First is the true cost, not the financial impact, but the hidden cost. Even at minimum wage I've probably wasted the equivalent of £40 a day watching the output drip up and down on the app, and that excludes 40 min typing this and MrsF's time saying "Are you STILL looking at that *&%*% phone?"
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The installer was very efficient - a good neat job, battery kit and brackets installed Weds, then it rained so panels up on Thurs morning and live at lunchtime. Paperwork returned, DNO letter obtained, and I went live on the export tariff yesterday (yup - 2 working days).
Panel output is lower than I thought, 400W panels and none have gone over 375W output, possibly cable losses, or optimiser losses? Also late evening we have the curtains closed because of the low sun streaming in, but the panels facing that direction are not putting much out, I guess the low angle means they're not efficient.
The Huawei dashboard is very confusing, consumption so far today is 3kWh, 5.38kWh of which is from the grid! This is probably due to me using overnight leccy to charge the battery (see later) as yield today is 1.4kWh, so it sort of adds up, but not in a way I can get my head around.
Setting up the battery was also time consuming, there is a Time Of Use mode so you can set it to charge from the mains at set periods, which makes sense if you are on a variable rate (see below).
We use about 9-11 kWh/day, so initially the 5kWh battery was just keeping us going overnight the first day or 2, then 2 cloudy days meant it wasn't full by evening and reached the discharge limit overnight. This is not a problem now we're on the Flux tariff (below).
Other considerations which I had accounted for included charge/discharge rates. We bought a more powerful inverter which allows us to draw 3kW from the battery, and charge at 2.5kW. Some have lower limits. What this means is when the kettle/oven goes on after dark the battery is able to power it, mostly, but anything over that is topped from the grid.
The financial returns on these systems is largely dependant on your export rate, and this is where Octopus have come into their own. Their normal tariff is roughly 30p supplied, 15p exported (compared to the abysmal 5p or so from many suppliers). However there is a very hard to find "Flux" tariff available on a beta test, where it is the same as their variable rate escept overnight you get 3 hours roughly half price leccy which is about the same price as you get for exporting during the day, with the trade off of 4-7pm the rate is 42p import/30p export. Consequently it is a no brainer to charge the battery up overnight at 18p so you start the day almost fully charged, and now the sun is hitting the panels and the battery is full I'm already exporting at 18.5p. More importantly it means that I should end the day with a (nearly) full battery, so as long as the battery lasts until 2am, all our leccy is supplied at the overnight or export rates. The last couple of cloudy days the battery wasn't fully charged by dusk and ran out overnight. However starting the day with a full battery should mean that even in winter it has enough charge to get us over this.
I've had my doubts about the financial viability of batteries, in raw terms they don't make sense as daily payback is restricted to the capacity multiplied by the import/export price difference. However if you consider all the times during the day you draw more than the panel output then actual total draw from the batteries can be more than their capacity each day. Add in cheap overnight charging and they become more viable.
One thing I have little doubt about is these iBoost systems, which measure when you are exporting energy and divert it to the immersion heater. With a battery they make virtually no sense, and with a Flux tariff they make none whatsoever. At circa £300 installed, compared to £20 or so for a timer where the water can come on at cheap rate 4-5am and then say midday from the battery (also charged at cheap rate) then why bother. And that's ignoring the "gas or leccy for hot water" debate which is for another post.
So there we are, no doubt I'll change my mind about much of this in the future, but for now all is looking good
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Paul