Flat By Tea Time: Why A Small Battery Is A Big Problem

Having noticed a few different hardware retailers and grocery shops making moves into the home battery market, it seems prudent to point out the pitfalls of cheap small systems.

Recently we detailed why bigger isn’t always better, but this time around I’d like to explain how we arrive at just right.

If your hip pocket nerve is the most reactive of your senses, I’d suggest taking the fingers you use to count your savings and slamming them in the car door. The pain will be a great way to focus attention, and you’ll realise paying for a replacement battery is difficult when you’re smarting from a self-inflicted injury.

Now I appreciate that might sound a little extreme but it’s a necessary evil, genuinely proportional to the problem.

You see in the SolarQuotes office we often field phone enquiries from people who have multiple quotes in hand. They ask advice on what is the best solution and you can hear the pleading in their voice, surely the cheap quote for a small 5kWh battery can’t be that bad?

Small Batteries Suit Very Small Users

The most modest bachelor I ever met, with a gas hot water service and a habit of watching TV under a blanket without the lights turned on, still used about 8 kWh per day on average. While solar would have knocked that consumption down considerably, a 5kWh battery would barely run his fridge and freezer.

I’m not saying it wouldn’t help out, cycled twice a day using a cheap off-peak retail plan, it would work hard and make a decent return on investment, I expect.

However the average Australian house uses double or triple that amount of energy, so there’s every chance a 5kWh battery would be flat by tea time, before the telly even goes on.

Little Power And Few Hours

A 5kW inverter and oversized 50kWh battery is a poor choice. A 5kW inverter and undersized 5kWh battery is arguably worse.

If there’s a grid outage, your inverter needs a decent reserve of energy on tap to meet peak loads like starting the fridge. Many have exceptions and can work with a minimum 6kWh, but doesn’t have advertised surge capacity; unless you have a larger battery of say 9 or 12kWh stacked behind them.

Sadly most batteries don’t specify a “C-rate” which explains the speed at which you can get power into or out of them, however some are honest enough to give you a simpler explanation. Tesla Powerwall 3 for instance can charge at 5kW and discharge at 10kW.

While Powerwall 3 has a very useful 13½kWh sized tank, a cheap 5kWh solution from the centre aisle will likely be a big disappointment. You would have to be an exceptionally modest energy user to cook your tea, run some heating or cooling through the night and then manage the morning coffee without importing expensive electricity.

Still, running a midnight recharge scenario might work, if the cheap hardware can automate it. To be honest the network providers and energy retailers would probably find that suits them the best.

This beautifully executed Fronius shows battery stacks can grow tall, so it’s best for the inverter to go beside them. Image credit Jae Taylor

Bottom Line Is Careful Design

People qualified to design and install batteries have been trained to size them correctly, not just shop for perceived bargains.

It’s why the industry would like to see sales companies ruled out unless they have trained staff. And it’s also why sometimes the people installing on the day find a fatal technical flaw in the sales process is hidden behind the switchboard, where only electricians can legally poke their fingers.

There’s a lot more to a solar power system and a home battery than just the invoiced cost. If you get the design wrong, not only will it be a waste of money, it’ll be a waste of time installing it.

More importantly it will be a waste of your Cheaper Home Battery Program rebate, which a household only gets to claim once.

Even if you started out with a small modular battery that can be upgraded to a larger capacity in future, that upgrade would miss out on the rebate.

Cheaper Doesn’t Mean Cheapest

The federal government has introduced the rebate to make batteries cheaper, not cheapest.

The guidance you need to follow isn’t so much the brand name of the scheme, but what it says on the back of the tin. The feds are offering to pay for about one third of your battery, so for every 1kWh of storage capacity you buy, they’ll kick in about $350.

Think of it like buying a car, from a recognised dealer who obviously has a service department and offers warranty support from a recognised international company. If you have a list price for a new vehicle and the government is offering genuine 30% discount, that’s a great deal.

If you buy a cheap battery, and it dies an early death, that’s poor value for my taxpayer dollar. It’s worth asking if you can afford to burn your initial investment; can you afford to spend twice as much again for a decent quality replacement?

Early Adopters Are A Cautionary Tale

I’ve had first hand experience with people at the leading edge of technology adoption, those who will spend irrational sums, because they just want the latest thing.

These energy enthusiasts were a pain in the arse from a retail solar perspective, but they did provide a useful learning base for all concerned: mainly that battery owners are demanding customers that make the phone ring.

They got first-of-type installs with small capacity and very modest returns because of the high cost, have been beset by technical issues, and in some cases have upgraded several times over.

This early adopter started with a small Victron system, changed to Sungrow then expanded his 48V BYD battery stack with two more modules. It desperately needs a fan-forced enclosure so the door can go back on.

Winter Means Off-Grid Needs To Be Even Bigger

Whether you have “enough” solar or not, there will be some days or weeks across winter when there simply aren’t enough sun hours to give you full self sufficiency.

It’s why an off-grid solar and battery system needs to be four times bigger and more capable than a hybrid, and will always realistically have a backup generator, but that should be explained in the design too.

Luckily for the millions of Australians who remain on grid, mains power provides a cheaper, quieter, more reliable way to recharge your battery. As commercial-scale wind power becomes our mainstay, along with bulk solar energy, the cheapest power ever generated is now available at midday. So even in winter, a decent-sized battery offers value by charging from the grid.

If you have a small roof and a large appetite, retail deals offering low or zero-cost happy hour rates mean you can charge a battery for next to nothing and save yourself from being violated by peak pricing after 3pm.

However you need an inverter big enough to suck down enough power to fill your battery inside the limited time window. 

So How Big A Battery Do You Need?

What sized battery is just right depends on a host of factors, from how much electricity you use, to when you use it, to what energy plan you’re on, to how much solar you have. The answer is different for every household: use our add battery calculator to find out, or if you are looking to get solar as well, then our solar and battery calculator should do the trick.