Around thirty days ago we had solar panels installed \o/. I think it's useful for other people who are thinking about solar to have more information, so this is a bit of a summary of how things have gone for us so far.

Ordering and installation

We ordered back at the end of November 2021 after getting some quotes from local installers and E.ON. These are the rough quotes we got from E.ON (all the quotes include the 5% VAT that was valid at the time):

• 15x390W panels (5.85kWp): £6400 (37%)
• 15x390W panels (5.85kWp) plus 5.2kWh battery: £9600 (59%)
• 11x390W panels (4.29kWp) plus 5.2kWh battery: £8600 (52%)
• 15x390W panels (5.85kWp) plus 8.2kWh battery: £10400 (63%)

kWp is "kilo Watt peak", the maximum output under certain conditions. The percentages at the end are the claimed potential energy independence, so an idea of how much of the electricity we'd potentially make use of vs export.

The local installer quotes were a bit more expensive, and crucially for us E.ON were offering 3 years interest free credit which made it a lot more affordable and meant we could go with the most expensive option, which otherwise would've been tricky. Given that having a battery and more panels means the payback time should be much smaller, that's a big advantage.

Systems over around 4kWp have to have permission from the Distribution Network Operator (DNO) before they are installed. This application is undertaken by the installers, but could take up to around 12 weeks to be approved - this was a big part of the delay between our order and the installation. There was also a delay of around three weeks because of supply problems getting the batteries from China. That delay ultimately worked in our favour though, because the installation happened after the change to 0% VAT. I dread to think what the lead time is these days though.

The installation itself was pretty straightforward. The scaffolding was erected, including having to block our back door, grr. The same day E.ON told us they were running behind because installers had been off with covid... so we had another week delay, grr. Once they were here though, the installers were great. There were some chaps dealing with the roof fittings on the first day, and an electrician starting the work with the inverter and battery as well. The battery had a small dent in the top and the electrician said that he'd been told not to use it, so we went along with that. He wasn't sure when we'd be able to get a new one, given the delays they'd been having, but in the end managed to get a new one the next day. On the second day the remainder of the DC wiring run was completed, the panels themselves were installed and the full system commissioned. Great.

The only bit that was annoying was when one of the roof installers showed me the panels and pointed out how they'd made the effort to put the panels on the roof so there was still space for another two in the future if we wanted. That was a surprise because I'd thought we were filling the roof completely, based on their measurements. The scaffolding came down the next day, it's unlikely we'll be getting an extra two panels to fill the gap.

The system and data logging

The inverter and battery we have are a GivEnergy Giv-HY 5.0 inverter and a Giv-Bat 8.2. These models are no longer available, there are now gen 2 inverters and 9.2kWh batteries.

This is the inverter (top), battery (bottom), and generation meter (top right):

The inverter comes with a WiFi dongle that acts as an access point in the first instance, so you can configure it to connect to your home WiFi, to send data up to the cloud. We got access to two mobile apps that use the data - one from E.ON and one from GivEnergy. The E.ON one gives a nice overview of different generation/usage views, and also offers to integrate with other gadgets like controllable lighting. The one from GivEnergy gives a similar view of energy, but also offers some control features about how the battery should be used. There are a few options, the obvious/easy difference is that in Summer you want to charge the battery up in the day and use it at night, whereas in Winter you may want to charge the battery overnight on a cheap rate, then use it in the day when you aren't getting sufficient solar generation.

I have both apps installed, but typically use neither of them. Instead, I grab my own data and save it to InfluxDB. Happily, the inverter has a modbus interface which you can query to get lots of data. Even more happily, some people have already put the effort in to get this working in a very easy way. I looked at two solutions, givenergy_modbus and giv_tcp.

givenergy_modbus is a Python module that allows you to very easily query the data from the inverter and battery and then do what you want with it. I like it because I can write the code to do what I want - logging to InfluxDB.

giv_tcp is a docker image that contains components to query data from the inverter and battery, and then... publish them to MQTT. Yep, this has Mosquitto included in the image, which made me smile.

I believe there is also a "proper" HomeAssistant plugin nearing completion, but I haven't looked at it because I don't currently use HomeAssistant.

The code I put together for using givenergy_modbus is on github.

Family friendly monitoring

I have a lovely dashboard with all of the data that I'm interested in, but it's not much use for others in the family. I've also got an MQTT enabled ambient orb, and each time I take a measurement from the inverter the colour of the orb is updated. The different states are:

• Green - battery is charging
• Red - battery is discharging
• Blue - battery is no charging (most likely exporting to grid)
• Pink - we are importing from the grid

In each case, the brightness of the colour indicates the intensity of the state.

The orb sits in the hall where everyone can see it, and honestly I think it's fairly useful during the day at least.

Other logging

I'm publishing spot generation values and AC frequency and voltage values to solar/1d4a3c4f-493c-405a-b7c5-a6c558d2b6ab/# on my test.mosquitto.org MQTT broker.

I've also recently set up an twitter bot @ralight_solar to publish generation graphs in a similar manner to @edent_solar.

Generation, usage, and export

Over the last 30 days we've generated 760kWh in total, with the best generation day producing 38.7kWh and the worst producing 7.8kWh.

This is what generation looked like over that time:

We've imported 8kWh during daytime, and 62kWh during nighttime.

Our hot water only comes from electricity and we are a large household, so we have higher than typical usage. We used around 470kWh in the 30 days (and barely any gas, by comparison). The numbers are not as accurate here, because the inverter doesn't provide daily load demand figures unlike the daily generation numbers.

It's difficult to make proper cost comparisons because we have an economy 7 meter and so pay different rates during the day and the night, but also because we have changed when we use electricity. Using an average price of 25p/kWh for the day and night, we would have been charged £117.50 without solar (ignoring daily standing charges). The real cost is more like £17.50.

I'd still like to reduce the usage further. We've had a few days with practically zero import and think that should happen more often. The biggest cause of import is still down to hot water during the night. I'd like to get a solar diverter like an iBoost to ensure we get as much generation pumped into the hot water during the day, but I'm not sure whether the solutions that exist right now are enough - I want to be able to guarantee that we have hot water in the morning. If there was sufficient generation during the day then there's no need to turn the hot water on at the night at all. I'm not sure the built in thermostats are good enough for what I want.

We exported 364kWh. We don't yet have a smart meter so can't easily get an export contract, but if we were with Octopus and on their 7.5p/kWh tariff then that would have produced £27.30.

Total saving for the month, including the unrealised export value would be around £127.30. I'm fairly happy with that.

Limitations

The biggest surprise for me was how the inverter works in different situations. The inverter is rated at a nominal 5kW AC output which is obviously less than our peak 5.8kW output. Our generation is clipped at 5.2kW. This isn't a massive problem, because if we've got that much generation we've already producing more than we'd be using - it does limit the export amount somewhat though.

The exception to this situation is if the battery is still charging. In that case, we can dump 2.5kW max into the battery and still pull up to the limit of the panels. You can see that in this example, where the output is >6kW until the battery charges at which point the output clips.

The other slight annoyance is that the inverter can only draw 2.5kW from the battery at once. This is annoying because there are plenty of devices around the home that draw >2.5kW - in particular for us, the hot water immersion. So if we use the immersion at night, it costs money regardless of how much battery capacity is available. Apart from the days where there were very low generation and hence we needed lots of water heating at night time, a great deal of the import we had was from the excess use over 2.5kW, even if the battery still had sufficient capacity. I'm hoping a solar diverter can add a ~2.5kW limit for night use when we get one. In the meantime, I only need ~700W generation to be able to use the immersion during the day, assuming I have some battery charge.

The second gen of the inverter, available now, can draw 3.6kW from the battery, so it's not a problem there.

Battery

We've had 4 days total where the battery was fully drained over night, and 3 days where the battery wasn't fully charged during the day. As we've changed behaviour regarding electricity usage it's very difficult to say, but it's absolutely clear that the addition of the battery makes a massive difference to the benefit we get from the panels.

I want my own solar panels!

Go for it, if you can. One thing to note is that our installers told us there is now huge interest - they're massively over subscribed with orders from their expected plans for the year. So set your expectations accordingly in terms of lead time.

Conclusion

These are the hard numbers from a single month, the real test will be to see how we fare over a full year.

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