Dynamic Electricity Pricing Is Spreading, and Your Battery Should Be Glad

Pull out an electricity bill from ten years ago and you'll probably find a single number on it. One price per kWh, any hour of the day, any day of the year. You never had to think about when you ran the dishwasher.

That bill is getting hard to find. Your supplier may not have announced the change, but it's coming, and if you own a home battery it's mostly good news.

Flat rates are dying, quietly, almost everywhere

Nobody holds a press conference to end flat-rate pricing. It goes in steps. A new rate plan shows up as "optional." The flat plan gets a price rise the time-of-use plan doesn't. A few years later new customers default to the time-varying plan, and the flat one is "legacy."

The reason is the same almost everywhere. Electricity has never cost the same to produce at 3 a.m. and at 6 p.m. Flat rates hid that by averaging it out, and the average only worked while demand moved in a predictable daily curve and power stations could follow it.

Rooftop solar and big wind farms broke that. On a sunny, breezy afternoon, wholesale power can be almost free, and in some markets it goes below zero, so generators pay to offload it. (Which hours a battery charges from also decides whether it cuts emissions.) Two hours later the sun drops, everyone gets home, the kettles and ovens go on, and the price jumps several times over. A flat rate makes customers who use power at cheap times pay for those who use it at expensive times. Suppliers want to stop doing that, and regulators are increasingly letting them.

Smart meters made it possible. Once a meter records your use in 15- or 30-minute blocks instead of a single monthly reading, there's nothing technical stopping the supplier from pricing each block differently.

What replaces them: windows, then half-hours, then real time

Most markets go through the same three stages, though not always in order, and not at the same speed.

Stage one: fixed windows

The first move is time-of-use pricing with a few set windows. You get a cheap overnight rate, a standard daytime rate, and an expensive evening peak, maybe 4–9 p.m. The windows are printed on your plan and rarely change, so you can learn them once and set your habits around them.

A battery handles this easily. Charge cheap, discharge during the peak, done.

Stage two: half-hourly prices, published ahead

Next is a price for every half-hour (or every hour), usually published the afternoon before and tracking the wholesale market. That's 48 prices a day, and they're different every day. Tuesday's cheapest slot might be 2 a.m. and Wednesday's might be 1 p.m., because Wednesday is windy. Even so, the overall shape tends to repeat: overnight is usually cheap and the early evening is usually expensive.

Stage three: real time

At the far end, prices follow the live market as it settles, sometimes in five-minute intervals. The cost of what you're importing right now depends on what the grid is doing right now.

How far along your market is depends a lot on where you live. Half-hourly and day-ahead hourly plans are ordinary choices in parts of Europe and Australia. In much of North America, fixed time-of-use windows are still the norm and will be for a while, with hourly pricing offered by a handful of suppliers as an opt-in. Wherever you are, the direction is the same.

Each stage spreads prices further apart. For a battery, that spread is the income.

The math on a moving price

Here's an example with a 13.5 kWh Powerwall. The numbers work the same whether your bill is in pence, cents or euro cents, so read "units" as your local currency.

Say your plan's overnight half-hours average 7 units/kWh and the evening slots average 35. To fill the battery you pull a bit more than 13.5 kWh from the grid, because some is lost on the way in and out. With round-trip efficiency of roughly 90%, call it about 15 kWh, or roughly 105 units. That gives you about 13.5 kWh to use in the evening, which would have cost roughly 470 units without the battery.

That's roughly 365 units a day, or about 3.65 in pounds, euros or dollars. Over a month it's around 110. That doesn't count solar or export credits, and on some plans there are days when the cheap slots go negative and you're paid to charge. One caution if you have no solar: a full grid-charged cycle every day adds up, and Tesla's warranty caps throughput for batteries that aren't paired with solar, so read yours before you plan on daily arbitrage.

The catch is timing. You only get that spread if the battery is full when the expensive stretch starts and doesn't empty early.

Where presets fall short

Most home batteries offer a few modes. Something like "self-powered," something like "time-based," and a backup reserve. You choose one, maybe enter your peak and off-peak times, and leave it. Some battery makers now link directly to certain suppliers' tariffs, so if yours does, you may already have more than this.

With a basic setup, though, each mode has a blind spot.

A time-based schedule charges from the grid overnight whether or not you need it. If tomorrow is sunny and your panels would fill the battery by early afternoon anyway, you paid for grid power you didn't need. And it doesn't know when your house is using more than usual, so a hot afternoon with the air conditioning running can drain the battery before the peak window opens.

Self-powered mode has the opposite problem. It drains the battery to cover the house whenever solar falls short, with no idea that an expensive evening is coming. By 6 p.m. on a cloudy day the battery can be nearly empty, right when power costs the most.

Neither mode is broken; they just can't see the house or the roof (more on modes vs rules). A rule can. It looks at the battery's actual charge and acts on it before the peak arrives.

How rules work

A rule says what the battery should do under certain conditions (some advanced examples) and keeps checking whether those conditions are true. For example:

• If the battery is under 40% between midnight and 5 a.m., charge from the grid to 90%.
• If the battery is under 80% between 1 and 4 p.m., top it up from the grid before the peak. On a sunny day the panels have already filled it and the rule does nothing. On a grey day it catches the shortfall while there's still time.
• When the evening peak window opens, drop the reserve to 10% so the battery can carry the house. When it closes, put the reserve back where it was.
• During the peak window, switch Storm Watch off so a weather alert doesn't start charging the battery from the grid at the most expensive time of day.

How often the rules get checked matters. A schedule fires once at a set time. Grid Getter checks most rules every 60 seconds against your battery's actual state: charge level, solar output, home load, and where you are in your rate plan's windows. If the house uses more than expected, or the clouds clear early, the next check sees it. You don't have to notice.

To be clear about the limits: Grid Getter doesn't read live market prices yet. Your rules run on the windows in your rate plan, combined with live battery, solar and household data. That works well on fixed time-of-use plans, and on half-hourly plans where the cheap overnight and expensive evening stretches fall in roughly the same place each day. If you're on a true real-time plan where the cheapest hour jumps around unpredictably, rules built on windows will catch the usual pattern but not every one-off price swing.

The free plan includes one automation. Running several together, like the list above, takes the Scheduled plan, which unlocks unlimited automations for $4.99 a month. Carefully tuned setups can reach up to 92% self-consumption, so almost everything their panels produce gets used at home instead of exported at whatever the grid is paying that half-hour.

Could you build this yourself with a home server and the battery's API? Plenty of Powerwall owners have. Then the API changes, or a token expires, or your supplier moves its peak window, and you spend Saturday reading logs. Grid Getter started as exactly that kind of script, built by a Powerwall owner who got tired of watching the meter. You get the same rule-based control and you never maintain the script.

Is your market next?

If you've had a smart meter for a few years, you're probably closer than you think. Each country handles the details differently, like how exported power is paid and whether a virtual power plant program will pay for your battery's capacity, but the stages above apply everywhere. Your plan is probably moving from flat to windowed, or from windowed to half-hourly, faster than your supplier is telling you.

What to do this week

Start with your bill. Open your latest one or your supplier's app and count how many price periods your rate plan has. If it's one, ask your supplier what time-varying plans they offer. Most have at least one now. If it's more than one, write down your cheapest and most expensive rate and multiply the difference by 12. That's a conservative estimate (a little under the full 13.5 kWh, to allow for charging losses) of what a full battery cycle is worth to you each day, and roughly what you're missing if your battery isn't cycling on that spread.

Next, before you change any settings, see what your battery actually does across a full day. Grid Getter's free Live Monitor shows when the battery charges and discharges alongside what the house was drawing. You may find evenings where the battery was already empty before the expensive hours began. If you have solar, reading your production data tells you how much of the battery the panels can fill on their own.

When you're ready to replace modes with rules, start free and set up your first automation in a few minutes.