Understanding Battery Capacity and Depth of Discharge

Headline capacity is not the number that matters

Walk through any solar quote and the conversation lands quickly on a big round number: 5 kWh, 10 kWh, 15 kWh. It is the figure on the front of the datasheet and the one most people remember. Yet the amount of electricity you can actually draw from that box on a dark December evening is smaller, sometimes noticeably so, and understanding the gap is the difference between a system that quietly earns its keep and one that disappoints.

Three things shrink the headline figure: depth of discharge, round-trip efficiency, and the reserve your installer keeps back for power cuts. None of them are marketing tricks. They are sensible engineering, but they all need to be understood before you decide how much capacity to buy.

What depth of discharge really means

Depth of discharge, usually shortened to DoD, is simply the proportion of a battery's capacity you have used. A 10 kWh battery that has given up 8 kWh is at 80% DoD, which is the same as sitting at 20% state of charge.

Every manufacturer publishes a maximum DoD, and the battery management software enforces it. The figures vary by chemistry:

• Older lead-acid cells, now rare in home storage, dislike going below about 50% DoD and age quickly if pushed harder.
• Lithium iron phosphate (LFP), the chemistry in most modern home batteries, will happily run to 90% and many models tolerate 100%.
• Some nickel-based lithium chemistries sit in between, often around 80% to 90%.

So a 10 kWh LFP battery limited to 90% DoD offers 9 kWh of usable capacity. Add a 10% round-trip loss through the inverter and cells, and roughly 8.1 kWh reaches your sockets. Still useful, but it is not 10.

Why shallower cycling can mean a longer life

Degradation comes from a mix of calendar ageing, temperature and how hard the cells work. Deep discharges stress the electrodes more than shallow ones, so the same battery will deliver more total cycles if you treat it gently.

A typical pattern is a battery rated for 6,000 cycles at 80% DoD managing only 3,500 at 100% DoD, but stretching beyond 10,000 if you keep to 50%. Because shallow cycles are so much more numerous, the total energy throughput over the battery's life is often higher with gentler use. That is why a well-set-up system may shift more kilowatt hours over a decade than a hard-cycling one, even with a smaller usable capacity each day.

Warranties are usually written as two limits: a cycle count or throughput figure in megawatt hours, and a calendar term, whichever arrives first. A ten-year promise of 60% of original capacity is weaker than it sounds if the throughput cap is reached in year seven, so read both numbers.

Sizing the battery to your actual daily use

The average UK household uses around 2,700 kWh of electricity a year, roughly 7.5 kWh a day, but that average hides a wide seasonal swing. Think 5 kWh a day in June and 11 kWh in January.

Solar generation swings the opposite way. A 4 kWp array in southern England might produce 18 kWh on a bright June day and barely 2 kWh in midwinter, while the same array in northern Scotland will see less again. A battery that feels generous in summer can be short in winter, just when you want it most.

The practical rule is to size for your evening and overnight load rather than your whole day. If you use 3 kWh between 5pm and midnight and 2 kWh overnight, an 8 kWh battery at 90% DoD covers that comfortably with a little headroom for a dull day. Many homeowners find 5 to 10 kWh of usable capacity is the sweet spot; beyond that, you are paying for capacity you cannot shift within 24 hours.

Settings, temperature and the day-to-day details

Once the battery is installed, a few settings have a real effect on how much you get from it:

• Backup reserve. Most systems hold back 10% to 20% so the lights stay on during a power cut. That reserve is not available for shifting cheap electricity, so factor it in.
• Charge windows. If you are on a time-of-use tariff, set the battery to charge in the cheapest hours and discharge during the expensive peak. A well-timed window is worth more than an extra kilowatt hour of capacity.
• Location. LFP cells lose a little capacity in the cold. A battery in an unheated garage at 2°C may deliver a few percent less than one in a insulated utility room, so keep it out of the worst of the frost.
• Top-end headroom. Some systems throttle charging near 100% to protect the cells, so the last stretch of charging can be slow. Leaving a small gap at the top is often kinder than chasing a full charge every day.

Running the numbers on whether it pays

Take a battery with 9 kWh usable. Charging it overnight at 10p per kWh costs 90p. Using that electricity instead of buying at 29p per kWh saves about £2.60, so the net gain is roughly £1.70 a day, or a little over £600 a year if you cycle fully every single day.

In reality you will not cycle fully every day, and there are losses in the round trip, so £400 to £550 a year is a more honest expectation for a well-sized system on a good tariff. With installed costs often landing between £400 and £700 per usable kilowatt hour, that points to a payback somewhere in the region of eight to twelve years. The case rests as much on using your own solar generation as on tariff arbitrage.

Before you sign anything, ask for four numbers in writing: the usable capacity, the depth of discharge the system will actually run to, the round-trip efficiency, and the throughput limit in the warranty. Get those right and the headline figure becomes a detail rather than a decision.