Battery life calculator

Amp-hours tell you almost nothing on their own. A 100 Ah battery at 12 V and a 100 Ah battery at 24 V hold 1,200 and 2,400 watt-hours, and only the watt-hour figure can be compared against an appliance, because appliances are rated in watts. Any comparison that stops at amp-hours is comparing the wrong thing, which is why specifications are increasingly quoted in Wh. It also explains why larger systems run at higher voltages: the same energy at four times the voltage draws a quarter of the current, which means thinner cable and far lower losses.

Four things whittle the rating down

Depth of discharge is the first cut and much the largest. Taking a lead-acid battery below 50% repeatedly shortens its life dramatically, so a 100 Ah lead-acid bank is a 50 Ah bank in practice, while lithium iron phosphate is content at 80 to 90%. That is most of the reason lithium displaced it despite costing more per amp-hour: the usable capacity per pound is far closer than the headline figures suggest.

Discharge rate is the second, and it is the one nobody is told about. Capacity is quoted at a specific rate, usually C/20 for lead-acid, and drawing the same battery at C/5 gives measurably less than the rating. That is the Peukert effect, it is pronounced in lead-acid and small in lithium, and it is another reason lithium holds up better in the real world than the specification sheet implies.

Conversion is the third. An inverter turning 12 V DC into 230 V AC runs at 85 to 92%, and it also draws idle current whether or not anything is switched on. For a small load over a long period that idle draw can dominate: a 20 W inverter overhead running a 10 W load wastes twice what it delivers, which is why DC-native appliances are so common in vans and boats.

Temperature and age are the fourth. A lead-acid battery at 0 °C delivers perhaps 80% of its rating, and most chemistries are specified down to 80% capacity at end of life. Stack all four and a nominal 1,200 Wh bank might deliver 700 Wh of AC energy on a cold day in its fifth year. If the system has to work, that is the number to size against.

Charging

The arithmetic here covers the bulk phase, where the charger delivers its full current. Real chargers then move to an absorption phase at falling current and, for lead-acid, a float phase, and the last 20% of a lead-acid charge can take as long as the first 80%. Lithium holds near-constant current almost to the top and finishes far more predictably. The C-rate figure is the useful sanity check on a charger: above 0.5 C stresses most lead-acid batteries, while lithium iron phosphate commonly accepts 1 C without complaint.

Sizing the system around the bank

Inverters are rated twice for a reason. The continuous figure is what one can sustain thermally; the surge figure is what it supplies for a second or two before folding back. A compressor starting draws a genuine short-lived multiple of its running power, so an inverter that meets the running load and misses the surge simply refuses to start the fridge, repeatedly and confusingly, because the load looks well inside its rating. The other half of the job is the DC side, where 1,200 W on a 12 V bank is about 111 A and over 330 on surge. That is why anything past roughly a kilowatt belongs on 24 or 48 V, and why cable and fusing on a 12 V system stop being an afterthought.

Solar sizing has a seasonal problem the annual figures hide. Peak sun hours vary far more by season than by location: a UK array sized on an annual average of 2.8 will produce about five times as much in June as in December. Grid-tied, that swing is absorbed by the grid and the annual number is the right basis. Off-grid, charging a battery, you have to size for the worst month, which typically triples the array, or accept a generator for the darkest weeks.