Storage is the most over-sold component in solar. This guide explains the specifications that matter, how to size a battery to your own load profile, how AC and DC coupling differ, what determines whether storage pays back, and the three situations where it clearly does.
Storage is the most enthusiastically sold and least rigorously justified component in residential solar. A battery does not generate a single extra kilowatt-hour. It moves energy you already produced from the afternoon, when you had a surplus, to the evening, when you don't.
Whether that shift is worth paying for depends almost entirely on the gap between what your utility charges you to import and what it pays you to export. In some markets that gap makes storage obviously worthwhile. In others it makes storage a slow way to lose money. This guide gives you the tools to work out which one you're in.
What a battery actually does for you
Three distinct benefits, and they are worth separating because people conflate them and then buy the wrong size.
Self-consumption. Without storage, surplus midday generation flows to the grid at whatever export rate you're offered, and you buy it back in the evening at the retail rate. A battery lets you use your own energy instead. The value of this equals the spread between those two rates.
Backup power. If your grid fails, a properly configured system keeps selected circuits running. Note that ordinary grid-tied solar without a battery shuts down completely during an outage — anti-islanding rules require it, so panels alone provide no blackout protection.
Tariff arbitrage. On time-of-use tariffs, you can charge cheaply overnight or at midday and discharge during the expensive evening peak, saving money even on days with no sun at all.
Most households value one of these far more than the other two. Identify which one before you size anything.
Battery chemistries
Lithium iron phosphate (LiFePO4)
The current default for home storage and deservedly so. Typical usable depth of discharge sits around eighty to ninety-five percent, cycle life commonly runs from three thousand to over six thousand cycles, and the chemistry is markedly more thermally stable than the alternatives. It tolerates partial charging without damage, needs no maintenance, and holds up well in heat.
It is heavier and bulkier per kilowatt-hour than nickel-based lithium, and costs more upfront than lead-acid. For a stationary home installation neither drawback matters much.
Nickel manganese cobalt (NMC)
Higher energy density, so a smaller and lighter package for the same capacity. Cycle life is typically lower than LiFePO4 and thermal management requirements are stricter. Still used in several well-known home batteries and perfectly serviceable — but the market has been shifting steadily toward LiFePO4 for stationary applications where weight is irrelevant.
Lead-acid
Cheapest upfront and still common in budget off-grid builds. The problem is that you can only safely use around fifty percent of nameplate capacity, so a nominal 10 kWh bank gives you 5 kWh of real storage. Cycle life is short, deep discharges cause rapid degradation, and flooded types need ventilation and regular electrolyte topping up.
Calculate cost per usable kilowatt-hour delivered over the battery's whole life rather than cost per nameplate kilowatt-hour, and lithium wins clearly in nearly every scenario. Lead-acid retains a place only where upfront capital is the absolute constraint.
Compare real products in our Tesla Powerwall vs BYD comparison and in the BlueNova vs Pylontech review.
The specifications that matter
Usable capacity, not nominal capacity. These are different numbers and only one of them is yours to use. A 13.5 kWh nominal battery with a 90 percent depth of discharge gives you about 12.2 kWh. Always compare on usable.
Continuous power output in kilowatts. This determines what you can run at once, and it is entirely separate from capacity. A large battery with modest power output cannot start an air conditioner regardless of how full it is.
Peak or surge output. Needed for motor startup — pumps, compressors, air conditioning.
Round-trip efficiency. Energy in versus energy out. Quality systems sit around ninety percent, meaning you lose roughly a tenth of everything you store. This loss is real and belongs in your payback calculation.
Cycle life and warranty terms. Warranties are usually expressed as ten years, or a total energy throughput in megawatt-hours, or a number of cycles — whichever comes first. Read which limit applies to you. A battery cycled daily reaches a cycle limit long before a calendar limit.
End-of-warranty capacity. Most guarantee around seventy percent of original capacity at the end of the term. That is a substantial decline and it should be in your model.
Operating temperature range. Garage and outdoor installations in extreme climates need units rated for it. Heat shortens lithium life significantly.
Scalability. Can you add modules later, and at what cost? Requirements grow, particularly if an EV arrives.
AC coupling or DC coupling
DC-coupled systems connect the battery to the same hybrid inverter as the panels. Solar charges the battery directly as DC with no conversion in between, which is more efficient — commonly two to five percentage points better round-trip. It is the natural choice for new installations, where you simply specify a hybrid inverter from the start.
AC-coupled systems give the battery its own inverter and connect it on the AC side of your existing setup. Solar DC becomes AC, then converts back to DC to charge, then back to AC to discharge — three conversions instead of one. Less efficient, but it retrofits onto an existing solar installation without replacing anything, and it lets you size battery power independently of solar capacity.
The practical rule: if you are installing solar now, choose a hybrid inverter and DC coupling. If you already have a working grid-tied system, AC coupling is usually cheaper than replacing a functioning inverter. Our solar inverter guide covers hybrid selection in detail, and the inverter database lets you filter for battery-ready models.
How to size a battery
Sizing storage is different from sizing panels. Panels are sized against total annual consumption. Batteries are sized against evening and overnight consumption, because that is the only part of your load a battery can serve.
Step 1 — Find your evening load
Look at your consumption from sunset to sunrise. Interval data from your utility or smart meter is ideal. Failing that, estimate: for many households, roughly thirty to forty percent of daily consumption happens after dark, and it is concentrated in cooking, lighting, entertainment, and heating or cooling.
A home using 20 kWh a day might use six to eight of those kilowatt-hours overnight.
Step 2 — Check your surplus
A battery cannot store energy you never generated. If your array produces 25 kWh on a good day and you consume 12 kWh during daylight, you have 13 kWh of surplus available. If your surplus is only 4 kWh, a 10 kWh battery will sit half empty most days and you have overpaid substantially.
This is the most common sizing error: buying storage larger than the daily surplus that would fill it.
Step 3 — Match the two, then add for backup
For pure self-consumption, size the battery to the smaller of your evening load and your daily surplus. Adding capacity beyond that point earns nothing.
If backup matters, add capacity for your critical loads through the outage duration you want to survive. Note that this is a genuinely different requirement — a self-consumption battery might be 5 kWh where a meaningful backup battery for the same house might be 15 kWh, because you are now covering a whole day rather than one evening.
Also check the power rating against your backup circuits, not just capacity. Running a fridge, lights and a router needs very little power. Running an air conditioner or well pump needs a great deal.
Run your own figures through the battery size calculator, and check your array is sized to actually produce the surplus using the system size calculator.
Start smaller than the salesperson suggests. Most modern batteries are modular. An undersized battery that cycles fully every day earns its keep; an oversized one that never fills is dead capital sitting on your wall.
Does a battery pay back?
Here is the honest arithmetic. A battery's annual saving is approximately the energy it cycles through in a year, multiplied by the spread between your import rate and your export rate, adjusted for round-trip losses.
That means everything hinges on the import-export spread:
Where export pays close to the retail rate — traditional full-retail net metering — the spread is near zero and a battery saves almost nothing on self-consumption alone. It may still be worth buying for backup, but be clear that you are buying insurance, not savings.
Where export pays a small fraction of retail — increasingly the norm — the spread is large and storage becomes genuinely attractive.
Where you face time-of-use tariffs with an expensive evening peak, a battery earns from arbitrage as well as self-consumption, which stacks the two benefits and shortens payback considerably.
Where outages are frequent, the value of avoided disruption may dominate the calculation entirely, particularly for home businesses, medical equipment or food storage.
Export rates have been falling in many markets, which is precisely why storage economics have improved. Our analysis of 2026 net metering changes covers what is shifting and why. Check your own tariff spread before anything else, then model the outcome with the payback calculator and ROI calculator.
When storage clearly makes sense
Your export rate is far below your import rate, and you have a genuine daily surplus to store
You face frequent or extended outages and the disruption has real cost
You are on a time-of-use tariff with a significant evening peak
You are off-grid, where a battery is not optional but structural — see the off-grid system guide
Your local incentives or grants cover a meaningful share of the cost
When it probably doesn't
You have full-retail net metering and a reliable grid — the grid is already acting as a free, lossless, unlimited battery
Your array is small and rarely produces surplus
Your consumption is concentrated during daylight hours, so you already self-consume most of your generation
You are buying primarily on the argument of energy independence without checking the numbers behind it
Installation, safety and lifespan
Batteries need a location that is dry, ventilated, temperature-stable and away from living and sleeping areas where local regulations require it. Garages, utility rooms and shaded external walls are typical. Heat is the enemy — a battery in an unshaded outdoor enclosure in a hot climate will age noticeably faster than the same unit indoors.
Proper isolation, earthing and surge protection are not optional; our grounding and surge protection guide covers what should be present. And if backup is part of your reason for buying, confirm at commissioning which circuits are actually on the backup supply and test it before you need it — plenty of owners discover the limits during their first real outage.
Expect ten to fifteen years of service from a quality lithium system, with gradual capacity fade throughout rather than sudden failure. Plan for the battery to be replaced roughly once during your panels' lifetime, and include that in any long-horizon financial model.
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