Where Does My Solar Electricity Actually Go?

Panels on the roof, an app on the phone, and a screen full of numbers that don't always seem to agree with each other. So where does the electricity from your solar panels actually end up?

Solar panels, an inverter, a home and the grid: a conceptual illustration, not a wiring diagram. Export depends on the system; conversion involves losses. Battery storage is not shown.

The short answer: it has three possible destinations, with some lost along the way. The things running in your house, a battery if you have one, or the grid. Which one gets what, and when, depends on what the sun is doing, what the house is doing at that moment, and how your system is set up.

The longer answer is more useful, because it's also the key to reading your monitoring app without tearing your hair out. So put the kettle on (it'll come in handy later) and let's follow the flow.

From the roof to the plug

Solar panels make direct current (DC) electricity. Your home runs on alternating current (AC), so an inverter, the box that converts one into the other, turns the panels' output into something the house can use. SEAI's page on electricity from solar covers the basics, and its homeowner's guide to solar PV goes into more detail.

From there, solar electricity can go three ways.

Into your appliances. Whatever is switched on at the time can use it directly: the fridge, the router, the washing machine, the kettle.

Into a battery. If you have home storage, surplus solar can be held for later, say for the evening after the sun has gone down. Batteries can be connected into a solar system in different ways, so exactly where the DC-to-AC conversion happens varies from one set-up to the next. The job is the same either way: store it now, use it later.

Out to the grid. If the panels are producing more than the house is using, and there's no battery taking it, the surplus can flow out through your connection to the electricity network, where the system is set up to export. That's exporting.

Possible solar routes depend on equipment and settings, not a fixed priority. Conversion occurs where needed, with losses in conversion and storage. A battery can later supply appliances; the grid can supply the home and, in systems configured for it, charge the battery. This is not a wiring diagram or connection approval.

You don't need a battery for the first and third of those. Without one, the house uses what it can at the time, and surplus can go out to the grid.

It's not always "house first"

Solar is sometimes described as a neat pecking order: the house gets first dibs, the battery gets whatever's left, and the grid gets the scraps. It's a handy picture, but it isn't a law of nature. It's closer to a setting.

Some systems with a battery let you choose how they behave. One setting might favour running the house, another might favour filling the battery at certain times of day, and some can charge the battery from the grid too. Some options only work with extra equipment, such as a meter at the grid connection, and not every system offers every option, so check your own system's documentation rather than assuming.

Timing matters just as much. Solar panels only generate in daylight, and what your house needs changes from minute to minute. As an illustration, boil that kettle at midday and the house may take more of what the panels are making for a few minutes. Once it clicks off, the same output might head into the battery or out to the grid instead.

So the honest answer to "where did my solar go?" starts with another question: when?

Batteries: not all of it is sunshine

Here's a bit that's easy to miss. A battery's charge doesn't come with a label saying where it came from.

SEAI's guide notes that batteries can store surplus solar for later, and that some can also be charged from the grid. If yours is set up to do that, the "battery charged" figure in your app may be a mix of solar and grid electricity. The same goes for what comes back out. A big battery total on a dull day is not, by itself, proof that the sun did all the work.

There's also the question of what was already in the battery when the day began. Energy stored yesterday might be used this morning, which makes a single day's battery figures harder to read on their own.

And some energy is lost along the way. SEAI points out that charging and discharging a battery involves losses, so you'll get back somewhat less than you put in. How much depends on the equipment.

One more thing worth knowing: the battery percentage on your screen isn't a measure of energy on its own. Fifty percent of what, exactly? That depends on the battery's capacity. Some datasheets list a usable capacity that's lower than the headline figure; others give the same number for both. The percentage shows how full the system reports the battery to be, but it isn't a kWh reading, or a guide on its own to how long the battery will keep the house going.

If you're curious about batteries more generally, we looked at the idea of running one without panels in Home Batteries Without Solar, Tell Me More.

Watts and kilowatt-hours: speed and distance

Solar apps can show two kinds of number, and mixing them up is an easy way to get confused.

Watts (W) and kilowatts (kW) are power. They tell you how fast electricity is being made or used right now. Think of it as the speedometer.

Watt-hours (Wh) and kilowatt-hours (kWh) are energy. They tell you how much has been made or used over a period of time. That's the distance on the clock, and kWh is the unit on your electricity bill.

The link between them is time. Something drawing a steady 1 kW for one hour uses 1 kWh. That's just the arithmetic of the units, not a measurement of any real appliance.

A steady 1 kW for one hour uses 1 kWh. Illustrative arithmetic, not measured household or appliance data. A single power reading cannot tell you the day's energy total.

Real homes are never that steady, of course. Usage jumps up and down all day, and solar output rises and falls with every passing cloud. Some apps and dashboards read a running energy total kept by the device itself; others estimate kWh from power readings taken at intervals, and an estimate like that can be thrown off by missing readings. Home Assistant's energy FAQ is a good plain explanation of the difference between power and energy readings, whatever app you use.

Say, purely for illustration, an app shows the panels producing 2.4 kW. That's a snapshot, not a reading from any real home. It tells you nothing about how much they've made today. For that, look for the kWh figure.

What your meter and apps can actually see

Every reading comes from somewhere, and where it's measured decides what it can see.

The smart meter sees the grid connection. According to ESB Networks, smart meters record electricity imported from and exported to the grid, including half-hourly readings. What the meter can't see is the solar you used directly in the house, because it never crossed the connection. So "imported from the grid" and "used by the house" are two different numbers, and with solar or a battery they can be very different.

The inverter and battery see their own corner. Inverter and battery monitoring reports what passes through that equipment. Depending on the system, the app may also have a sensor at the grid connection, which is how it can show import and export alongside solar.

"Home use" may be worked out, not measured. Some apps show a household consumption figure. Sometimes that comes from its own measurement. Sometimes it's calculated from the other readings. It's worth knowing which, because a calculated figure is only as good as the numbers behind it.

Two apps aren't always two opinions. If you have more than one app or dashboard, they may be showing the same underlying readings in different clothes. When they agree, that might just mean they're reading the same sensor, not that two separate measurements have confirmed each other.

If you pull everything together in a platform such as Home Assistant, its documentation shows how solar, battery and grid data can come from different places, such as a manufacturer's integration or a separate sensor. One dashboard for everything is very handy. It doesn't make the readings underneath any more accurate.

A checklist for reading your solar app

Before drawing conclusions from your screen, run through a few questions.

Is it power or energy? W or kW means right now. Wh or kWh means over a period.

Over what period? Today, this hour, this half-hour, or since installation? If you're comparing two apps, check they use the same day, the same time zone and the same idea of midnight, especially around the clock changes.

Where is it measured? At the panels, the inverter, the battery or the grid connection? Each one sees a different part of the picture.

Measured or calculated? Is household use measured directly, or worked out from the other figures?

Is battery charging shown separately from discharging? And can the app tell solar charging from grid charging, or does it lump them together?

Same source or separate? Are your apps reading independent meters, or the same data passed along?

Any gaps or resets? Running totals can reset, readings can go missing, and uploads can arrive late. Any of those can make a day look odd.

None of this involves opening up equipment or adding sensors. It's about understanding what's already on the screen, and your equipment's own documentation is the best place to check what each figure means.

When the numbers don't add up

If you add up solar, battery and grid figures, you may find they don't quite match what the app says the house used. A mismatch on its own can't tell you whether energy was lost or something is faulty.

Some of the gap can be real. Energy is lost converting electricity and storing it in a battery, and the equipment itself may use a little power to run.

But a mismatch can also be about the numbers, not the electricity. The readings might be taken at different points in the system, cover slightly different time periods, or have missing or delayed data. A counter might have reset. A figure might be an estimate. Something might be counted twice, for example battery output that's already included in the inverter's total and then added again on its own.

And if the app calculates household use from the other readings, it will tend to balance neatly by design. That tells you the sums were done, not that the measurements are right.

There's no universal "acceptable" mismatch that applies to every system, so be wary of any rule of thumb that claims otherwise. If a gap stays large or keeps growing, that's a good reason to look into the equipment's documentation or ask whoever installed it.

The Irish picture

Solar panels only produce electricity in daylight, and SEAI points out that in Ireland a large share of a year's generation comes in the summer months. So the balance between the house, the battery and the grid can look very different in July than it does in December.

That's another reason not to read too much into a few days of figures, in either direction.

Your smart meter's half-hourly import and export readings are a useful reference point, as long as you remember what they can and can't see. Payments for exported electricity, tariffs and grants are separate stories with their own rules. We covered the battery side of that in SEAI's €600 Battery Grant.

So, where did it go?

Into your appliances, into a battery if you have one, or out to the grid, with some lost in conversion and storage. It can be a mix of whichever routes your system has, shifting through the day and through the year.

Your app can tell you a lot about that mix, as long as you know what it's measuring. Next time a number looks strange, ask three things: is it power or energy, where was it measured, and over what period?

Get comfortable with those, and the screen full of numbers starts to look a lot less like a puzzle and more like a picture of your home.

Note: At the time of writing, Techknow Ireland has no commercial relationship with any manufacturer or provider of solar, battery, inverter or energy monitoring products or services.

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