Home Battery
Prices
On the 19th of March I got a home battery system installed. The government has a rebate scheme so it had a list price of about $22k for a 40kWh setup and cost me about $12k. It seems that 40KWh is the minimum usable size for the amount of electricity I use, I have 84 cores running BOINC when they have nothing better to do which is 585W of TDP according to Intel. While the CPUs are certainly using less than the maximum TDP (both due to design safety limits and the fact that I have disabled hyper-threading on all systems due to it providing minimal benefits and potential security issues) given some power usage by cooling fans and some inefficiency in PSUs I think that assuming that 585W is accounted for 24*7 by CPUs is reasonable. So my home draws between 800W and 1KW when no-one is home and with an electric car and all electric cooking a reasonable amount of electricity can be used.
My bills prior to the battery installation were around $200/month which was based on charging my car only during sunny times as my electricity provider (Amber Electric) has variable rates based on wholesale prices. Also the feed in rates if my solar panels produce too much electricity in sunny times often go negative so if I don’t use enough electricity. I haven’t had the electric car long enough to find out what the bills might be in winter without a home battery.
Before getting the battery my daily bills according to the Amber app were usually between $5 and $10. After getting it the daily bills have almost always been below $5. The only day where it’s been over $5 since the battery installation was when electricity was cheap and I fully charged the home battery and my car which used 50KWh in one day and cost $7.87 which is 16 cents per KWh. 16 cents isn’t the cheapest price (sometimes it gets as low as 10 cents) but is fairly cheap, sometimes even in the cheap parts of the day it doesn’t get that low (the cheapest price on the day I started writing this was 20 cents).
So it looks like this may save me $100 per month, if so there will be a 10% annual return on investment on the $12K I spent. This makes it a good investment, better than repaying a mortgage (which is generally under 6%) and almost as good as the long term results of index tracker funds. However if it cost $22K (the full price without subsidy) then it would still be ok but wouldn’t be a great investment. The government subsidised batteries because the huge amount of power generated by rooftop solar systems was greater than the grid could use during the day in summer and batteries are needed to use that power when it’s dark.
Android App
The battery system is from Fox ESS and the FoxCloud 2.0 Android app is a bit lacking in functionality. It has a timer for mode setting with options “Self-use” (not clearly explained), “Feed-in Priority” (not explained but testing shows feeding everything in to the grid), “Back Up”, “Forced Charge”, and “Forced Discharge”. Currently I have “Forced Charge” setup for most sunny 5 hours of the day for a maximum charge power of 5KW. I did that because about 25KW/day is what I need to cover everything and while the system can do almost 10KW that would charge the battery fully in a few hours and then electricity would be exported to the grid which would at best pay me almost nothing and at worst bill me for supplying electricity when they don’t want it. There doesn’t seem to be a “never put locally generated power into the grid unless the battery is full” option. The force charge mode allows stopping at a certain percentage, but when that is reached there is no fallback to another option. It would be nice if the people who designed the configuration could take as a baseline assumption that the macro programming in office suites and functions in spreadsheets are things that regular people are capable of using when designing the configuration options. I don’t think we need a Turing complete programming language in the app to control batteries (although I would use it if there was one), but I think we need clauses like “if battery is X% full then end this section”.
There is no option to say “force charge until 100%” or “force charge for the next X minutes” as a one-off thing. If I came home in the afternoon with my car below 50% battery and a plan to do a lot of driving the next day then I’d want to force charge it immediately to allow charging the car overnight. But I can’t do that without entering a “schedule”. For Unix people imagine having to do everything via a cron job and no option to run something directly from the command-line.
It’s a little annoying that they appear to have spent more development time on animations for the app than some of what should be core functionality.
Management
Amber has an option to allow my battery to be managed by them based on wholesale pries but I haven’t done that as the feed-in prices are very low. So I just charge my battery when electricity is cheap and use it for the rest of the day. There is usually a factor of 2 or more price difference between the middle of the day and night time so that saves money. It also means I don’t have to go out of my way to try and charge my car in the middle of the day. There is some energy lost in charging and discharging the batteries but it’s not a lot. I configured the system to force charge for the 5 sunniest hours every day for 5KW as that’s enough to keep it charged overnight and 5KW is greater than the amount of solar electricity produced on my house since I’ve been monitoring it so that forces it to all be used for the battery. In summer I might have to change that to 6KW for the sunniest 2 or 3 hours and then 4KW or 5KW surrounding that which will be a pain to manage.
Instead of charging the car every day during sunny times I charge it once or twice a week, I have a 3.3KW charger and the car has a 40KWh battery so usually it takes me less than 10 hours to fully charge it and I get at least 5 hours of good sunlight in the process.
There are people hacking on these devices which is interesting to get direct control from computers [1], and apparently not banned from the official community for doing so. I’m not enthusiastic enough to do this, I’ve got plenty of other free software things to work on. But it’s good that others are doing so.