How to size a solar battery bank
Battery sizing goes wrong in a specific, predictable way: people buy the nameplate number and discover they own roughly half of it. The fix is to do the whole calculation in usable kilowatt-hours and only convert to nameplate at the very last step.
Decide what you are actually backing up
The first question is not how big, it is what for. There are three distinct jobs and they produce wildly different answers:
- Essential backup. Lights, fans, fridge, router, phone charging. Typically 0.5–2 kW. Modest battery, big improvement in quality of life.
- Evening shift. Store the afternoon surplus and run the house on it until bedtime. Sized against a share of your daily consumption, usually 30–60%.
- Whole-house, multi-day autonomy. Genuinely off-grid. Sized against several days of full consumption with no sun. Expensive, and rarely the right answer if a grid connection exists.
Trying to back up the air conditioning is what turns a sensible battery into an unaffordable one. A single 1.5-ton unit running overnight is 14 kWh on its own — more than most residential batteries hold in total.
The usable-energy calculation
A 1.8 kW essential load that you want to run for six hours needs 10.8 usable kWh. If instead you are sizing for an evening shift, use a fraction of daily consumption: a 26 kWh/day house shifting 40% of its use needs 10.4 usable kWh. Take whichever of the two is larger.
Depth of discharge: the step everyone skips
No battery lets you use all of its nameplate capacity. Draining it fully destroys it, so the management system stops you. Depth of discharge is the fraction you are actually allowed to take out.
| Chemistry | Usable depth | Typical cycle life | Round-trip efficiency |
|---|---|---|---|
| Lithium iron phosphate (LFP) | 90 – 95% | 4,000 – 8,000 | 92 – 96% |
| Lithium NMC | 80 – 90% | 2,500 – 5,000 | 92 – 95% |
| Sealed lead-acid (AGM/gel) | 50% | 400 – 1,200 | 80 – 85% |
| Flooded lead-acid (tubular) | 50% | 800 – 1,800 | 78 – 82% |
Our 10.8 usable kWh becomes a 12 kWh lithium bank — or a 21.6 kWh lead-acid bank. That is the entire argument in one line. Lead-acid is cheaper per nameplate kilowatt-hour and almost always more expensive per usable kilowatt-hour over its life.
Round-trip efficiency
Energy is lost going in and coming out. At 94% round-trip, storing 10 kWh returns 9.4 kWh. At 80% you get 8 kWh. Over thousands of cycles this is not a rounding error — it is a permanent tax on every unit your panels produce that goes through the battery rather than straight to a load.
It is also an argument for using power when you generate it. Running the washing machine at 2 pm costs nothing; running it at 9 pm off the battery costs you the round-trip loss plus a slice of the battery's finite cycle life.
C-rate: the limit that catches people out
Capacity tells you how much energy a battery holds. C-rate tells you how fast you are allowed to take it out, and the two are independent. A battery can have plenty of energy and still be unable to run your load.
| Chemistry | Typical continuous C-rate | A 10 kWh bank can deliver |
|---|---|---|
| Lithium LFP | 0.5C – 1C | 5 – 10 kW |
| Sealed lead-acid | 0.2C | 2 kW |
| Flooded lead-acid | 0.1C – 0.2C | 1 – 2 kW |
So a 10 kWh flooded lead-acid bank holds 5 kWh of usable energy and can only release it at about 1 kW. Ask it for 3 kW and the voltage collapses, the inverter cuts out, and you have an expensive cupboard full of nothing. Lead-acid also suffers from Peukert's effect — the faster you discharge it, the less total energy you get out, so a hard discharge shrinks the capacity as well as straining it.
The calculator checks this for you and warns when the bank you have sized cannot physically carry the essential load you asked it to carry.
Converting to amp-hours
Batteries are still often sold in amp-hours, which is meaningless without the voltage.
A 12 kWh bank at 51.2 V is 234 Ah. The same 12 kWh at 12 V would be 1,000 Ah — and would need cable thick enough to moor a boat, because the current is four times higher for the same power. This is why anything above a few kilowatt-hours is built at 48 V or 51.2 V. Higher voltage means lower current, thinner cable, smaller losses and less heat.
When comparing two quoted batteries, convert both to kWh before you look at the prices. A "200 Ah" battery tells you nothing on its own.
Temperature and siting
Heat is the enemy of every chemistry. Lead-acid life roughly halves for each 10 °C above 25 °C. Lithium tolerates heat better but degrades faster and loses warranty coverage above its rated range, and LFP must not be charged below freezing without a heater.
Put the bank somewhere shaded and ventilated, indoors where possible. A battery on a sun-baked roof next to the inverter is a battery with a short life. Flooded lead-acid additionally vents hydrogen and needs real ventilation and periodic topping up with distilled water — it is not a fit-and-forget product.
Costing it honestly
Compare on cost per usable kilowatt-hour delivered over the warranty, not on sticker price.
Run that for a cheap lead-acid bank at 500 cycles and a mid-range LFP at 6,000 cycles and the ranking usually inverts. Lead-acid still wins where the use is genuinely occasional — a few outages a month — because calendar ageing dominates and you never reach the cycle count that would justify lithium.
Sanity checks before you buy
- Is the usable capacity, not the nameplate, bigger than the energy you need?
- Can the C-rate deliver your essential load continuously?
- Can the inverter's charger refill it in the hours of sun you actually get?
- Is the warranty written in cycles, years, or throughput kWh — and at what depth of discharge?
- Does it sit somewhere that stays under 30 °C?
- If it is lead-acid, who is topping up the water, and how often?