Fogstar Energy 15kWh: The Best Solar Battery for the UK

Back Nikola Nedoklanov 8 min read November 13, 2023 1560 words
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This article is my own opinion based on my experience using the battery, the research I did and the constraints I solved for in my system.

What is the Best Solar Energy Battery on the UK Market?

The Fogstar Energy 15kWh 48V Rack Battery Bundle , which comes with an amazing server rack for free.

Fogstar Energy 15.3kWh 48V Battery and Rack

Fogstar Energy 15kWh 48V Rack Battery Bundle
Capacity15kWh or (3x 100Ah)
Cycle Life4000 Cycles
Design life15 years
Peak power15kW
Rated voltage51.2V


Why is Fogstar Energy 15kWh 48V Rack Battery Bundle so great? Let’s dive in and find out.

The Right Size Battery for Home Energy Storage

The average electricity consumption of a UK home is around 8kWh a day. I need around 10kWh a day, so the battery I chose needed to cover most of that and output enough power to handle my peak consumption. I estimate this at 4kW.

Plenty of Power Based on Your Needs

The 15kWh bundle is built up from 3 x 5.12kWh server rack batteries. Each of the infividual server rack batteries can output 5.12kW. This is enough to cover the estimated peak consumption.

The three batteries together can go as high as outputting 15kW peak power if your inverter can handle it and the load demands it. This is easily done within the server rack cabinet, which includes a busbar for the of batteries inside it.

Modularity and Gradual Build-up

You can buy the batteries one at a time and buy the server rack separately when you get two or more batteries. In a gradual build-up of storage, you are safe knowing that the same battery you already have in your rack can be bought as an upgrade. This is useful because mixing batteries with different specifications is not advisable.

The Rack

The rack is sturdy and safe; it comes assembled and has wheels for easy manoeuvring into place. It includes a door that can be locked, which is crucial for the safety of pets and little children. There are additional nice touches to it, too, like the busbar is isolated, and only the terminal connectors are bare. Wide openings at the top and bottom with flexible glands let you run your thick battery cables easily and keep them neat.

Make the Most of The Free Rack in the Bundle

I bought the bundle and gave one of the batteries to someone else for a different project. I kept the server rack, which now houses my two batteries.

Designed in the UK

The design of the battery caters for the local climate conditions of the UK. This manifests in details such as a built-in heater. The batteries heat themselves when a temperature of <5°C is detected, enabling charging in temperatures of -20C. The batteries also come with built-in support for the protocols used by the most common inverters. The build quality and overall finish are solid.

Value For Money Battery

The price and quality of Fogstar are unmatched. There is plenty of competition, and prices keep going down on various competitors. However, before comparing what you pay, think about what you get. Considering all the benefits outlined in this article and the price still being reasonable, I regard this as a value-for-money battery.

Support is Local to the UK

Fogstar is headquartered in the UK, and their support and design are there too. I turned to them twice, first asking for some advice in the early stages and later with a more concrete question, and in both cases, my experience has been positive.

Support was an important factor in my decision-making on batteries. They are simply too expensive to risk being out of support or warranty.


I read some of Fogstar’s blog posts on LiFeP04 batteries and wanted to find out more from other people. Naturally, I turned to the online community to see what people have to say about Fogstar as a company and their home energy storage products in particular.

The Insights

I found Fogstar batteries have strong positive reviews on the social media solar energy groups I went to for research. These groups contain tens of thousands of users and lively discussions on relevant topics.

People there are knowledgeable, some of them are professionals others are hobbyists, but they go into serious levels of detail, and share valuable first-hand experience and knowledge. The consensus was loud praise for Fogstar’s product, and it seems that Fogstar is the recommended brand to people asking for advice on what to buy.

Disadvantages of Fogstar Energy 15kWh Server Rack Battery?

There are several points about this battery that you ought to know when considering if it is the right one for you.


At £2999.99 for 15kWh, this battery seems to be falling behind on value for money compared to Fogstar’s Seplos Mason battery at the current price of £2,532.86.

Both batteries are modular and can be connected in parallel to add extra capacity. Both come in a neat box or server rack. The big difference is that the Seplos Mason is a DIY kit. This will probably discourage some less experienced DIY-ers.

The Seplos Mason kit requires assembling all the cells, the and the busbars that connect them. It is not a simple job. On the other hand, the 15kWh server rack battery requires a relatively lower amount of work, even if you do DIY.

Battery Balancing

In the late autumn months, I discovered that when the energy generation is dropping and the battery isn’t charging to 100% State of Charge (SOC), the two batteries’ SOC starts deviating.

To mitigate this problem and make the most of my battery investment, I switched to a dual-rate tariff, and now I charge my battery at night. Thus giving them enough energy to reach the 100% SOC and perform top balancing.

If a favourable tariff is not accessible for you, then a modular battery like the Fogstar Energy 15kWh 48V Rack Battery Bundle may not be the best for you in the winter.

Update November 2023

I contacted Fogstar’s customer service about this and they advised to install an update to the BMS software. After installing the update the SOC of the two batteries has been staying much closer together. Here’s what Fogstar’s customer service said abou the update:

The racks are regularly rebalanced and rematched at 100%. Over winter with a lack of solar, we do get more customer inquiries as per balancing issues - and it seems the BMS gets a little lost when it's regularly floating within the 20%-99% range. I've witnessed this over several BMS JK, PACE, JBD, etc, so it seems we are not alone in this and it could just be a limitation of current-day BMS units and parallel connections. Of course, this is only a BMS reporting error, and as long as the voltages are the same - the batteries are in balance, but it's nice to see the same or similar SOC between them.
—Ben, Fogstar Customer Service

Why Invest in a Battery for a Solar Energy System?

What does the non-insignificant cost of a battery bring to the table? Not only does a battery keep your home supplied with energy during dark hours, but it also supplements your solar power during peak demand during the day. The second part is seriously underappreciated, but it takes care of the intermittent nature of solar production and even reduces the size of solar panels capability required for your system. This is a real game changer in a solar energy system. Even when solar energy is not present, or not enough, batteries can still help when coupled with a .

With the added efficiency batteries bring to your home energy system, they can pay for themselves, save you money or even potentially earn you a small income.

Diagram of monthly energy purchase from the grid for June 2023. It shows higher amout of purchased energy for the days before June 5th. The days after that have significantly lower consumption. Blue bars on white background.
My June 2023 electricity purchased from the grid. I installed the battery on June 5th. Before that I only had solar panels.


Batteries are likely the single most expensive item in your solar energy system. Choosing the right battery early in your project is crucial for the long-term financial benefit and enjoyment you will get out of owning your system. The modularity of the solution and the reasonable cost make Fogstar’s batteries front-runners in any comparison. What sealed the number one spot for me is the local support, the design features and the community of owners helping each other make the most out of these batteries.

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