Solar panels with batteries: when do they make sense?

A grid-connected (on-grid) system uses the grid as backup and applies net metering; it is the most cost-effective for saving and carries no batteries. A hybrid system adds batteries so it keeps running during outages. An off-grid system does not depend on the grid and runs on panels and batteries alone.

The right choice starts with defining the goal: cutting the bill, having backup during blackouts, or bringing power to where there is no grid. Each goal points to a different configuration, and confusing them is the most frequent cause of oversized installations.

It is worth noting that a hybrid system is not simply 'on-grid plus batteries'. It requires a different inverter, capable of operating disconnected from the grid and of managing charging, and a separation in the panelboard between the loads that will have backup and those that will not. It is a design decision worth taking before installing, not afterwards.

With net metering, the surplus you export during the day is deducted from the energy you draw from the grid at night. In practice the grid plays the role of a battery at no extra cost, with no conversion losses and no limited service life.

Adding batteries purely to save usually worsens the return, because they increase the investment without increasing the saving proportionally. Every kWh that passes through a battery suffers charge and discharge losses, and also consumes a fraction of the equipment's service life. That kWh comes out more expensive than the same kWh offset through the grid.

So if your only goal is to cut the bill and your grid is stable, the norm is not to fit them. The conversation changes completely if what you want is continuity.

Understanding two parameters avoids most misunderstandings when quoting storage: energy and power.

Energy is measured in kilowatt-hours (kWh) and determines how long you can sustain a load. Power is measured in kilowatts (kW) and determines which loads you can switch on at once. A battery with a lot of energy but little power will run a small load for many hours; one with a lot of power and little energy will start demanding equipment but run flat quickly. You need both values to match your case.

There is a third decisive parameter: usable depth of discharge. No battery delivers 100% of its nominal capacity without penalising its life. Lithium iron phosphate cells typically allow discharges of around 80 to 90% of their capacity, while for lead-acid the recommended figure is around 50%. That means two batteries with the same number on the label can deliver very different amounts of energy.

And a fourth: round-trip efficiency, that is, how much of the energy that goes in comes back out. In lithium it usually sits around 90-95%; in lead-acid it is lower. That difference is paid every day for the whole life of the system.

Batteries add real value when what you are after is continuity, not just savings. In those cases the calculation is not about return on investment, but about the cost of having no power.

Think of it this way: if a four-hour outage spoils the contents of a cold room, interrupts milking or leaves medical equipment without power, the value of the battery lies not in the kWh it saves but in the loss it prevents. That framing leads to very different decisions.

Lithium iron phosphate batteries have a longer service life measured in cycles, allow deeper discharges, take up less space, weigh less and require less maintenance. Their up-front cost is higher.

Lead-acid batteries are cheaper to start with, use a very well-known technology and remain reasonable on small installations or for occasional use. In exchange they last fewer cycles, make use of less of their capacity and some variants require periodic maintenance and good ventilation.

For most new installations with daily backup, lithium usually offers a better total cost over the life of the system despite the up-front price. For occasional backup a few hours a year, lead-acid can still make sense. If you want the full technical detail, we work through it in the dedicated comparison.

Storage adds a significant amount to the investment. Batteries also have their own service life, shorter than the panels', so they will have to be replaced at some point during the life of the system.

When assessing batteries, look at the cost per unit of energy genuinely used over their life, not just the purchase price. The sum is simple: multiply the nominal capacity by the usable depth of discharge and by the number of guaranteed cycles; that gives you the total kWh the battery will deliver. Divide the price by that figure and you get the cost per usable kWh, which is comparable across technologies.

That sum often reverses the apparent result: a cheaper battery with a shorter life or a lower usable depth of discharge can work out more expensive over time. Bear in mind too that cycle life is specified down to a residual capacity — usually 70 or 80% of the original — not until the battery stops working.

Temperature matters more than it seems. Sustained heat accelerates the ageing of almost every chemistry, so in hot regions where the battery bank is located — ventilated, in the shade, never under uninsulated metal roofing — has a real effect on how many years it lasts.

A common strategy is to size the batteries only for the priority loads — the ones you genuinely need during an outage — rather than for the whole house or farm. That balances backup against cost.

Make a list of which equipment has to keep running and for how long. For each one, note its power in watts and the hours it should operate during a typical outage. The sum of the power ratings tells you what power the system needs; the sum of the energy (power times hours) tells you what capacity in kWh you need.

Then add two corrections. Divide the energy required by the usable depth of discharge of the chosen technology, because you cannot use the whole nominal capacity. And consider whether the system will be able to recharge during the outage: if the blackout happens at night and lasts until morning, the panels will help; if it is a prolonged cut on cloudy days, they will not.

That exercise, done honestly, almost always shrinks the bank compared with the initial intuition. Very few people need to back up the air conditioning; almost everybody needs to back up the fridge, some lights and communications.

Before investing in batteries it is worth checking whether the problem you want to solve has a cheaper solution.

If what you need is for a few electronic devices not to shut down during brief cuts, a dedicated uninterruptible power supply may be enough and costs a fraction. If you need high-power backup during long, infrequent outages, a generator is still competitive on cost per kW installed, though it demands fuel and maintenance.

Batteries shine when the backup is frequent, silent, automatic and of moderate power. Outside that profile, it is worth comparing. And nothing stops you combining: some installations use batteries for continuous critical loads and a generator for occasional peaks.

These are the slips that most often force an installation with batteries to be redone or extended.