Back in 2020 I wrote a post about Simulating a Tesla Powerwall. Much has changed since then. Electricity prices went up and down(!) Energy providers started offering contracts with dynamic hourly prices based on the hourly market value of electricity. The deadline for net metering has been postponed in the Netherlands and now has a final deadline, January 1st, 2027. And related to my setup at home, I managed to change the timing of heating the hot-water storage tank through a hidden management interface. 🕵🏻♂️ 🙌🏼
Sessy
Since July 2024 we own 2 Sessy Home Batteries (LiFePO4), providing 10kWh capacity, 4400W max charging, 3400W max discharging. In this post I will share my experience and results of the past 6 months. The reason we chose a Sessy is because its independent (not bound to the electricity supplier) and can be fully controlled via its API’s. We also switched to a contract with dynamic hourly prices.
Out of the box Sessy comes with a couple of modes:
- Zero on the meter (maximize solar self-use)
- Dynamic mode (ROI, trade on the dynamic hourly market)
- Open mode (control the API)
- SessyConnect (trade on the imbalance 15 minute market)
- Eco-mode (mix of zero on the meter + dynamic)
- GridEase (Zero on the meter + external control (by Eneco) to correct net imbalance)
I’ve tried the first 3. I have to say, it’s magical to see the net usage of the electricity meter flatline in “Zero on the meter” mode 😮 and maximize self-usage of the electricity produced by my own solar panels. I’ve played around a little with Open Mode using Home Assistant, but in the end I chose Dynamic mode because:
- Dynamic mode comes out of the box. No need to fiddle around with Home Assistant (or any other tool) that barely outperforms the out of the box option.
- Dynamic mode maximizes profit.
- Some say solar produced energy is “free” and choose “Zero on the meter” to maximize self-usage. In my view solar produced energy has value, the current hourly rate the market is willing to pay for it. Definitely with the current net metering policy.
- With “Zero on the meter” mode Round Trip Efficiency (RTE) becomes worse when it’s charging/discharging at a low Wattage. In my house more then half of the time electricity usage is ~200W. With “Zero on the meter” mode and 2 Sessy’s, each one would deliver ~100W. RTE would be quite bad. Since firmware version 1.8.1 (November 27th, 2024) this has been improved btw. now Sessy’s are activated 1-by-1 based on the required power.
- The current net metering policy (until 2027) allows you to receive purchase fee and energy taxes for the electricity you deliver to the net, limited to the amount of electricity you consumed.
Analyzing the data
Every minute I’m collecting the data of the electricity meter, solar inverter and now also the Sessy’s. This data is stored in a time series database. After collecting data for 6 months I was curious about some things:
- What are the costs / profits and kWh stats in the last 6 months?
- What would the costs / profits have been ..
- .. without the dynamic contract?
- .. without the Sessy’s?
- .. without the Sessy’s and without a dynamic contract?
- .. if we wouldn’t have invested in Solar and Sessy’s at all?
- What is the Round Trip Efficiency (RTE) of my Sessy’s?
- With dynamic mode we didn’t choose to maximize solar self-usage. The Sessy’s
are charging during the cheapest hours and discharging during the expensive
hours. But still ..
- .. how much of it was solar energy that was stored?
- .. how much electricity did we directly consume from the Sessy’s?
Last 6 months in numbers
We have an energy efficient A++ (50~75 kWh/m²) terraced house, ~120m², 5600Wp solar and heatpump with a ~150l hot-water storage tank.
These are the kWh numbers of the past 6 months:
The time series data of the electricity meter showed there was 2017 kWh Net Usage and 2176 kWh Net Delivery (rounded values), the cost/profit (incl. tax) is:
- Costs: €481.91 (€0.2389/kWh calculated average)
- Profit: €580.07 (€0.2646/kWh calculated average)
- Result: €98.16 profit
Now let’s assume 6 months ago we would not have switched to a contract with dynamic hourly prices, but we would have signed up a fixed price contract. The cost/profit would look like this:
- Costs: €0.00 (€0.2642/kWh, 0 kWh because of net metering policy)
- Profit: €7.95 (2176 kWh - 2017 kWh x €0.05/kWh)
- Result: €7.95 profit (fixed price contract)
💡 A contract with dynamic hourly prices was definitely the right choice!
Deeper dive into the data
To get a better view on what the costs / profits and kWh numbers would have been without the Sessy’s, we need to know more details about the Sessy charge and discharge numbers.
The time series data shows the Sessy’s charged a total of 1222 kWh and discharged a total of 1032 kWh.
- This a round trip efficiency of 84.45%
- 52.78% was directly charged from solar
- 13.17% was directly consumed from the battery (self-usage) when there was no solar
This is also visible in the time series data:
Without Sessy’s
Without Sessy’s we would have had 1576 kWh Net Usage and 1925 kWh Net Delivery. Calculating this against dynamic hourly prices would have resulted in (incl. tax):
- Costs: €416.72 (€0.2644/kWh calculated average)
- Profit: €340.34 (€0.1768/kWh calculated average)
- Result: €76.38 costs (!)
If we would have signed up for a fixed price contract (without Sessy’s):
- Costs: €0.00 (€0.2642/kWh, 0 kWh because of net metering policy)
- Profit: €17.45 (1925 kWh - 1576 kWh x €0.05/kWh)
- Result: €17.45 profit (fixed price contract)
💡 Without a home battery system, for us it wouldn’t have made sense to go for a dynamic hourly contract.
Without Solar and without Sessy’s
If we wouldn’t have had solar and a home battery at all, total consumption would still be 2146 kWh, but in this case all the electricity would have come from the net.
With a dynamic hourly price contract, this would have been:
- Costs: €547.42 (€0.2550/kWh calculated average)
With a fixed price contract, this would have been:
- Costs: €567.34 (€0.2642/kWh)
💡 It’s interesting to see that the dynamic hourly price contract is cheaper in this case.
Summarized
It’s important to mention that for all the above calculations the net metering policy is still in place. This also has impact when you have a dynamic hourly price contract. The situation will completely change starting January 1st, 2027. Then we would probably choose to maximize self usage and go for “Zero on the Meter”.
| Mode | Cost | Profit | Result |
|---|---|---|---|
| Sessy ROI + Solar + Dynamic Contract | €481.91 | €580.07 | €98.16 |
| Sessy ROI + Solar + Fixed Contract | €0.00 | €7.95 | €7.95 |
| Solar + Dynamic Contract | €416.72 | €340.34 | -€76.38 |
| Solar + Fixed Contract | €0.00 | €17.45 | €17.45 |
| Dynamic Contract (no Sessy and Solar) | €547.42 | €0.00 | -€547.42 |
| Fixed Contract (no Sessy and Solar) | €567.34 | €0.00 | -€567.34 |
But currently having the Sessy’s resulted in €80.71 more profit than without Sessy’s + a fixed price contract. When you extrapolate this number and account for the costs of the Sessy’s it takes ~41 years to fully cover the initial investment. 😬 This will probably change when the net metering policy stops.