.5 DoD and 0.85 RTE: bank = 2.0 ÷ 0.425 = 4.71 kWh = 392 Ah at 12 V. That is four 100 Ah AGMs, twice the weight, and you still cannot discharge them as far. Lithium is not a luxury on a vehicle whose floor is already full.

Stay on 12 V if the van already is. Jumping to 24 V for 2 kWh a day adds a DC-DC charger problem and does not save enough copper to matter. The 12 V cable size page is the one that bites on the inverter run, not the house voltage.

Worked example: a cabin, 8 kWh a day, three winter days

Lights, well pump, laptop, a small fridge, no electric heat. Three days, 48 V LFP, heated cupboard (derate 1.0):

Usable = 8 × 3 = 24 kWh
Bank = 24 ÷ 0.76 = 31.6 kWh
At 48 V = 31,600 ÷ 48 = 658 Ah
Built from 100 Ah 12 V blocks: 4 in series for 48 V, 7 parallel strings = 28 batteries. That is too many parallel strings. Buy 48 V racks or 280 Ah cells instead, so you have one or two strings, not seven.

Same cabin, unheated shed, flooded lead-acid at 0 °C (DoD 0.5, RTE 0.80, temp 0.80): derating product = 0.32. Bank = 24 ÷ 0.32 = 75 kWh. That is the chemistry lesson in one line. Either insulate and use LFP, or stop pretending the shed is a battery room.

Now cut autonomy to two days and keep a generator for the third: usable 16 kWh, LFP bank 21.1 kWh, 439 Ah at 48 V. You just removed a third of the lithium and replaced it with a machine you already wanted for cloudy weeks. That is the decision the form will not take for you.

48 V is the right voltage at this energy. The same 31.6 kWh at 12 V is 2,630 Ah and the inverter cables become a fire problem. Below about 1 kW of continuous load, 12 V is simpler; above about 3 kW, 48 V is close to mandatory. This cabin sits in the 48 V camp even before the well pump starts.

C-rate, inverter idle, and the load people forget

A bank that can store 2 kWh is not the same as a bank that can deliver 2 kW. LFP is happy near 0.5 C continuous; many 100 Ah drop-ins are listed at 100 A, some at 50 A. A 2,000 W inverter on 12 V is about 190 A with inefficiency, which is why the van example belongs on one 200 Ah-class battery with a high discharge rating, not on two cheap 100 Ah paralleled units whose BMS will shut down under the kettle. Cabin banks at 48 V see a quarter of that current, which is another reason voltage is a safety choice as well as a copper choice.

Inverter idle is a daily kWh item, not a footnote. A 3 kW low-frequency inverter can idle at 20 to 40 W, which is 0.5 to 1.0 kWh a day with nothing plugged in. That is half a van's entire budget. Either use an inverter with a good idle figure, or switch it off at the DC breaker when you leave the cabin. The load calculator will not invent this line unless you add it.

Charge current has to refill the bank in the hours of sun you actually get. A 658 Ah 48 V cabin bank wants roughly 0.2 C, about 130 A of charge, if you hope to recover three days of use in two short winter days. That number feeds the charge controller, not the battery form. Undersize the controller and the bank never comes back up between weather systems, which feels like "not enough battery" and is actually not enough array and controller.

What to do with the number

Confirm daily kWh on a winter day, not an annual average.
Pick days from the list above, then price one fewer day plus a generator.
Pick chemistry. If the bank will see frost, solve charging temperature before you order LFP.
Pick voltage from the inverter you actually want, then run the calculator.
Buy as few parallel strings as you can. Parallel is how banks unbalance.
Size the inverter, then the charge controller, then the array. The bank is not the last component; it is the one the others have to match. Controller sizing, including the cold Voc mistake, is the next guide.

If you already own the bank and want hours, that is the runtime calculator, which also applies Peukert on lead-acid. Sizing and runtime are opposite directions through the same physics.

DoD and efficiency figures are typical manufacturer and IEEE 485 planning values, not a warranty. Cabin and van loads are examples. Have a qualified person review anything that feeds an AC installation or sits in a dwelling.

Frequently Asked Questions

How many batteries do I need for a van?

For 2 kWh a day and one day of autonomy on LFP, about 220 Ah at 12 V, which is one 230 to 280 Ah battery. Four 100 Ah AGMs will also cover it and will weigh more than the fridge.

Is three days of autonomy better than two?

Only if you have no backup and you cannot shed load. The third day of a 8 kWh cabin is 10 kWh of extra lithium after derating, which is usually more expensive than a small generator you run twice a winter.

Why divide by depth of discharge?

You are working backwards from energy you must have to the nameplate that contains it. If you only use 80%, 10 kWh of useful energy lives inside 12.5 kWh of nameplate. Multiplying by 0.8 gives a bank that is too small.

Can I mix old and new batteries?

Not in parallel. A new string will carry most of the current until it ages into the old one. Replace a bank as a set, or keep chemistries and ages on separate controllers.

Should a cabin bank be 24 V or 48 V?

At 8 kWh a day, 48 V. Current on the inverter cables is a quarter of the 12 V figure. 24 V is a compromise for mid-size systems that already have 24 V kit; it is not a destination.