DIY Solar – is it worth it?

Our energy prices are ridiculous and even taking into account recent “world events” I find it hard to understand why electricity is so expensive. So I figured I’d try to do something about it by installing my own battery and solar system on my shed. The idea being to power my servers from the sun.

After doing a bit of research I decided a 300W panel, 2Ah of lead acid AGM batteries and a hybrid inverter. This is based off the idea that

• My server setup uses 100W of power constantly
• I have an energy tariff that gives me 7.5p/kWh for four hours at night and 38p during the remaining 20 hours of the day

So if I can avoid using expensive peak rate electricity, I can charge the system up at night, then use solar to maintain the batteries during the day, hopefully giving me enough power to last the evening.

This should save me about £22 a month if the system works properly.

I’m trying to power the following devices from the system:

• An SFF PC that runs as a media server
• A 4U rack mounted AMD APU based NAS with 4 HDDs
• Several Raspberry Pis that function as other servers

I went with AGM batteries because of their cost and relative safety. I’ve not dealt with large current carrying batteries and cables before, so didn’t want to risk exploding a lithium battery pack. A lead acid battery going wrong is easier to contain, its composition isn’t flammable.

The panel is a 300w roof mountable panel that comes with standard connectors and outputs voltages that a “proper” system runs at. It’s not a 12V system.

The inverter is a hybrid inverter that can switch between mains and DC by itself depending on battery charge and solar output. It will also switch over to AC if overloaded. It isn’t a grid tie inverter so doesn’t push its own energy into the grid.

To monitor the system I discovered a pretty good Home Assistant integration and monitoring tool that let me create graphs and see the energy production.

Does the system work? yes it does, surprisingly well given my shed faces West – the Eastern face of the roof looks directly at next door’s house and gets no sun. I’ve had it running for six months and when in full sun the panel generates about 160w. The inverter easily copes with the demands of my servers and will work as a UPS should the power fail.

Does it work well enough to run my servers off grid all day? No, no it does not. And here we discover a problem…

Lead acid batteries are not that great. You can only discharge them 50% before they’re considered “flat”, but that’s quite an aggressive thing to do to them, so to prolong their life you discharge them to 80%. This is why lead acid batteries are used in UPS systems, they like being topped up to 100% charge constantly.

So this system won’t save me any money at all, but it is a nice science project. I got to learn all about setting up a solar installation, and when the sun is out the system works for five or so hours before the batteries run down too much. If I had an actual off grid place, this setup would work really well.

If I want to actually save money on my power bill I need to do more than shave off 2kWh per day of electricity, I need to be fully off grid during the whole day. But that means having a real system installed in my house.

Which is the topic of a future post…