During the colder months, we regularly receive questions such as “My battery is barely working, is it still functioning?”, “Why is my efficiency lower than in summer?” and “Is my Plug-In Battery defective?”. Good to know: lower efficiency in winter is normal. This is mainly because the battery often charges and discharges at low power during this period, and because the available solar power is different from that in summer. The temperature and ventilation of the room in which your home battery is located also play a role.
In this article, we address the most frequently asked questions, explain when your Plug-In Battery is efficient or less efficient, and what you can do if your efficiency remains structurally low.
Efficiency or yield: what does it mean?
When storing electricity, some energy is always lost. That is normal.
Part of the energy you put in is lost to:
The inverter (converting alternating current to direct current);
The electronics and controls.
That is why you do not get back exactly the same number of kWh that you put in.
That's why you don't get back exactly the same number of kWh that you put in. Want to know exactly how this works? Read our article: Energy loss in home batteries, how does it work exactly?
Why does efficiency drop, especially at low power levels?
The Plug-In Battery works most efficiently when it can charge and discharge at higher power levels. In winter, you still benefit from your battery, but because there is less solar power, charging and discharging often takes place at lower power levels. This can result in lower efficiency.
What happens in winter?
In winter, you have less solar power and more often only a small surplus. As a result, the battery charges more often slowly or in small amounts. And when there is little power, the electronics use relatively much, leaving less to actually store.
An example
For example, do you have 10 solar panels on your roof? Then in the summer you regularly generate 1,000 to sometimes as much as 4,000 watts. Exactly how much depends on your inverter and the type of panels you have. If you subtract your household consumption from that, you often have enough left over in the summer to charge your Plug-In Battery.
In winter, there are fewer sunny days. And even on sunny days, your panels generally produce less. As a result, you often end up with less than 1,000 watts. The further your surplus is below 800 watts, the less efficiently the battery will work. For example, if you have 200 watts left over, your battery can still charge, but it will be less efficient than with a surplus of 800 watts.
In short: when there is little power, the system itself uses a relatively large amount of energy. This leaves less power available and reduces efficiency.
Why is the efficiency higher in summer?
In summer, your battery charges and discharges more often and at a higher capacity. This means that the fixed loss is relatively smaller. In addition, the battery charges faster due to the higher solar yield, which means you often need to charge for less time. The result: you have more energy left over and the efficiency is higher.
Why does it sometimes seem like nothing is happening with my battery?
In winter, it is quite normal for your battery to be less active. This is usually due to:
Too little solar energy
Your home uses more (heating, lighting, etc.) and solar panels deliver less.
Small fluctuations are not always corrected immediately
Very small surpluses or shortages change quickly. Continuous adjustment can then be inefficient.
Temperature
The Plug-In Battery operates within 0–40°C and performs best between 10–30°C. At lower temperatures, the battery may (temporarily) charge and discharge less efficiently and appear to drain faster. At higher temperatures, additional cooling is often required, which results in more loss and therefore lower efficiency.
The battery starts charging from 70 watts of feed-in.
When is a Plug-In Battery most efficient?
The battery is generally most efficient when:
It can charge and discharge with high current (i.e., close to maximum power).
It is at a normal indoor temperature (roughly 10–30°C).
It transfers most of the energy in relatively short, powerful charging and discharging moments instead of remaining at low power for a long time. The higher the power and the shorter the charging time, the smaller the fixed loss is in relative terms.
This is because the inverter is relatively most efficient at a higher proportion of its maximum power.
Do I need to store my battery during the winter?
No. For optimal service life and reliable operation, we always recommend leaving your home battery connected.
Why is this important?
Your battery is equipped with a smart Battery Management System (BMS). This system continuously monitors the condition of the cells. As soon as the energy level threatens to become too low, the battery automatically recharges itself a little.
You can compare this to a car battery: if it is left unused and without power for too long, the battery can ‘drain’ and eventually fail to start. By keeping the battery connected, you avoid this risk.
Charging for storage
Are you moving or not going to use the battery for a long time? In that case, we have a handy solution. Select the ‘Charge for storage’ function in the app. The battery will then charge itself to an ideal level of 60%. You can then safely take the battery out of service for several months. This keeps the chemistry in the battery stable and ensures that it is ready for use again when you need it.
At what wattage does the Plug-In Battery start charging/discharging?
The battery charges (and discharges) as soon as there is sufficient surplus (or shortage) to make a meaningful correction, up to a maximum of 800 watts per battery. In practice, you will sometimes see that the battery does not respond immediately to very small surpluses (e.g., a few dozen watts). This is also normal: adjusting for very small differences can be inefficient overall, so charging only starts when there is a surplus of 70 watts in your total power graph.
Can I also charge at cheap times (dynamic contract) instead of just solar energy?
The Plug-In Battery is designed to automatically store solar power for later use. Do you want to charge at cheap times and discharge when electricity is expensive? That is a smart strategy that we at HomeWizard are currently developing. We will of course keep you informed when there is news!
My yield is lower in winter. Is that normal?
Yes, that is normal. In winter, there is less solar power and you often only have a small amount left over. The battery then charges slowly or in small increments. And because the system always uses a little power, when there is little power available, this has a relatively greater impact on your yield. This results in a lower yield. You can follow the steps below for a fair test.
Step 1: Are you really comparing the same situation?
Yield is influenced by, among other things:
Charging/discharging capacity (low vs. high);
Temperature;
Many short steps vs. longer blocks;
Measurement period (start and end level of the battery);
Whether you get your yield from the app or from external measurements.
In practice, 70-80% efficiency may be normal under certain circumstances.
Step 2: Perform a ‘fair’ efficiency test.
Want to compare efficiency properly? Don't look at a single day or week, but at a period in which the battery starts and ends at roughly the same charge level (e.g., a month or an entire season). For example, if you start at 100% and end at 60%, the efficiency will appear lower because some of the energy is still in the battery (and vice versa), so only compare import and export when your starting and ending points are comparable.
Step 3: When is it suspicious?
Contact support if you:
Structurally remain well below 70%, even when charging or discharging your battery with a lot of power (note: with few solar panels or low consumption, a lower efficiency may also be normal);
Experience significantly less usable capacity;
Notice strange dips or strikingly abnormal charging/discharging behavior;
If one battery in a set of several performs very differently from the rest.
We will then work with you to determine whether there is an anomaly or defect.
Step 4: What do we need for a quick diagnosis?
When you contact us, please send us:
Screenshots of import/export and efficiency in the app;
The test period;
The battery's starting and ending percentage;
Temperature and location;
Whether you use one or more batteries.
This will allow us to assess the situation quickly and accurately and help you further.
A frequently heard term that you may already be familiar with is Round Trip Efficiency, or RTE. There is a lot of confusion about this, because different platforms compare RTE with actual efficiency. Want to know more about the difference between efficiency and RTE? We explore this in more depth in this blog: Why RTE is not the same as efficiency.
Still having trouble?
Are you still having trouble or is your return consistently low? Click on the green button at the bottom right to contact us. We are happy to help you.