A Solar Battery’s Secret Lever: The Minimum Reserve Level

The minimum level of charge a battery will keep up its sleeve can be customised to best suit your priorities – whether it’s maximising energy trading profits, blackout protection or reducing degradation. You can even tweak it temporarily in response to unusual conditions – as I found during a power price surge earlier this week.

A Price Spike That Drained My Battery

I had a Sungrow SBR home battery (pictured above) installed a couple of months ago and joined Amber, to see what life on the wholesale price rollercoaster was like.

It had been a pretty uneventful ride until last Sunday night, when wholesale electricity prices skyrocketed in part due to a severe wind drought – an event that prompted Pauline Hanson to falsely claim that us South Australians were paying $5 just to boil a kettle (despite almost all non-Amber households being shielded from price spikes behind flat retail rates).

Amber’s AI-driven energy management system, which is still getting to know my home’s consumption patterns, got a bit excited about cashing in on the very attractive prices, and ramped up exports of our battery’s stored energy for profit.

I was happy enough with this – until realising that it had left our battery at just over 25% capacity ahead of a cold evening. Suddenly it looked like I was going to have to buy back the energy I’d just sold, with the prices still sky-high.

The June 21 price hike, as shown in the Amber app.

Time For A Fireside Chat

My partner raised a skeptical eyebrow when I intercepted her as she was on the way to turn the ducted heating on, and I suggested out of nowhere that it was a great night to get our little-used fireplace going.

She wasn’t buying it, and I had to begrudgingly admit that after forking out thousands on our new energy storage system, we’d better both put an extra jumper on rather than turn on the ducted reverse cycle.

But then I had another thought: why not instead lower our battery’s minimum reserve level so it uses more of its stored energy than usual to get us through the evening with the heater in full swing, without supercharging our energy bill by drawing from the grid?

A Battery’s Minimum Reserve Level

A home battery generally has a programmable minimum reserve level that the system doesn’t drop below, ensuring homeowners will be able to keep the lights on and the fridge humming in case of a blackout.

This is usually set between 10% and 30% of your battery capacity. Setting aside at least 2 kWh is advisable, but what percentage level you choose should really depend on the size of your system, what your priorities are, and what kind of backup features your system boasts.

The higher you go, the longer your payback period will be, as some of your battery capacity is going to sit unused most of the time. But likewise, the lifespan of your battery is extended as there’s less battery degradation.

Go for a lower reserve however, and you’ll put the battery to work more often and enjoy better financial returns – less drawing from the grid, more exporting for a profit.

Modern battery chemistry will reliably work much harder than legacy lead acid for instance, but you will also put more strain on the battery and risk degradation, especially if you routinely cycle below 20%.

Allowing charge levels to drop very low in exceptionally cold weather is also bad for the battery, although this applies more in freezing conditions we don’t see often in Australia.

You will obviously be more vulnerable to running out of power during a blackout, which might be more of a going concern if you’re in a fringe of grid area that experiences them more frequently.

Manufacturers often specify recommended level to set your battery at, as well as maximum depth of discharge (DoD), which can in turn impact warranties, so consider that before mucking around with your minimum reserve level.

Sungrow set a floor on their battery so you can never use the last 5%, whereas other makers allow 0%, but immediately force a grid charge to recover a safe level.

How Do You Change A Battery’s Minimum Reserve Level?

The answer depends very much on your brand of battery, but a good installer will ask you what your preferred level is during the install and do it for you – and might even take you through how to adjust it yourself.

In my case, in typical Sungrow style I had to navigate a slightly convoluted adventure through obscurely-named options buried in menus and submenus of the brand’s iSolarCloud app (to be fair, many other brands subject users to similarly Byzantine journeys).

The menu path for me in the app was:

• Hit the small hexagon symbol on the right of the home screen and choose the ‘device’ option;
• Select ‘energy storage system’ (the inverter);
• Swipe right through several menus to get to ‘settings’, then select ‘general settings’;
• Find and select ‘backup mode’ and enable it;
• This will make the ‘reserved battery SOC for off-grid (%)’ field appear (as shown below);
• Enter the percentage number preferred for the minimum reserve level at (the ‘read-back value’ below this number shows what it is currently set at);
• Then hit ‘apply settings’ and ‘confirm’.

All about as intuitive as trying to locate a needle in a haystack in a hurricane, but once you know the steps, it is straightforward enough.

As I’m with Amber, I had to use the Amber app to additionally select ‘devices’, ‘SmartShift settings’, and then enter the new minimum reserve level percentage in the appropriate field, so Amber’s AI could know where I had set it to (it won’t sell to the grid below 25% anyway unless you order Amber to).

Amber actually has a pretty good rundown on how to tweak the minimum reserve level on other brands, from Sigenergy to Fox ESS to Anker.

You Might Only Do This Once

In most cases, this function is something to typically set and forget when the battery is first installed, but it does give options when you’re facing unusual circumstances. With the battery under more strain than usual in winter conditions and energy prices up, this presented the opportunity for us to stay warm through the night without paying through the nose by having to draw from the grid.

So I adjusted our settings from the 20% level we usually run at down to 10%, and restored the old setting once things were back to normal.

There’s risk and reward here – times of high energy prices reflect a grid under strain, so you there’s potential for a higher-than-usual chance of a blackout.

But with energy prices exorbitant enough that I was inclined not to use much electricity anyway, I figured we might as well.

There’s other circumstances you might want to temporarily change your minimum reserve level – perhaps wild weather is forecast and you want to set it higher to see you through a prospective blackout, or a letter in your letterbox has specifically warned ahead of time of a scheduled power cut.

In our case, by reducing our minimum reserve level temporarily, we were able to use our heating without racking up a great big bill during a price spike – and our fireplace has still not gotten a call up.

For more on home energy systems, read our guide to solar batteries.