What Size Solar Battery Do You Need? A Sizing Walkthrough

Ask “what size battery do I need?” and the only responsible first reply is another question: for what? A battery bought to ride out a multi-day outage, a battery bought to dodge expensive evening electricity rates, and a battery bought to soak up solar production the utility will not pay fairly for are three genuinely different sizing problems that happen to share the same hardware on the wall. People run into trouble when they skip straight to shopping for a kilowatt-hour number without deciding which of those jobs they are buying for, and they end up either paying for capacity that sits idle or discovering the unit runs flat halfway through the first outage. Pick the job first. Once the goal is settled, the arithmetic that follows is honestly simple — mostly multiplication and one clear-eyed inventory of what you actually run.

Name the job before you size anything

The three common jobs each point to a different number, so it is worth seeing all three before deciding which is yours. Backup power sizes to your critical loads multiplied by the hours you want to survive without the grid. The battery here is insurance, and capacity buys duration — more kilowatt-hours means more hours before the lights go out, full stop. Time-of-use savings sizes to something completely different: the energy you consume during the expensive rate hours of a typical day. The battery is doing arbitrage, charging when power is cheap and discharging when it is dear, and capacity beyond your evening usage earns you nothing extra because there is no more expensive energy to displace. Self-consumption sizes to your typical evening-and-overnight usage, bounded by how much surplus your solar array produces during the day, and the battery’s job is to convert cheap or unpaid exports into full-value avoided imports after the sun goes down.

Those three logics can pull toward very different capacities from the same household, which is exactly why naming the job matters before any number gets written down. A backup-focused buyer who honestly lists their critical loads often lands on a surprisingly small battery, because “critical” turns out to mean the refrigerator and some lights rather than the whole house. A time-of-use buyer sizes to a specific window — say the four or five expensive evening hours — and anything bigger is dead weight. A self-consumption buyer is constrained on both ends at once, needing enough capacity to hold the evening’s usage but not more than the array can actually refill in a day. Get the job wrong and you can easily buy double what you need, or half.

Plenty of households, reasonably, want more than one of these at once — backup security and a dent in the evening rates from the same unit. That is fine, and it is common, but it does not mean adding the numbers together. It means sizing for the larger of the two answers and knowing which one is driving the result, because the two jobs make different demands. A backup battery wants to sit mostly full, holding a reserve for an outage that might come at any moment. A time-of-use battery wants to cycle deeply every single day, draining each evening and refilling each afternoon. A unit doing both has to hold back an outage reserve while still leaving enough usable capacity to make the daily arbitrage worthwhile, which is a real constraint that shapes both the capacity you buy and how you configure it. So the honest first step is not “how many kWh,” it is “which job leads, which job follows, and how much do they overlap.” Everything downstream depends on that answer, and it costs nothing but a few minutes of clear thinking before you look at a single spec sheet.

It is worth being honest about the money at this stage too, because the job you pick determines whether a battery pays for itself or simply buys something you value. A backup battery, sized to critical loads, rarely pays back in pure dollars — it is insurance against the cost and misery of an outage, and you justify it the way you justify a generator or any other resilience purchase, by how much a dark, powerless house would actually cost you and how often your grid goes down. A time-of-use battery has a genuine savings case, but its strength depends entirely on the spread between your cheap and expensive rate periods: a wide spread makes nightly arbitrage worthwhile, while a narrow one leaves the battery cycling hard every day to capture pennies, and no amount of capacity fixes a rate structure that does not reward shifting. A self-consumption battery lives or dies on the gap between your retail rate and your export credit — where the utility pays you nearly retail for exports, there is little to gain by storing them; where it pays a fraction, the battery recaptures the difference on every kilowatt-hour you would otherwise have sold cheap. Naming the job, then, is not just an engineering step. It tells you which benefit you are buying and therefore which yardstick to measure the purchase against. A buyer who wants backup but evaluates the battery on savings will be disappointed by the payback and miss the point; a buyer who wants savings but buys a big battery for imagined outages will overpay for capacity that sits idle. Match the yardstick to the job before the spec sheets come out, and every later decision about capacity, chemistry, and configuration falls into place around a goal you have actually defined.

Building the load list, and why solar changes the answer

For backup sizing specifically, the work is a load list: write down what genuinely must stay powered through an outage, find each item’s power draw from a label, a manual, or a cheap plug-in watt meter, and convert each one to daily energy with the simple formula of watts times hours of use per day divided by 1,000, which gives kilowatt-hours per day. The discipline is in the word “genuinely.” Everything you add to this list, you are paying to back up. Here is a worked example with every figure assumed for illustration, laid out as a compact tally.

Critical load	Assumed draw and use	Daily energy
Refrigerator	~150 W avg (cycling) × 24 h	1.5 kWh
Internet router and modem	20 W × 24 h	0.5 kWh
Lights (LED, several rooms)	100 W × 6 h	0.6 kWh
Phone and laptop charging	—	0.3 kWh
Gas furnace fan (winter)	400 W × 8 h	3.2 kWh
Sump pump (intermittent)	—	0.5 kWh

That totals about 6.6 kWh a day of critical load — a far cry from the 25 to 35 kWh a typical home burns when nobody is rationing anything. That gap is the central insight of the whole subject. Backup is cheap if you are honest about what “critical” means, and brutally expensive if “critical” quietly expands to mean “everything, including the central air conditioning.” One large absence from that list is deliberate and worth staring at: electric resistance heating, electric water heaters, ovens, and central AC each draw enough power to drain a typical home battery in a matter of hours rather than days. Backing those up takes multiple battery units or a generator, and pretending otherwise is how people end up disappointed; how long a battery can power a house runs those bleaker numbers for the loads most likely to blow your budget.

Once you have a daily critical load, the raw duration math is a single multiplication: daily load times the days you want to cover. The 6.6 kWh example rides out one overnight-to-morning outage with margin, covers a full 24 hours at about 7 kWh, and reaches two days at roughly 13 kWh. But if you have solar, the battery never faces the outage alone, and this changes the target dramatically. The array recharges the battery each day — even under clouds, at reduced output — so a battery that covers a single evening-to-morning stretch, paired with panels producing anything at all the next day, can ride out a multi-day outage that would flatten the same battery working by itself. This is why solar-plus-storage sizing usually targets “one night plus a reserve” rather than “three full days,” and it is why battery sizing and solar sizing genuinely have to be done together rather than one after the other: the array’s daily contribution is part of the battery’s effective capacity during an outage. How home solar batteries work covers the mechanics of that daily charge-and-discharge cycle, and understanding it is what keeps people from massively overbuying storage out of outage anxiety. The battery does not need to survive the whole outage on its own if the sun is going to help refill it every morning.

Two subtleties in the load list trip people up and are worth handling deliberately. The first is that some critical loads are intermittent or surge-heavy rather than steady, and the daily-energy figure understates their real demand on the battery. A sump pump or a well pump might run only a few minutes an hour, so its daily kilowatt-hours look tiny, but each start draws a brief surge several times its running wattage, and that surge is a power problem the battery has to meet even though it barely registers in the energy total. Note those loads separately so you check them against the battery’s power rating later, not just its capacity. The second is that the critical list changes with the season, which means you should size against the harsher season rather than an average one. The winter load list in the worked example carried a 3.2 kWh furnace fan that simply does not exist in summer; a home in a hot climate would instead face the question of whether to put a window air conditioner or a single mini-split on the list, which can double the daily critical load. Build the list for the season that stresses the battery hardest — usually winter for heating-dependent homes and summer for cooling-dependent ones — because a battery that comfortably covers your mild-weather critical loads can still fall short on the January night or the August afternoon when you need it most, and those are precisely the outages worth preparing for.

From nameplate to what actually reaches your loads

There is a gap between the kilowatt-hour number on a battery’s spec sheet and the energy that actually reaches your refrigerator, and three deductions live in that gap. The first is depth of discharge — how much of the nameplate capacity the battery will actually let you use. Most modern home lithium batteries allow 90 to 100 percent, but some chemistries and older designs reserve 10 to 20 percent to protect cell life, so you size on usable kilowatt-hours, which any reputable spec sheet states separately from the nameplate. The second is round-trip efficiency: storing energy and pulling it back out loses roughly 8 to 12 percent to heat in the power electronics and the chemistry, so ten kilowatt-hours in yields about nine out. The third is your own reserve setting — a battery cycling daily for time-of-use savings typically holds back a slice, often around 20 percent and user-configurable, so that an outage never catches it empty. A 13.5 kWh unit cycling with a 20 percent reserve offers only about 10.8 kWh for the daily savings work. Stack those three deductions and a rule of thumb falls out worth committing to memory: plan on roughly 80 to 85 percent of nameplate capacity doing real work. If your sizing math says you need 10 kWh delivered to your loads, shop for about 12 kWh of nameplate, not 10.

One more spec gate stands between the right capacity and the right battery, and it is the one people skip most: power, measured in kilowatts, not just energy in kilowatt-hours. A battery can store plenty of energy and still be unable to release it fast enough to start a well pump or run two large loads at the same moment. Energy is how much total work the battery can do; power is how fast it can do it, and a well pump’s startup surge or a simultaneous AC-and-oven moment is a power problem, not an energy one. Check the continuous kW rating against your largest simultaneous loads before you commit, because a battery that passes the kilowatt-hour test can still fail the kilowatt one. The distinction trips up a lot of buyers, and kWh versus kW for home batteries unpacks it properly; for sizing, just treat both numbers as gates the battery has to clear, not one.

Put the whole chain together on the two households from earlier and the method becomes concrete. The backup-focused home with 6.6 kWh a day of critical load, wanting a full 24 hours of autonomy plus margin and with no solar yet, needs about 7 kWh delivered, which after efficiency and depth-of-discharge losses means roughly 8.5 to 9 kWh usable, which one mid-size unit in the typical 10-to-13.5 kWh nameplate range covers comfortably, with the winter furnace-fan case as the stress test to size against. The time-of-use home, assuming an expensive rate window from 4 to 9 p.m. during which it consumes about 9 kWh, needs 9 kWh deliverable, which points to roughly 11 kWh of nameplate cycling capacity plus whatever outage reserve you want layered on top — one larger unit or two small ones, and beyond that size the extra capacity would sit idle most evenings and only stretch the payback. The solar battery calculator automates this entire chain of loads, hours, efficiency, and reserve, and pairing it with the solar panel calculator confirms your array can actually refill each night what the evening drains.

A few questions come up so often they are worth answering head-on. Can you start small and add capacity later? Usually yes — most major battery ecosystems support stacking additional units — but confirm the specific model allows expansion and that mixing battery ages is supported, because buying the second unit later costs more than buying both at once even as it costs far less than overbuying capacity you never touch. Does battery size depend on solar system size? Loosely but genuinely: the battery has to be small enough that the array’s daily surplus can refill it, since a 5 kW array cannot reliably feed 30 kWh of storage, and large enough to hold the surplus you would otherwise export cheaply, which is why the two should be sized together. And is a bigger battery always safer for outages? No — diminishing returns arrive fast once solar recharge is in the picture, because doubling capacity does not double outage endurance when the array refills the battery daily; it mostly just adds cost. The margin is better spent making sure you have truly covered your critical loads and, if outage risk is severe, on a generator interlock as the deep backstop behind the battery.