DIY SOLAR POWER UPDATE – Winter & Spring

In September last year, I built my own DIY Solar power system. It’s just three panels, but they are Trina solar 400W panels, meaning on a clear sunny day I should be making 1.2kW. I built my own, admittedly janky, battery pack too using LiFePo4 prismatic 280Ah cells, for a total of around 7kWh of capacity. I’ve been making use of that power since, and I thought I’d give you a bit of an update on the system, and probably more importantly, how much power I’ve been generating, using, and how much money I’ve saved.

As a quick recap on my setup, like I said I’m using three full size solar panels rated for 400W each. They are all in series hooked up to a Renogy 60A MPPT charge controller. That converts the 110V or so from the solar panels to the 28V or so in the battery, and the MPPT part means it’s rather efficient at doing that. That gets dumped into my DIY battery pack, via a 150A battery management system. I’ve also added a separate balance board as this BMS isn’t great. If I were doing this again I wouldn’t get one of these – actually I probably wouldn’t build my own battery but still. That power then gets converted into alternating current via a pure sine wave inverter, specifically one rated for 3kW which works out to around 13A at 240V, which is the maximum a UK plug can carry. That runs from my shed into my house via an armoured cable, and terminates with a Samsung SmartThings plug which records how much power I’m using – both as a total and instantaneously. 

Not all that much has changed about the system. I’ve added some flashing to the middle of the panels to make it watertight, and I’ve added some insulation for the winter which helped a fair bit, and since the weather has been getting better I’ve been adding more devices on and off to make use of the power I’m generating. I’ve found that over the course of the day the parasitic draw from the charge controller, BMS, inverter especially, and things like the MQTT bridge that reports data to home assistant and the WiFi range extender I have running in the shed to let the MQTT bridge actually connect all add up to something like like 1kWh per day. That might be a bit of an exaggeration, but if I keep that number as my go-to I am comfortable knowing I won’t run out of charge. 

I’ve had home assistant recording data since I installed the solar system, and since then I’ve used a little under 300kWh. The fridge is the thing that is plugged in pretty much 24/7, and over the winter I’ve been letting the washing machine and dishwasher use the power, although as we get into summer I’ll be disconnecting them and focusing on using more constant loads, like the 800W air conditioner that will be the only thing keeping me alive for like three months this year. Taking a look at the usage graph, you can see the spikes from the washing machine and dishwasher drawing 2kW for a few minutes, although let me zoom in to the last two days to show you more typical usage. To start off the day you’ll see the fridge turning on and off. It draws about 80W for about 20 minutes every hour. You can see when I get up at 10AM because my triple monitors switch on drawing another 100W, then at 2PM I decided it had been sunny enough to switch my UPS over. My UPS runs my main PC, my overkill NAS, and my little QNAP NAS too, and that brings the total to around 600W. We run the dishwasher at half three which momentarily spikes the usage up to 2.5kW, then it’s stable at around 600W or so until around 9PM when I switch my UPS back to the grid. That adds up to about 6kWh of energy, or about £2 saved at the current cost of electricity.

Now that is on a pretty sunny day – looking at the panel power you can see for a good hour or two it was generating 1.4kW (yes, higher than the rated spec). The charge controller reckons it stored 5.3kWh to the battery, which yes that means we dipped into existing stored energy to use all that power, but that’s what it’s there for! Today wasn’t nearly as sunny so it’s been mostly off and charging. 

Over the winter it really didn’t generate all that much. The data here is a little incomplete because I actually switched the inverter off, and therefore the WiFi range extender, but we didn’t use any power during that time either. That wasn’t because none was being generated, that was a few external reasons, but even if we could it would have been the fridge at most. Occasionally it gathered enough power to run the washing machine or dishwasher, but especially in the depths of winter those days were a bit few and far between.

I think the most interesting graph here is the Samsung SmartThings plug energy summation – basically a total of how much energy I used over time. Now the plug has a bit of a bug where it resets itself after around 100kWh of usage and I didn’t capture the first 40kWh of usage in the first couple weeks, but what’s telling is how long it took to use 100kWh over winter, versus how quickly we used the same power in spring. The current jump is even faster! If I include the first 40kWh used in autumn it took me about two months to use 100kWh. Then another 5 or 6 months to use the next 100kWh, and now it’s more like 100kWh per month – and that’s only going to get shorter over summer! If I can use my 800W air conditioner for 12 hours a day using solar, I’ll use nearly 300kWh in a single month. That’s basically £100 per month saved! 

As for how much I’ve saved so far, well I kind of just gave the game away. I’ve used about 300kWh so far, which means I’ve saved around £100. That doesn’t sound like all that much – and granted compared to what I’ve still had to pay the greedy corporation that supplies my grid power, it really isn’t much – but that’s not bad considering that’s basically over winter and it was off for about a month too. While I’ll be bringing you a full one year update in autumn, my projections land me at about £500 per year of offset costs. That’s probably a little optimistic, sure, but I doubt I’ll be that far off, and since I spent about £3,000 on the system all in, that means I’ll see a return on investment within 6 years. That’s not bad at all – and that’s assuming energy prices stay where they are for that whole time too. We’re expecting Ofcom to continue to fail to do their job of protecting consumers by allowing the energy price cap to rise again, meaning prices will rise, which I guess means my ROI time will only decrease. Sadly it also means the remainder of my usage on the grid will also go up so it’s a bit of a bad trade. Still, I’m very happy to have the system. I’m using the panels that are on their frame as a bit of a sun shade and have a rowing machine under it that I’m enjoying using. I overheat badly when exercising so having it outside helps a lot – and the fact that it’s sheltered on all sides means I’m protected from the weather, including rain.