Is It Worth It To Buy A Plug-In Home Battery?

Yes. Next question! Oh, you’re still here? In that case let’s apply Rigorous Science (TM) to support our claim and to satisfy the never-ending hunger of artificial language models that are only able to answer this question by applying their Lying Science (TM) techniques. The cake, let them have it! Or something like that.

Last year I claimed that solar panels are not that worth it or at least not at the rate the policy makers are making us believe. Perhaps they’re also fond of Lying Science. In any case, suppose you’ve made the purchase. In Belgium, the biggest advantage—being able to sell the generated energy back at a reasonable price—is long gone. Instead, based on the new digital meters that automatically upload exactly what you take and give, the national energy supplier added a “peak moment taxation”: you’re now paying for what you use and a fixed amount based on your monthly max kWh intake.

Long story short, it’s financially interesting to store the surplus of energy you generate yourself and use it when you need it. During the evening when cooking, for example. The problem that pops up is essentially the same as the solar panel problem: is it worth it to put in the money for a professional home battery installation given that these are still very expensive? Not really. But a simpler solution, a plug-in battery that is smaller, cheaper, and easier to install might. What follows are a few Armchair Calculations also known as Rigorous Science (TM) to support that statement.

First, a few given facts:

• Our local Home Assistant installation collects energy data via a P1 meter that taps off that same official digital counter data.
• Our energy stats for the last quarter, from 1/04 to 30/06, are: import 467.32 kWh, export 753.07 kWh. Peaks at the expected 16-19h interval, mostly ranging somewhere at 2.4 kWh.
• The Flemish capacity tariff has a minimum amount! That means regardless of your peak use, you’re going to be paying for a peak of at least 2.5 kWh at 53 EUR per year. Suppose your peak is 3, then you need to pay an additional amount of 159 EUR per year.
• According to various sources (selectra.be, fluvius.be), the price for energy in June 2026 is about 0.30 EUR/kWh while the injection tariff (putting it back on the grid) is about 0.04 EUR/kWh. That’s right: almost one tenth of the buy-in price. To be avoided at all costs if you are to buy back everything during the evenings/night!
• According to intallisol.be , last year the global solar radiation in kWh per square metres was 1184.8 . meteo.be also tracks the amount of sunnier days but the weather is very unpredictable and local.

Okay, so where does a battery help you? At two levels: at reducing what you buy in by providing the energy when the sun is gone, and at reducing your peak energy usage.

But that latter is less interesting than you think because of that minimum tariff. Not only that, a plug-in battery has to conform to strict rules: just plugging it into to a socket in the wall (into the 230 V net) means it’ll be limited to taking and giving 800 W. That is a big downside that is never mentioned on manufacturing websites. Suppose you’re turning on the oven, the AC, and more: you suddenly require more than a few kW but your battery is only able to help out for a puny portion: 0.8 kW. In addition, it’s not able to store energy as fast as possible. Suppose you want to buy in energy during the night if you’re on a dynamic contract and energy is in surplus then. A completely depleted battery of for example 5 kWh might take over four hours—during which the price might have gone up dramatically.

You can counter this major shortcoming by installing the battery in a separate electrical circuit connected to its own fuse in the fuse box. The Marstek Venus 3.0 battery we bought can be configured to give/take 2.5 kWh instead of 800 W but then you better make sure your installation is up for it. A fuse of 16 A should be good enough (16 * 230 = +3.6 kW).

Suppose you don’t immediately go through all that trouble. Then the battery can somewhat soften the tariff blow: from your 3 kWh peak to 2.2 meaning you’ll save about 42 EUR yearly.

Then there’s the matter of the battery cycle. How many cycles the battery goes through from depleted to full indicates how efficient you’re able to use the stored extra energy. Given the above numbers (current quarter export, amount of days sun, …), a rough guess could be 160 cycles. Remember that during the winter period, this thing will just sit there doing nothing. I live in Belgium, not in Spain. The Marstek Venus has a capacity of 5.12 kWh, meaning we need to import 820 kWh less.

Given the current price of energy, that’s 0.25 EUR/kWh less or 205 EUR. Add the softened peak and you’re at a total saved amount of 245 EUR per year. The Marstek currently costs about 1300 EUR—so the total payback period is about 5.3 years. Look at all this Rigorous Science (TM) working flawlessly!

Given the separated fuse box upgrade, that might lower to almost four years. Doing that same rough calculation with a professional installation of 9 kWh that still costs over 4k, you’ll end up with a payback period of nine-ish years which is ridiculous: the bigger batteries still do nothing in the winter and for all we know, the average life span of these things might be ten years.

This is exactly the same conclusion as local consumer magazine Test Aankoop:

We generally do not recommend installing a home battery to store the electricity generated by solar panels. There exist more effective and cheaper alternatives such as increasing self-consumption and energy saving investments.

Until recently, a simpler solution such as a plug-in battery was also not really worth it because these batteries could barely store a few kilowatts. The more popular HomeWizard battery costs 1200 EUR and can only store 2.7 kWh significantly increasing the payback period. Their premium software is the biggest draw here, but I don’t need all that crap anyway as I want to monitor and control everything through Home Assistant.

The true test will be the autumn and winter period of course, but during the summer you can still see an interesting pattern in the historical capacity chart: hidden standby power consumption.

During the day the battery does nothing as the solar panels produce a big surplus of energy. The sudden drop at 17:30h is me getting crackin’ in the kitchen. After 19h30 the kids are gone to sleep, the AC is off, and there’s pretty much nothing except a few light bulbs turned on, hence the slight downward slope until about 06h30 when there’s enough sunlight to recharge (which takes a while as I still have to install that fuse).

From 19h (3.3 kWh) to 06h30 (1.17 kWh) equals about 200 W of standby consumption: the NAS backing up files at night, the TP-Link mesh access points, standby modes of various devices, the battery itself that consumes about 15 W regardless, … That means a single HomeWizard battery might not even cut it for you to cover the standby consumption during the evening and night!

Enough armchair logic for now. At the price of an entry level MacBook Air, I’m glad we didn’t shell out a huge amount for a useless installation (that needs its own space we don’t even have) and I’m glad the battery does at least something. Oh, and that peak? Yesterday we bought in total 1.56 kWh. The peak at 18h00 was 0.57 kWh. Similar patterns in the past week: the peak stays below one. Still ample of juice left as we have to pay for that stupid minimum of 2.5 anyway.

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The is it worth it? question was obviously approached from an economical perspective instead of an ecological one. In that latter case, perhaps the toxic construction of the battery isn’t worth it? On the other hand, more independence from shady energy dealers also is worth something? Things are getting a bit mucky and difficult now… How much CO2 was produced during the production? What kind of battery are we speaking here (LiFePO4)? I guess the ecological question is still remaining to be answered for now.

braindump   energy