How Many kWh Is a Tubular Battery? Ah to kWh for 1, 2 and 4 Battery Banks

People ask how many kilowatt hours a tubular battery holds because the real question is practical: how long will the inverter keep the lights, the fridge and the office running when the grid goes. The label does not answer it. Tubular batteries are sold in amp hours, inverters in kVA, and loads are measured in watts, so the buyer is left converting between three units that nobody at the shop counter bothers to reconcile.

The conversion itself is one line of arithmetic. What makes it worth an article is that the number it produces, the nameplate energy, is not the energy you get. Between the label and the socket sit depth of discharge, discharge rate, temperature, age and inverter losses, and together they take away more than half of it.

This article does the arithmetic properly for the ratings most people are actually holding: 12 volt tubular batteries of 150 Ah and 200 Ah, in banks of one, two and four.

The Formula: Volts Times Amp Hours

Energy is voltage multiplied by charge. For a battery that gives one rule.

Watt hours equal nominal voltage multiplied by the amp hour rating. Divide by 1,000 to get kilowatt hours.

A 12 volt battery rated at 200 Ah therefore holds 12 × 200 = 2,400 watt hours, which is 2.4 kWh. A 12 volt 150 Ah battery holds 12 × 150 = 1,800 watt hours, or 1.8 kWh.

Two conventions hide inside that sum. The first is the voltage. A 12 V lead-acid battery is six cells of 2 V nominal each, and its real terminal voltage wanders from well above 13 V on charge to below 12 V near the end of a discharge. Using the nominal figure is standard practice and close enough for sizing. The second is the amp hour rating, which is only true at a stated discharge rate and a stated temperature. We come back to both, because that is where most of the disappointment with battery banks comes from.

The conversion from amp hours to kilowatt hours, worked for a 12 V, 200 Ah tubular battery. The result is nameplate energy, not usable energy. Source: MIMAH engineering practice.

1 Tubular Battery Is How Many kWh?

One 12 V tubular battery holds 12 multiplied by its amp hour rating, divided by 1,000.

150 Ah: 12 × 150 = 1,800 Wh, so 1.8 kWh on the nameplate.

200 Ah: 12 × 200 = 2,400 Wh, so 2.4 kWh on the nameplate.

Single-battery systems are the small 12 V inverters that run a few lights, a fan, a television and a router. Keep the nameplate figure in perspective: if the battery is cycled every day and expected to last, the energy you should plan to take out of it is about half of that, which is 0.9 kWh from the 150 Ah battery and 1.2 kWh from the 200 Ah. The reason is covered under depth of discharge below.

2 Tubular Batteries Is How Many kWh?

Two identical batteries hold twice the energy of one, whichever way they are wired.

Two 150 Ah batteries: 3.6 kWh nameplate.

Two 200 Ah batteries: 4.8 kWh nameplate.

Most two-battery installations feed a 24 V inverter, so the batteries are connected in series. The bank is then 24 V at 200 Ah, and 24 × 200 = 4,800 Wh. Wire the same two batteries in parallel for a 12 V inverter and the bank becomes 12 V at 400 Ah, and 12 × 400 = 4,800 Wh. Same answer. Planned at half depth, the usable energy is 1.8 kWh for the 150 Ah pair and 2.4 kWh for the 200 Ah pair.

4 Tubular Batteries Is How Many kWh?

Four 150 Ah batteries: 7.2 kWh nameplate.

Four 200 Ah batteries: 9.6 kWh nameplate.

Four batteries usually mean a 48 V inverter with all four in series: 48 V at 200 Ah, and 48 × 200 = 9,600 Wh, or 9.6 kWh. Some 24 V systems use the same four batteries as two series pairs connected in parallel, which gives 24 V at 400 Ah: 24 × 400 = 9,600 Wh. Again the energy does not change. Planned at half depth, the usable figure is 3.6 kWh for four 150 Ah batteries and 4.8 kWh for four 200 Ah batteries.

Series or Parallel Changes the Voltage, Not the Energy

Series connects the positive of one battery to the negative of the next. Voltages add and the amp hour rating stays the same.

Parallel connects positive to positive and negative to negative. Amp hours add and the voltage stays the same.

Either way the kilowatt hours are the sum of the batteries, so the wiring choice is decided by the inverter's input voltage, not by any gain in capacity. What the voltage does change is current. A 1,200 W load drawn from a 12 V bank pulls 100 A from the battery before inverter losses; from a 48 V bank it pulls 25 A. Lower current means thinner cables, smaller fuses, less heat in the terminals and less voltage drop, which is why larger inverters run at 48 V. Our guide to choosing an inverter covers how the battery voltage fits into that decision.

Parallel strings bring one practical warning. Batteries in parallel rarely share current perfectly, and the imbalance gets worse with unequal cable lengths, loose terminals, or a mix of old and new batteries. Keep parallel strings to a minimum, use equal cable runs, and never add a new battery to an ageing bank and expect the two to behave as one.

Series raises voltage and parallel raises amp hours, but the energy in the bank is the same either way. Source: MIMAH engineering practice.

Nameplate Is Not Usable: Depth of Discharge

Depth of discharge is how much of the rated capacity you take out before recharging. It is the single biggest correction between the label and reality.

Tubular lead-acid batteries tolerate deep discharge better than flat-plate lead-acid, which is why they are the lead-acid type worth using for solar and inverter duty. They still trade depth against life. The manufacturer's cycle life curve shows the number of cycles falling as each cycle gets deeper, and design practice for tubular banks cycled daily commonly sits at about 50 percent depth of discharge. A backup bank that sits full most of the month can be taken deeper on the occasional long outage, at some cost to life.

Applied to the banks above, 50 percent depth of discharge gives the energy you should actually plan around: 0.9, 1.8 and 3.6 kWh for one, two and four 150 Ah batteries, and 1.2, 2.4 and 4.8 kWh for one, two and four 200 Ah batteries.

This is also the correction that decides most comparisons with lithium iron phosphate, which is routinely cycled much deeper. Our comparison of lithium and tubular batteries covers the technical differences, and cost per usable cycle covers why a cheaper tubular bank is not always the cheaper purchase.

The C-Rating Trap

An amp hour rating is only meaningful alongside the rate it was measured at. Tubular batteries are commonly rated at C10 or C20, and the datasheet should say which.

C10 is the capacity delivered when the battery is discharged evenly over 10 hours. A 200 Ah battery rated at C10 delivers 20 A for 10 hours.

C20 is the capacity delivered over 20 hours. A 200 Ah battery rated at C20 delivers 10 A for 20 hours.

Lead-acid batteries deliver less total charge when discharged faster, a behaviour usually described by Peukert's law. The practical consequence is that the same battery earns a higher number at C20 than at C10, and two batteries both labelled 200 Ah are not the same battery if one is rated at C10 and the other at C20. Compare labels only at the same rate.

The trap springs when the real load discharges the bank faster than the rating assumed. A 1,000 W load on a 48 V, 200 Ah bank, with the inverter at 90 percent efficiency, draws 1,000 ÷ 0.9 ÷ 48, which is about 23 A. That is a little above the C10 current of 20 A, so the bank will deliver somewhat less than its C10 rating, and noticeably less than a C20 rating. A single 12 V, 200 Ah battery running a 500 W load draws 500 ÷ 0.9 ÷ 12, about 46 A, more than twice its C10 current. That battery will fall well short of its label, which is why single-battery systems so often disappoint.

Temperature and Age

Temperature. Capacity is rated at a reference temperature stated on the datasheet. Cold reduces the capacity available. Heat makes slightly more capacity available in the short term and shortens life, and the life penalty is steep. On hot sites without a shaded, ventilated battery room, tubular life figures taken from a temperate datasheet will not be met.

Ageing. Capacity falls as the battery ages, and end of life is commonly defined as the point where it can no longer deliver 80 percent of its rated capacity. A bank sized exactly to the load when new will fall short well before it is worn out. Leave margin for it.

Formal sizing methods account for all of this. IEEE 1013-2019, the recommended practice for sizing lead-acid batteries in stand-alone photovoltaic systems, sets out a method that starts from the load and the required autonomy rather than from the battery label. When comparing datasheets, a battery whose performance is quoted against IEC 61427-1, the IEC standard for batteries in off-grid photovoltaic storage, at least tells you the test conditions behind the numbers.

From kWh to Hours of Runtime

Runtime is usable energy, multiplied by inverter efficiency, divided by the load.

Hours of runtime equal usable kWh multiplied by inverter efficiency, divided by the load in kW.

For inverter efficiency, use the datasheet figure at your actual load. Efficiency is usually lower at light load, and the inverter also draws its own idle power whether or not anything is switched on. The examples here assume 90 percent.

Four 200 Ah batteries: 9.6 kWh nameplate, 4.8 kWh usable at 50 percent depth, and 4.8 × 0.9 = 4.32 kWh delivered to the sockets. On a 500 W load that is 4.32 ÷ 0.5, about 8.6 hours. On a 1 kW load it is about 4.3 hours, before the C-rate correction above.

Two 200 Ah batteries: 2.4 kWh usable, 2.4 × 0.9 = 2.16 kWh delivered, and about 4.3 hours on a 500 W load.

One 200 Ah battery: 1.2 kWh usable, 1.2 × 0.9 = 1.08 kWh delivered, and about 2.2 hours on a 500 W load on paper. At 46 A the real figure will be lower.

From nameplate to runtime for a four-battery bank, assuming 50 percent depth of discharge and 90 percent inverter efficiency. Discharge rate, temperature and ageing reduce the result further. Source: MIMAH engineering practice.

Working It Backwards From the Load

Sizing properly runs the arithmetic in reverse. Start with the energy the load needs, then work back to the battery.

Suppose the overnight load needs 3 kWh at the sockets. Divide by inverter efficiency: 3 ÷ 0.9 = 3.33 kWh must come out of the battery. Divide by the planned depth of discharge: 3.33 ÷ 0.5 = 6.67 kWh of nameplate capacity. Divide by the bank voltage: 6,670 Wh ÷ 48 V is about 139 Ah. Four 150 Ah batteries at 48 V (7.2 kWh) clear that on paper. Once discharge rate, site temperature and ageing are allowed for, four 200 Ah batteries (9.6 kWh) are the safer choice.

The battery is only half the system. Whatever comes out of it each night has to be put back the next day, plus charging losses, by the grid, a generator or a solar array large enough to do it in the hours available. Our guide to sizing a solar system for a business covers that side, and hybrid system design covers sites where a generator or an unreliable grid shares the load.

MIMAH's renewable energy team designs off-grid and hybrid systems with battery storage across Sudan, Nigeria and Egypt, sized from measured loads rather than from the label. If you have a bank that is not lasting as long as it should, or a quotation you want checked, get in touch.