Reading the Tesla app's energy graphs
The Tesla app shows you a great deal about your home's energy, but it takes a little practice to read. Once you know what each flow means, why the totals never quite balance, and what a good day looks like on your tariff, the shape of a day tells you most of what you need to know.
The four flows and their signs
Everything in the app is built from four measurements taken at the gateway.
- Solar is what the panels made. It is never negative.
- Home is what the house used. Also never negative.
- Battery runs both ways. Charging takes energy in; discharging gives it out. The app draws the two in different directions and shows both totals for the day.
- Grid also runs both ways. Import is energy bought; export is energy sold. Again, two totals.
They are tied together by one identity. Everything coming in must equal everything going out:
solar + battery out + grid import = home + battery in + grid export
Every odd-looking day is a case of one of those six numbers being larger or smaller than you expected, and the identity tells you where the difference went.
Why the numbers never add up exactly
Try the identity on a real day and it will be out by a little. That is normal, for several reasons.
- Losses. Energy is lost turning DC into AC and back, and inside the cells. Across a day the battery gives out less than it took in, and on an AC-coupled Powerwall 2 the solar inverter has its own losses before the gateway sees anything.
- The equipment's own draw. The gateway, the battery's electronics and its thermal management use power around the clock. Depending on where the sensors sit, this can appear as home use, disappear into the gap, or both.
- Rounding and sampling. Readings are taken at intervals and shown to a tenth of a kWh. Fast changes between samples are smoothed over.
A gap of a few percent is nothing to worry about. A gap that grows suddenly is worth a look; it is usually a sensor that has been moved or a circuit added outside the gateway's view. Round-trip efficiency: where the missing ten percent goes covers the loss side.
What "self-powered" means, and what it does not
The self-powered percentage is the share of your home's use that came from solar or the battery rather than the grid. A day with 20 kWh of home use and 4 kWh of grid import scores 80 %. It is a measure of independence from the grid, and as that it is honest.
It is not a measure of money, and it is easy to chase it in the wrong direction. On a cheap-window tariff the cheapest day is one where the battery fills from the grid at night, which counts as import and lowers the score. On a Flux-style tariff the best day sells the battery into the evening peak and buys back cheaply later, and scores worse than a day that hoarded everything. In winter the score is low whatever you do, because there is no solar to be self-powered with. Treat it as one view of the day, not as the target.
The live view and the daily bars
The live view shows power: how many kW are flowing along each line right now. The daily view shows energy: how many kWh flowed over the whole day. A kW is a rate; a kWh is a quantity. A 2 kW draw for half an hour is 1 kWh. The live view is for checking what is happening this minute; the day chart and the bars are for judging whether the day went well.
The day chart stacks the flows over time, so you can see when the battery charged, when it ran the house, and when the grid stepped in. The week, month and year views turn each day into a bar, which is the quickest way to spot a day that behaved differently from its neighbours.
Spotting a problem in the shape of a day
Some shapes are worth recognising.
The battery is flat by early evening
The battery ran the house from sunset and reached its reserve before the expensive hours. Either it did not have enough to begin with, or nothing told it to save some for later. In Self-Powered mode this is expected behaviour. In Time-Based Control it usually means the tariff on the Powerwall does not show the evening as expensive. How Powerwall time-based control works explains the fix.
Import at midday
The sun is up, the battery is not full, and the grid is still supplying the house. Common causes: a burst of home use above what the panels and the battery can supply together, the battery already at its charge-rate limit, or the reserve having been raised above the current charge, which makes the Powerwall top up from the grid.
Export while the battery is empty
Solar is going out to the grid while the battery sits at its reserve. In Time-Based Control this can be deliberate if the Powerwall's tariff shows a high sell price now and cheap buying later. If that is not what you intended, the tariff on the device is telling it the wrong story.
A flat-topped solar curve
Solar rises, hits a ceiling and runs along it for hours. That is an export limit clipping generation, and it is covered in The 3.68 kW export limit.
A cheap-window day and a Flux day
The same house on two tariffs produces two quite different charts, and both are correct.
| Time | Cheap-window tariff (Go-style) | Peak-export tariff (Flux-style) |
|---|---|---|
| Night | A block of grid import as the battery fills | A block of grid import, sized to leave room for solar |
| Morning | Battery runs the house; small solar | Battery runs the house; solar tops it up |
| Midday | Solar covers the house, surplus exported | Solar fills the battery, surplus exported |
| 16:00–19:00 | Battery runs the house, little export | A tall export spike as the battery sells |
| Late evening | Battery runs the house to the reserve | Small import or the last of the battery |
The Go-style day is judged by how little day-rate import it shows. The Flux day is judged by how tall and well-timed the evening spike is. Neither is judged by the self-powered score.
Beyond the graphs
Why the app's kWh differ from the meter
The Tesla app measures at the gateway's own sensors. Your supplier bills from the smart meter. They are different instruments in different places, and they disagree by a little for ordinary reasons: sensor accuracy of a percent or two, the gateway's own draw sitting on one side of the meter and not the other, circuits that bypass the gateway, and the day boundary, which the app draws at local midnight while the meter records in half-hours. The meter is the bill. The app is a very good guide to what happened and why, but it is not what you are charged on.
What a money view adds
A kWh chart treats every kWh as equal, and on a smart tariff they are not. A kWh imported at 3 am on a cheap-window tariff might cost a quarter of one imported at 6 pm. A kWh exported at the Flux peak earns several times one exported at noon. The energy graphs cannot show that difference; only a view that prices each half-hour at its real rate can. That is where spent, earned and net figures come in, and where a poor-looking energy day can turn out to have been a good one. How much does a home battery actually save in the UK? takes that further.
In SunHarvest
Home shows a live flow diagram with the same four flows, updating every 30 seconds on Pro and trial, and the Energy page draws each day with that day's solar forecast dashed alongside so you can see how far the forecast was out. Every five-minute bucket is priced at your real tariff, so the day also shows Spent, Earned and Net in money, with Saved on Pro comparing the day against the same household with no solar and no battery. When a day looks wrong, the change log shows who changed each setting and when, whether that was you, a rule, the AI planner or the Tesla app, which is usually the quickest way to explain an odd shape.
General information, not financial or electrical advice. Tariff terms, prices, warranties and connection rules change and differ by supplier, region and installation; check the current documents for yours. SunHarvest is not affiliated with Tesla or Octopus Energy.