Battery efficiency during grid charge-discharge
I have been looking at the " Output referred efficiency" of my system charging and discharging to and from my batteries.
I am using as 3.6 kW Sunsynk eco with 10.24kWh battery capacity (2xSSLB1) and a 5.8kW solar array.
I realise that the efficiency curve varies depending on charge levels, charge and discharge rates, temperature etc.
I have been working out my efficiency level from the charge and discharge data for February. The figures I am using are a bit rough an ready, those reported in the web interface, but at least it is giving me a ball park.
The charging has been excursively from the solar array, no charging from the grid.
Using empirical data over 10 days in mid February, my efficiency works out at 87.59% That is to say for every 1kWh I charge I get 876Wh out, the rest being lost as heat and other losses in the inverter and battery system.
As it is February, here in the UK the charge discharge cycles are very mixed, the odd day I get 100% SOC most of the time it is well below this, so I am probably looking at the worst case scenario.
I am interested in what efficiency levels other people are getting?
I am using the following formula; Discharge kWh divided by Charge kWh all multiplied by 100 to get percent
Why is this important you may ask? Well I am currently looking at a tariff where off peak rates and cheap rates are marginally different. Knowing my efficiency level allows me to calculate the actual cost of import, store, then use later.
Out of intrigue, what are you trying to achieve from the calculations you're doing? Regardless of efficiency losses, I can't see anything that's remotely close to Flux in terms of potential payback for a typical PV + battery system, assuming that there's no existing FiT on the system. Is it simply a case of trying to find an optimal charging/discharging profile for Flux (i.e. avoiding 'excessive' charging, as the offset isn't truly paid at the rate advertised due to efficiency losses) or are you comparing Flux to other Octopus tariffs to find the best option? For example, I switched over from Octopus Go, so was initially questioning whether increasing the cost of the cheap tariff from 12p to circa 20p was a smart move, given that 90% of my usage sits within that period. However, the combination of cheaper daytime rates outside peak and a five-fold increase in the export rates were going to be worth the switch at this time of year.
It's also enabling us to use the system in a more relaxed way as it's less essential to try to utilise the excess solar generation at the point of generation rather than allowing it to be exported. When paying back at only 4p, the opportunity cost of not being able to utilise it was for more of an irritation. I hope you get enough feedback from users to make more sense of the numbers.