Usable Battery Energy

Nameplate is not usable

A battery's rated capacity is reduced twice before it reaches a load. First by depth of discharge: the fraction of the rated capacity you may actually draw without destroying cycle life. Second by round-trip efficiency: the energy lost as heat on the way in and again on the way out.

For lithium iron phosphate the product is around eighty-five percent. For flooded lead-acid it is around forty. That is why a ten kilowatt-hour daily load needs an eleven point eight kilowatt-hour lithium bank or a twenty-five kilowatt-hour lead-acid one — and why comparing the two on price per nameplate kilowatt-hour is the wrong comparison entirely.

Energy and power are different questions

A bank with ample energy can still be unable to start a motor. Energy in kilowatt-hours determines how long something runs; power in kilowatts determines whether it can run at all, and surge capacity determines whether it can start.

Sizing on energy alone is a common omission. A battery adequate for a day's consumption may still trip on the inrush of a well pump or a compressor, and that is a continuous and peak kilowatt question the energy figure does not touch.

Both figures decay

Capacity falls with age and with temperature. Most warranties guarantee seventy to eighty percent of nameplate at end of term, so a bank sized exactly for today's load is undersized by year eight. Cold reduces available capacity substantially in the moment, and lithium cannot be charged below freezing at all without a heater.

None of this is modelled by the equation. It is why an off-grid system sized to exactly meet its load is a system that will not meet it for long.

Chemistry sets how much of the nameplate you may actually use

Depth of discharge is not a preference; it is a property of the cell. Drawing a lead-acid battery below about half its rated capacity shortens its life sharply, so the working assumption for a lead-acid bank is that roughly half the nameplate is available.

Lithium iron phosphate tolerates a far deeper draw — commonly eighty to ninety per cent — which means a lithium bank of a given nameplate delivers close to twice the usable energy of a lead-acid one. Comparing the two on nameplate capacity is comparing the wrong number, and it is the usual reason a lithium replacement of nominally equal size behaves like a much larger bank.

The permitted depth is also not a single figure per chemistry. It trades against cycle life: a bank cycled shallowly lasts disproportionately longer than one cycled deeply, so the ECONOMICALLY usable fraction is generally smaller than the technically permitted one. A system designed to its maximum depth is designed to its shortest life.

Capacity falls as you draw harder, and the effect is chemistry-specific

A battery's rated capacity is stated at a particular discharge RATE, conventionally over twenty hours for lead-acid. Draw it faster and the total energy you get out is less — not merely delivered sooner, but genuinely less.

For lead-acid the effect is large and is described by an empirical relationship with an exponent fitted to the cell. A bank rated at a twenty-hour rate and discharged in one can deliver substantially below its nameplate, which is why an inverter sized for a short heavy load needs a bank sized on that load rather than on the energy alone.

Lithium chemistries are far less sensitive, which is part of why they tolerate variable loads well. But they have their own limit in the opposite direction: a maximum continuous current, set by heat rather than by capacity, and enforced by a battery management system that will disconnect rather than allow it to be exceeded. The constraint has moved from a soft derating to a hard cut-off.

Temperature, conversion losses and standby

Cold reduces available capacity in every chemistry, often substantially below freezing, so a bank in an unheated space has a winter capacity and a summer capacity. Lithium adds a harder rule: charging below zero degrees plates lithium metal and causes permanent damage, which is why cold-climate installations either heat the battery or refuse to charge until it warms.

Between the bank and the load sit conversion losses. An inverter is not free, a charger is not free, and the round trip through both is the fraction of stored energy that reaches the appliance — a figure in the eighties or low nineties of a per cent for good equipment, and worse at low loads where fixed losses dominate.

Standby consumption is the loss people forget, because it is small and continuous. An inverter left energised to serve an occasional load can consume a meaningful share of a small bank over a day doing nothing at all. For an off-grid or backup system the relevant question is not only how much energy is stored but how long the system will sit armed before it is needed.