In trade mode, there are a number of conditions specified.
The system ignores the time intervals with the highest price and exports 50% of the maximum grid export threshold. Instead, the system tries to charge the batteries first, with an SOC currently sitting between 70 and 80%, and with discharging from the battery allowed.
Initial data:
from 06:00 to 11:00, conditional maximum export tariff is 15 cents
from 11:00 to 20:00, conditional export tariff is 11 cents
from 20:00 to 06:00, export price is 0.001 cents.
Where is the logic? That’s right, there is none whatsoever! Since trade mode lacks a single constant that would allow it to build an accurate generation forecast, this constant is the actual power of the PV arrays.
There’s no need to try and charge the batteries quickly; there’s a strict charge limit and it needs to be met! The priority in commercial mode is not storage, but selling during strictly defined intervals with maximum efficiency in using the PV arrays for maximum output.
The current forecasting revision significantly reduces the efficiency of the photovoltaic system. By approximately 15-20%.
Personally, my view is that it’s necessary to introduce the actual power of the photovoltaic system into the forecasting calculation formula-not of the MPPT controllers, but specifically of the PV modules.
Cheers, everyone.
UPD: For a more accurate forecast, it’s even better to take both parameters into account: MPPT power and PV modules power.
I see the same charge-first behavior on my system, and from what I can tell it is by design: DESS builds its schedule from the forecast and prefers to go into the highest-price window with a full battery, so it charges first and only exports whatever surplus the forecast expects on top. The 50% export you are seeing is most likely the scheduler holding reserve for later hours rather than a hard cap. On the forecast accuracy point, double-check that the installed PV power in your DESS settings matches the real array size, since DESS scales its VRM-based forecast from that number.
Thanks for the detailed report. I looked at the installation I believe is yours, VRM ID 973427: Trade mode, sell prices 0.001 / 8.13 / 6.13 in the 20-06 / 06-11 / 11-20 blocks. Please correct me if that’s the wrong one. Below is what I found for 8 October (local time). There are three separate effects.
The battery charges first because of the periodic full charge setting.
Your DESS settings have periodic full charge on with an interval of 1 day. Grid-to-battery charging is also disabled. Together these mean DESS must bring the battery to 100% from solar alone every afternoon. On 8 October it started at about 67% and reached 100% around 15:15.
Your buy price (12.32 / 14.32) is also always higher than your best sell price (8.13). So apart from the full charge, solar stored for the evening and night is worth more than solar exported at 6.13.
Unless your battery manufacturer requires a daily full charge, consider a longer interval; for most lithium batteries every 1-2 weeks is enough. That frees battery capacity for export in the 06:00-11:00 block.
The solar forecast is still too low.
DESS only plans battery export in the 06:00-11:00 block if it expects solar to refill the battery before the full charge. Your solar data in VRM only starts on 4 October, and with so little history the forecast was 20-30% below actual production (33 kWh forecast vs 42 kWh actual on 8 October). So DESS held back energy that was later exported at 6.13 instead of 8.13.
Your estimate of the forecast error is in the right range. The forecast learns from your installation’s history, so it should improve over the coming weeks. If it is still clearly too low after 2-3 weeks, please post here again.
Your system drops off the grid when exporting close to 5 kW.
This is probably the main reason you see only part of the maximum export. In the debug log for 8 October, the inverters disconnected from the grid 27 times. Each time it happened while exporting about 4.3-4.9 kW, and often the grid voltage was near 250 V just before. Most disconnects lasted 20-35 seconds; one lasted 30 minutes (12:30-13:01). In total the system was off-grid for about 49 minutes during the export hours, about 3 kWh of planned export on that day alone.
While disconnected, nothing is exported and the solar goes into the battery instead. That is why it looks like the system “charges the battery first” even when DESS asks for full export. DESS itself is requesting the right export power; the inverters just can’t hold the grid connection at that power.
Please ask your installer to:
check the grid code settings of the inverters (the overvoltage limits and reconnect conditions);
measure the voltage at the connection point while exporting about 5 kW;
read the disconnect reason on the GX (VE.Bus alarms and events) right after it happens;
as a test, lower the grid export limit to 4 kW. Steady export at 4 kW will very likely give more energy than repeatedly tripping at 5 kW.
A note on the prices.
The prices you quoted (15 and 11) differ from what’s configured in VRM (8.13 and 6.13). If 15 and 11 are your real tariffs, please update the price schedule, because DESS can only optimize against the prices it is given.
@mikaelkorhonen, you mentioned the same charge-first behavior on your system. Could you share your VRM installation ID? You’ll find it in the url of your VRM dashboard. Then I can check whether the same settings, forecast or grid effects apply to you.
I’m the system architect, and yes, I’m aware of the 250 V issue. That is the threshold at which the inverter shuts down. I set strict limits for synchronisation and voltage quality. I could export more, but problems with the grid operator force me to limit the system’s export power to prevent the voltage from rising, even though my grid export limit is twice as high. The time window during which the system can export relatively consistently matches the model and strategy configured for DESS, thereby helping to smooth out peaks in grid demand. I want to sell as much power from the battery as possible in the morning and evening, as its capacity is sufficient.
My solar array is oversized by 60%.
Also, yes, I want to calibrate the batteries daily, but that is no reason for DESS to throttle the capabilities of my PV system. I simply want to achieve an energy balance, at least by this method, if this cannot be done using the standard Venus OS settings.
Just add an extra constant to the DESS calculation or forecasting formula to account for the size of the PV array or its oversizing percentage.
Even better, provide a third DESS option with a manual model and strict limits. Alternatively, expand the functionality of Venus OS.
Thank you.
UPD: The prices entered in the model are completely irrelevant. I’m not interested in DESS accounting, they are simply constants that help me establish the limits DESS must operate within. The main objective is to maximise export to a problematic grid. Neither my client nor I have much leverage, or any real desire to fight the electricity supplier by having to explain the EN 50549 standards and requirements to them.
Thanks, that helps. If the goal is maximum export within what the connection can take, three settings work against you:
Prices: DESS optimizes on them. Buy (12.32/14.32) is always above sell (max 8.13), so DESS values keeping energy over exporting it. If the prices are levers for you, bring the sell price closer to buy to make it export more.
Grid export limit: set it to what the connection really holds (e.g. 4-4.5 kW). At 5 kW it tripped 27 times on 8 October, and the solar went into the battery each time.
Battery charge limit: it’s set to 4 kW, but the system charges at up to about 6 kW. With the real value, DESS knows it can refill faster and will sell more from the battery in the morning.
With a daily full charge and capped export, some solar curtailment in the afternoon is unavoidable. Points 1 and 3 reduce it.
The high purchase price is another deterrent to put DESS off ever drawing power from the grid again. I have a large portfolio of systems based on Victron products, and I know all the “joys” of DESS. That’s why I prefer to nip it in the bud.
This system is self-sufficient for consumption at any time of year, even under the worst-case solar irradiance scenario, including with the future connection of an EVCS, which is planned but has not yet been commissioned due to the lack of a vehicle.
I have already implemented your recommendation to reduce export, lowering it to 4 kW, and I’ll give DESS some time to learn. After that, I’ll compare the effectiveness of these measures.
As for disconnections, there can be no compromise here: the upper limit is 250 V, with reconnection at 240 V. I will not allow voltage above 250 V or below 210 V into a client’s home. We will of course raise a complaint with the supplier for breaching the agreed export conditions and limits, as well as for non-compliance with the standards, but it is unlikely that anything will change.
With the addition of a new load (EVCS), I think the situation will return to normal, and there will no longer be such an excess of energy, which should allow us to balance the system even further.