Last Updated 1 year by Lukas
I’d like to share my experience of owning a passive house. I’ll also describe what I would have done differently if I had known what I know now. Together we will explore what are reasonable expectations of a passive house. How to avoid the pitfalls I’ve fallen into, and how to get the most enjoyment and benefit from your house. As in the previous article, which described systems I had personal experience with, we will go system by system and evaluate them. This article is based on almost three years of using, learning and fine-tuning the passive house.
What are the benefits and drawbacks of systems used inside passive houses?
Hybrid Solar system with battery, water heating and island mode.
- System parameters
- Peak power 5,4 kWp (On Single string)
- Inverter RCT Power Storage 6 kW
- Battery LiFePo4 11,5 kWh (Usable is 90% of capacity)
- Heating into water through SSR relays and 6 kW three phase heating rod (Continous routing)
- PLC that controls whole system (island mode, current/voltage monitoring per phase, water heating)
- Advantages
- RCT as a company makes both the inverter and the battery pack. They are well connected.
- RCT inverter has two string inputs and can try to optimise power from them itself (works similar to installing an optimiser on the panel itself).
- RCT inverters can link phases together and provide a single 6kW output. This is good if you want to back up larger machines in case of islanding.
- RCT has passive cooling and it’s very quiet.
- Heating in water gives you ability to cheaply store extra energy system produces and because routing is done through 3 solid state relays, you can route even small fraction of power.
- PLC gives you ability to control system with higher level of freedom and integrate it together with other home systems.
- The whole system monitors power consumption and routing. You can get very nice overview and plan changes/improvements based on that.
- Disadvantages
- RCT is a single company and because you are using a complete system from them (inverter + battery). It is difficult to change, there is a limited number of people who can service it. There is also higher price associated with components and currently the maximum supported size of battery is 11.5 kWh.
- If you use a bespoke PLC, you are tied to the service company. This makes changes very difficult and expensive. Simply put, you limit your options unless you are prepared to pay for a complete redesign.
- As a passive inverter, switching to island mode takes about 30 – 60 seconds. You can be sure that all your equipment will go down. You will have to invest in a UPS to protect sensitive electronics (routers, PCs, …).
- An RCT with a maximum power of 6kW can only provide 2kW per phase, so you need to think about what you want to back up. Please also note that when appliances are started after a power failure, they have a temporarily higher power consumption. This may force your RCT to shut down and restart in island mode (when the grid is present, it draws more power from the grid).
- You will need to fine-tune the basic settings of the RCT as the primary market for it is Germany. This is only necessary if you want to use it in the most efficient way. It can work out of the box with default settings. It’s just not as effective as it could be.
- Changes / Improvements made to the system over time
- First system upgraded to 7 kWp on a single string (end of 2021). Later upgraded to 2 x string with 10 kWp total power (end of 2022). In the future I plan (if possible) to increase to maximum of inverter around 14 kWp
- RCT Power Storage 6 kW changed to RCT Power Storage 10 kW
- Fine-tune of internal settings (based on RCT manual / consultation with support)
- Use battery in interval of SOC 7% to 97%. In other words 90% of capacity
- Change battery charging strategy from ‘Internal’ to ‘Constant
- Internal strategy can discharge battery to grid according to weather forecast (not very effective in Czech Republic)
- Request to increase the maximum allowed export power into the grid
- You have to go through the process with CEZ, it’s like brand new connection of micro-energy source
- Please be prepared you will need official documentation and approved stamps to pass through the process
- Change default settings of battery discharging into grid and consumers to 9 000 W
- Please check RCT documentation , maximum is based on number of battery cells
- Add random time before retrying to start into island mode and maximum number of attempts
- So that it doesn’t start again and again after a short time if the island mode start fails.
- Upgrade FW within inverter and battery management system after consultation and reporting of problems to RCT support.
- Make sure you only upgrade the inverter when there is enough power from the sun!
- Install multiple UPS devices to protect sensitive HW (router, PC) due to RCT switching time
Given my experience with the solar system so far, I’d definitely buy it again. However, if I were to design/buy a similar system in the future, I’d do it another way. I will choose a system that is widely used and tested, where there is a large community around it. It will be a system with known and proven designs to store energy in battery or water. I’ll avoid any proprietary and bespoke systems that would lock me in or create barriers to change. I’ll also take a step back and assess how such systems fit into the overall design of the house and other systems installed there. This will help me avoid surprises and long evenings spent fixing and fine-tuning. It’s also good to assess how fast the inverter is when it switches to island mode. If the time is in seconds, it’s probably too slow and your appliances will restart.