20kWh Battery: Capacity Ladder, Runtime and Inverter Sizing

Searching for a 20 kWh battery returns two kinds of result: product listings for single units that are not actually 20 kWh, and articles that calculate how long 20 kWh lasts without telling you what 20 kWh is built from. Neither answers the question a buyer at this stage is actually asking, which is narrower: what does a 20 kWh bank consist of, how much of it can I really use, and what inverter does it need?

This guide answers those three. It gives the capacity ladder from 5 kWh to 50 kWh with the module configuration behind each rung, the arithmetic that turns a nameplate figure into delivered energy, and the inverter sizing that decides what you can run at once. Written from the battery supply side, which is disclosed rather than hidden.

Capacity at a glance

Module capacities are nameplate figures at 51.2 V. Delivered energy depends on your depth-of-discharge setting and system round-trip efficiency; the arithmetic is given below so you can run it for any capacity.

Nameplate, usable, delivered: three different numbers

Direct answer: Multiply nameplate capacity by your depth-of-discharge setting, then by system round-trip efficiency. At 80% DoD and 90% round-trip, a 20.48 kWh nameplate bank delivers about 14.7 kWh per cycle.

LFP chemistry tolerates deeper discharge than older chemistries, and 90 to 95% is within technical limits for occasional use. The reason to plan at 80% for daily cycling is cycle life: shallower daily cycles buy years. Run the deeper discharge when you need it in an outage, not as your everyday setting. We cover what actually governs cycle life in how long LiFePO4 batteries last.

Why published usable-capacity figures disagree

Direct answer: Other guides put the usable energy of a 20 kWh system anywhere from 12 kWh to 19 kWh. The spread is not disagreement about the batteries — it is that each source stops at a different point in the chain, and most do not say which point.

All four are defensible. A figure of 18 to 19 kWh is genuinely available from the pack if you discharge to 90–95% every day, and LFP tolerates that within technical limits. We plan at the third row for two reasons, and both are worth stating because they make our number look worse than the alternatives.

First, daily deep cycling costs cycle life. Reserving 20% is what turns a datasheet cycle count into years of service, so we treat 90–95% as the outage setting rather than the everyday one. Second, round-trip losses are real and they happen after the battery: energy that leaves the cells still has to pass through conversion before it reaches an appliance. A number that excludes them describes what the battery can release, not what your fridge receives.

If you are comparing quotes or articles, the useful move is not to pick the biggest number but to ask which of these four each source is quoting, then recompute them all on the same basis with the formula above.

The Capacity Ladder: what each tier is built from

Direct answer: At 51.2 V, common modules are 100 Ah (5.12 kWh), 200 Ah (10.24 kWh), 280 Ah (14.34 kWh) and 314 Ah (16.08 kWh). Nearly every residential target is a combination of these, which is why real bank sizes land on 20.48 or 30.72 rather than a round 20 or 30.

Delivered figures use 80% depth of discharge and 90% round-trip efficiency for consistency; substitute your own values with the formula above. Capacity options are covered on our 48V LiFePO4 systems page.

Two things fall out of this table. First, 20 kWh is not a product, it is two 200 Ah modules — which is why searching for a single 20 kWh unit returns so little. Second, the same target can often be reached two ways: 32 kWh is either two 314 Ah modules or three 200 Ah modules, and the choice comes down to wall space, per-module cost and how you plan to expand later.

What a 20 kWh bank costs, and why the per-kWh figure matters more

Direct answer: Published installed pricing for a 20 kWh LFP bank runs roughly $700 to $800 per kWh, or about $14,000 to $16,000 all-in, with the upper end applying when the system needs its own inverter rather than working with one you already have. Compare on cost per kWh rather than total, because the totals describe different capacities.

Figures compiled from published installer and review pricing, not from our own quotations. Installed cost varies substantially by market, site condition and installer — treat these as planning ranges. The sealed-unit reference row is included because it is the comparison most buyers are actually making.

Two things are worth reading out of that table. The first is that the inverter question moves the number by roughly $100 per kWh, which is why “do I already own a working inverter” belongs in the first conversation rather than the last. The second is that a modular LFP bank generally lands below a sealed all-in-one on a per-kWh basis, largely because the sealed product bundles an inverter into every unit. Whether that is the better buy depends on what you value — we set out the trade honestly in our breakdown of what home storage actually costs.

One caution on scaling the arithmetic: cost per kWh is not flat across the ladder. Smaller banks carry a higher per-kWh figure because fixed costs (labour, permitting, enclosure, protection) are spread over less capacity. A 5 kWh install is not a quarter the price of a 20 kWh install.

What has to be true for a bank to work

Direct answer: A working bank needs matched modules, a compatible inverter, correct conductor sizing, and a communication path between the battery management system and the inverter. Missing any one of them produces a system that is either unsafe, under-performing, or silent.

Item 03 is the one most often discovered late. A bank that charges is not proof that the inverter is reading the battery — that agreement has to be verified at commissioning. Our inverter and battery compatibility hub covers what closed-loop pairing requires, and conductor sizing for the current your bank can actually deliver is covered in the 48V cable size chart.

The Inverter Ceiling

Direct answer: Size the battery to your daily energy in kWh and the inverter to your simultaneous load in kW. These are independent decisions and buyers routinely get the first right and the second wrong.

The mismatch shows up in a specific way: the system runs fine for weeks, then trips the first time the air conditioner and the oven come on together. That is not a battery fault and adding capacity will not fix it. If you are still choosing, our hybrid inverter page covers the pairing side, and the off-grid battery calculator works the capacity half.

How long will 20 kWh actually last?

Direct answer: Divide delivered energy by your average load. A 20.48 kWh bank delivering about 14.7 kWh runs roughly 7 hours at a steady 2 kW, or covers a full day for a household using 10 to 15 kWh — the typical range for a US home of three to five people.

Runtime figures assume the delivered energy calculated above. Actual results vary with load profile, temperature and settings — treat these as planning arithmetic, not guarantees.

The last row is where the two ceilings meet: drawing 4 kW is only possible if the inverter is rated for it, regardless of how much capacity sits behind it.

Building the number from your own appliances

Aggregate figures are only useful once you can assemble one. The table below gives typical residential consumption so you can build a daily total rather than guess at it — and, in the right-hand column, the running draw that determines whether your inverter can carry the load at all.

Typical residential ranges for planning. Actual consumption varies with appliance age, efficiency rating, climate and usage pattern — your utility bill is a better source than any table for the daily total.

Add the daily figures for what you intend to back up and compare the total against delivered energy; add the running draws for what might run simultaneously and compare that against the inverter rating. Most homes clear the first test at 20 kWh and fail the second if the inverter was chosen carelessly.

How much solar recharges a 20 kWh bank?

Direct answer: To put back the roughly 16.3 kWh a 20.48 kWh bank consumes in a full cycle, you need about 4.5 kW of array in a location averaging 4.5 peak sun hours. Lower-sun sites and winter months need more; the array does not scale with battery capacity but with the energy you actually cycle through it.

A 0.80 derate covers soiling, temperature, wiring and inverter losses. Figures assume the entire array output goes to the battery, which it does not in practice — daytime loads are served first, so a grid-tied system needs less array than this to keep the bank cycling.

The seasonal point matters more than the average. An array sized to the annual mean will recharge the bank comfortably in summer and fall short in December, which is why off-grid designs size to the worst month and grid-tied designs do not have to. Off-grid sizing is covered in our off-grid solar systems guide.

The 5 sizing mistakes

Direct answer: Five errors account for most banks that disappoint their owners, and four of them happen before anything is installed.

When a bigger bank is the wrong answer

Direct answer: Above roughly 30 kWh, adding capacity stops solving the common residential problems. If your issue is tripping on simultaneous loads, buy inverter capacity. If it is multi-day autonomy, add generation or a generator. If it is cost per kWh, buy fewer, larger modules rather than more small ones.

We supply batteries, so this is the section where we argue against selling more of them. Three situations where capacity is not the fix:

The third row deserves emphasis because it runs against the modular instinct: four 100 Ah modules and one 200 Ah module reach similar capacity, but the smaller build costs more per kWh in enclosures, wiring and labour. Modularity is worth paying for when you genuinely intend to expand in stages, and worth avoiding when you already know the final size.

What is changing in 2026

Direct answer: Two shifts matter for capacity planning: module energy density has risen so single modules now reach 16 kWh where 5 kWh was standard, and the federal Section 25D credit ended for homeowner cash and loan purchases after 31 December 2025, which changes how buyers weigh a larger bank.

The density change is the quieter one and it alters the ladder. A 32 kWh bank that once meant six modules is now two, which reduces wall space, wiring runs and installation labour. If you are comparing quotes built on different module sizes, the per-kWh hardware price is only part of the difference — the install cost differs too.

The incentive change pushes the other way. Without a 30% federal credit on a purchased system, the marginal cost of extra capacity is fully out of pocket, which makes the “size it properly rather than generously” discipline worth more than it was. That makes the discipline of sizing to measured load, rather than to a round number, worth more than it was a year ago.

Frequently asked questions

Get a configuration sized to your loads

Send these four items and we will return a specified configuration (module count, nameplate and delivered capacity, and the inverter rating it needs) rather than a brochure.

1. Average daily consumption in kWh, from twelve months of utility bills if possible
2. Peak simultaneous load — the circuits and appliances that must run together
3. Grid-tied or off-grid, and whether solar is already installed
4. Market — for the correct certification scope

Related reading

For the modules behind these tiers, see our 200Ah module (the building block of most 20 kWh banks) and the 314Ah module for higher density in fewer enclosures. Smaller builds start at the 100Ah module. For whole-home design rather than capacity alone, see the whole-home backup guide.