Renewable energy
Batteries
Batteries are where the money goes if you buy new. Salvage, rewire, and test cells first where you can.
Batteries on the cheap
LiFePO4 batteries are getting pretty cheap now, which is great if you want something that handles deep cycles properly. The flip side is that SLA (sealed lead acid) batteries are very cheap too, and you can sometimes get them for free if you are clever. I pick up free medical-grade 48V battery packs and rewire them down to 24V with a few wiring changes.
You can also build smaller packs from salvaged LiPo and 18650 cells pulled out of old laptop and medical battery packs. Test each cell, throw away the weak ones, and pack the good ones into a case or holder. A multi-slot smart charger that handles 18650s makes the sorting much easier.
Get yourself a nice LiPo balance charger as well. When you are building a pack from salvaged cells, balance them properly at the start before you trust the pack. One weak cell will drag the rest down fast if you skip that step.
If you run SLA, remember this: you absolutely have to keep them charged. Let an SLA go flat and it is dead, not recoverable. LiFePO4 is more forgiving, but SLA will punish you fast if you neglect it.
You can also pick up integrated solar and battery units very cheaply direct from China on sites like AliExpress. All-in-one portable power stations with a panel, MPPT and LiFePO4 battery built in. Handy for a small standalone setup, camping, or the separate night-charging system covered on the night power page, without wiring everything from scratch.