A battery is not sized by the array on the roof - it is sized by the loads it must carry and for how long. Two houses with identical panels can need wildly different banks: one wants an evening buffer of a few kilowatt-hours, the other wants two sunless days of full living. Sizing is therefore a chain of divisions, not a single multiplication, and each link - inverter losses, depth of discharge, cold mornings - makes the bank bigger than the raw load suggests.
This guide walks the chain in order: measure the daily energy, decide the days of autonomy, apply the efficiency and discharge fractions, and land on a bank size in kWh and amp-hours at a chosen voltage. The worked example is a 10 kWh-per-day household, carried all the way through, and it matches the arithmetic the Solar Battery Sizing Calculator runs.
Bank size = daily use × days of autonomy ÷ inverter efficiency ÷ depth of discharge. Planning defaults: inverter efficiency 90%; depth of discharge 80% for lithium (LiFePO4), 50% for lead-acid. A 10 kWh/day house wanting 2 days of cover at those defaults needs 10 × 2 ÷ 0.9 ÷ 0.8 ≈ 28 kWh of storage - about 583 Ah at 48 V. Whole-house banks live at 48 V; small systems sit at 12 or 24 V.
Start With the Load, Not the Battery
Every honest sizing begins with a measured or carefully itemised daily energy figure in kilowatt-hours. The measurement route is the better one: read your meter at the same hour on two ordinary days, or pull the daily history from a smart meter or monitoring app if you have one. A month of daily figures beats any table of typical values, because it shows your cloudy-Tuesday floor and your laundry-and-guests ceiling, and the battery has to serve both.
Itemising works when measurement is not possible yet. The honest version multiplies each appliance's power by the hours it actually runs: a modern refrigerator at roughly 1–1.5 kWh per day, a LED bulb at a few watt-hours per evening, a washing cycle at 0.5–1 kWh per load, a laptop working all day at 0.2–0.3 kWh. The dishonest version rounds everything optimistically. Air conditioning, electric space heating, electric water heating, pool pumps and tumble dryers are the loads that decide bank size - leave them out of the itemised list and the arithmetic will describe a different house than the one you live in.
Season matters as much as the list. Winter typically raises lighting and, in many homes, heating loads while the solar yield falls - the two worst effects arriving together. If the plan is year-round off-grid cover, size against the darkest month's consumption, not the annual average.
How Many Days of Autonomy?
Autonomy is the number of sunless days the bank carries without grid or generator help. It is a design choice with a direct multiplier on the bank size, so it deserves a conscious decision rather than a default.
Grid-tied backup systems - the bank exists for outages, not for daily cycling - usually plan for hours, not days: one evening and one morning of essential loads. Off-grid homes plan for 2–3 days of autonomy because a winter weather pattern can hold cloud for that long, and because a bank that bottoms out on day two has no margin for the cold snap that follows. Hybrid systems that expect the grid or a generator to catch the shortfall commonly plan 1–2 days and treat the third cloudy day as the generator's job.
The trade is plain: each day of autonomy is another full day of banked energy sitting in the rack, and the marginal day is the least-used one. Most off-grid designs settle at 2–3 days; most backup designs at half a day or less; and the honest way to pick is to look at what happens when the bank runs dry - an inconvenience, or a genuine hardship.
Depth of Discharge: The Chemistry Decides
Depth of discharge (DoD) is the share of the bank's nameplate energy the design allows itself to use. The battery tolerates deeper discharge than the plan uses; the plan holds back to protect cycle life. The chemistry sets the sensible number.
| Chemistry | Planning DoD | Notes |
|---|---|---|
| Flooded lead-acid | 50% | The classic conservative figure; deeper cycling shortens life quickly |
| AGM / gel | 50–60% | Sealed lead-acid family; follow the datasheet |
| Lithium iron phosphate (LiFePO4) | 80% daily, 90–100% occasional | Modern default for home storage; flat discharge curve, thousands of cycles |
| Other lithium chemistries | 80–100% | Varies by chemistry and manufacturer; the datasheet is the authority |
The divisor works in the intuitive direction: planning at 50% instead of 80% grows the bank by a factor of 0.8 ÷ 0.5 = 1.6 for the same delivered energy. That single fraction is why lead-acid banks of a given usable capacity are physically larger than their lithium replacements, and why the chemistry decision is made before the arithmetic, not after it.
The Chain of Losses
Energy leaving a battery bank for your sockets passes through the inverter, which converts DC to AC at 90–95% efficiency at typical mixed loads - 90% is the standard planning default, and inverters spending their life near their ceiling run worse than that. That loss lands first in the chain: the bank must deliver more kilowatt-hours than the loads consume.
The sizing chain deliberately stops there, and it is worth being clear about what it ignores. Round-trip losses inside the battery itself (a few percent on lithium, 15–20% on lead-acid), charge controller conversion losses, wiring voltage drops and cold-weather capacity derating are all real, and a full system design adds margin for every one of them. The planning arithmetic leaves them out to stay transparent - the calculator's output is a sizing floor, not a commissioning document.
Temperature deserves its own word, because it attacks from both directions. Lead-acid capacity drops noticeably in the cold; lithium charging is restricted below freezing unless the module has built-in heating; and a bank installed in a hot roofspace ages faster than one in a temperate room. The installation location is part of the sizing decision.
Volts, Amps and Amp-Hours
Storage is specified in kWh, but batteries are still bought, wired and fused in volts and amp-hours, so the conversion belongs in the plan. It is plain arithmetic: amp-hours = kWh × 1000 ÷ bank voltage. The worked bank below is 28 kWh; at 48 V that is 28,000 ÷ 48 ≈ 583 Ah; at 24 V it would be 1,167 Ah and at 12 V, 2,333 Ah - same energy, very different copper.
The reason whole-house banks standardise on 48 V is current. Power equals voltage times current, so doubling the voltage halves the current for the same kettle or saw. Lower current means thinner cables, smaller breakers, less heat at every termination and less voltage sag on motor starts - the 3 kW morning kettle that draws 250 A at 12 V draws 62.5 A at 48 V. Below roughly 5 kWh of storage, or for systems that never invert more than a kilowatt or two, 12 and 24 V remain entirely sensible and keep the equipment simpler.
Worked Example: A 10 kWh-Per-Day Household
The example household uses 10 kWh per day (measured, not guessed), wants 2 days of autonomy, runs a modern inverter at 90% efficiency and chooses lithium storage planned at 80% DoD. The chain runs:
- Energy through the inverter: 10 kWh/day × 2 days = 20 kWh of AC energy; ÷ 0.90 = 22.2 kWh that must leave the bank.
- Bank nameplate: 22.2 kWh ÷ 0.80 = 27.8 kWh ≈ 28 kWh of storage.
- Amp-hours at 48 V: 27,800 ÷ 48 ≈ 583 Ah.
- Reality check: the same loads on lead-acid at 50% DoD need 22.2 ÷ 0.5 = 44.4 kWh - about 925 Ah at 48 V, a materially bigger installation.
Four numbers, and each division is honest about why the bank grew: the inverter takes its share before the loads see a watt, and the DoD hold-back reserves a fifth of the bank permanently. The Solar Battery Sizing Calculator runs exactly this chain - enter daily use, days, efficiency, DoD and voltage, and read the bank in kWh and Ah.
The Charging Side Has a Say
A bank that size needs an array that can refill it. The honest check is a day of arithmetic in reverse: the panels must cover the day's loads plus the charging energy plus the losses on both, within the sun hours of the worst planning month. A 10 kWh/day household with a 28 kWh bank is not charging that bank from empty every day - but the array still needs comfortable headroom above the 10 kWh of daily loads, and off-grid designs usually carry generous panel overcapacity for exactly this reason.
Charge rate is the other constraint, and chemistry sets it. Lead-acid banks like a gentle charge - roughly 0.1–0.2 times capacity (C/10 to C/5) - so a 925 Ah bank wants a charging current in the 90–185 A range at the bank voltage, which is a serious array or a generator session. Lithium accepts 0.5C and often more, so the same energy goes back in a fraction of the time. The panel count guide covers the generation side of the partnership.
Where the DIY Line Sits
Bank arithmetic is safely a desk exercise; bank installation is not. A 48 V, 28 kWh store can deliver fault currents that arc, weld and start fires, DC breakers and fuses are specialist items, and the interplay of battery management system, inverter and charge controller is manufacturer-specific. Rules for who may install fixed electrical work vary by country and region, and battery storage sits firmly in the certified-installer category almost everywhere. Use this guide to size the system and interrogate quotes; leave the lugging, crimping and commissioning to the qualified.
Frequently Asked Questions
How long will a 10 kWh battery run a house?
At an 80% depth of discharge and 90% inverter efficiency, a 10 kWh bank delivers about 7 kWh of usable AC power. That runs a typical fridge (1-1.5 kWh per day), lighting, internet and a few hours of television for roughly two to three days, or one heavy evening - cooking, laundry, air conditioning - on its own. Add heating, pumping or electric water heating and the runtime shrinks fast, which is why the sizing starts from your measured daily use, not the battery's nameplate.
What depth of discharge should I plan around?
Lead-acid chemistry - flooded, AGM or gel - is planned at 50% because deeper cycling shortens its life sharply. Lithium iron phosphate (LiFePO4) tolerates 80% as a comfortable daily default, and many modules permit 90-100% for occasional use. The exact recommendation varies by manufacturer, so treat the datasheet, not this rule of thumb, as the authority - and remember that the planning number you enter is a floor: the bank grows if you discharge deeper than planned.
Should the battery bank be 12, 24 or 48 volts?
Small systems - a shed, a campervan, a few lights and a fridge - run happily on 12 V. Whole-house banks almost always land on 48 V, because doubling the voltage halves the current for the same power, which means thinner copper, smaller fuses and fewer losses at the connections. A common planning line: below about 5 kWh of storage, 12 or 24 V is reasonable; above it, 48 V is the practical default. Higher-voltage stacks (high-voltage residential systems) follow the inverter manufacturer's architecture rather than open arithmetic.
How do I convert kWh to amp-hours?
Amp-hours = kilowatt-hours times 1000, divided by the bank voltage. A 28 kWh bank at 48 V is 28,000 divided by 48, about 583 Ah. The conversion is pure unit arithmetic - energy to charge at a fixed voltage - but it is also where series and series-parallel wiring decisions live: the same 583 Ah at 48 V can be built as strings of lower-voltage modules, and the wiring plan must follow the manufacturer's string rules, not just the totals.
Can I add more batteries to the bank later?
Often yes, with conditions. Lithium banks with a proper battery management system usually expand cleanly with matching modules from the same family, within the inverter's limits. Lead-acid banks are fussier: mixing old and new, or different capacities, tends to equalise badly and drag the whole string to the weakest cell. If expansion is likely, plan the cabling, fusing and inverter rating for the final size on day one - retro-fitting headroom later is the hard part.
Planning guidance for residential battery storage. Round-trip losses, temperature derating, wiring drops and local installation rules are deliberately excluded from the sizing chain - a real design adds all of them, and fixed electrical work belongs to certified installers.
Where to Read Next
- The Solar Battery Sizing Calculator — the chain from daily kWh to bank kWh and amp-hours, at any system voltage.
- How many solar panels fit on your roof? — the generation half of the partnership: usable roof area, setbacks and panel math.
- The electrical service load guide — how a house's demand is actually calculated, and what a bank must cover.
- The wire size guide — why the amp-hour side of the arithmetic matters: current, voltage drop and conductor sizing.
- The Roofing & Solar category — every roof, panel and storage guide and calculator in one place.
The bottom line: size the bank from the load, not the panel count - daily kWh times days of autonomy, divided by inverter efficiency, divided by depth of discharge. At the common defaults (90% / 80% lithium), two days of cover for a 10 kWh-per-day house is a 28 kWh, 583 Ah bank at 48 V. Every excluded loss makes the real bank bigger, and the installation itself is certified-installer work.