12V DC Heating Elements

 This is a follow up on a previous post concerning survival level applications for PV and battery packs. The idea here is to find the lowest price to give people warmth and light using minimal circuitry.

I won't go into the weird sauce of how I connected my battery. But the big idea there is that I'm not using any inverters for me DC heaters. Inverters and even voltage regulators introduce inefficiencies and we want people to get as much bang for the buck as possible.

First of all, here is a picture of the heating elements for under 20 bucks on Amazon:

Second, these elements get very hot... like over 140 degrees F... enough to make food safe to eat.

With that out of the way, the logic of the system is this:

1. A 400 Watt solar array charges a .25 kWh battery pack in less than an hour in full sun (using a BMS circuit to step the voltage down)

2. The 12V 100W heating elements will then drain the battery overnight providing more than enough heat for a single person (not enough to heat a 12' x 12' room) for about 2.5 hours

So following this logic we could say 4, .25 kWh battery packs would be more than enough so that one person could sleep comfortably all night and even into the day (8 to 10 hours).

Let's talk money. The battery pack costs $35 for .25 kWh of energy. The heating elements cost $20. The solar panels cost roughly $250. So all in, this system cost me $310. If we wanted it to last all night, we're looking at more like $350.

So for $350 a person could sleep comfortably overnight every night for 5 years. Then it's just the cost of replacing the battery pack after that (maybe $40), for another 5 years of service.

But you may have noticed that the solar panels were only put to work for a single hour... so this person would have 7 more hours of daylight to use the panels for other systems. I'm thinking the heating elements could be plugged in directly to the solar panels during the day to provide daytime heat during colder months. Essentially this person could survive under a canopy in a sleeping bag in Yellowstone National Park all winter long... provided they had food for $350.

400 Watts of PV w/ Grid Inverter

 Something very news-worthy has happened in the past 6 months. There is a glut of PV panels manufactured and now the price has come down as retailers try to shed their inventory. I could at this moment go on the internet and find a 100 Watt PV panel listed for $50. So naturally I sprung for 3 new panels and put them on my roof.

As you scale up your home brew rooftop PV installation, there are problems to look out for. First of all, wires and connectors will get hot. But then the grid inverter itself will get hot. 

One thing to look out for is loose and frayed connectors. If the electrical contact surface area is small and a lot more current is pushed through that contact, it will heat up more than the rest of the wire. So essentially beef up your wires and connectors as you scale up your system.

Let's talk money. So the three new panels were a little over $180 and the first panel was $100. So $300 in and then another $200 for the grid inverter brings us to a total of $500 DIY rooftop solar installation. Now we could talk ROI which is something like 5 years, but that's a bit boring for a normal person's income.

Far more exciting is what doors 400 Watts (in the middle of the day) of electricity opens. Easily, this powers computers and laptops and TVs... but that's a 1st world problem and not very interesting. I've been looking into heated flooring and heated blankets and you can easily find a 100 Watt system like that.

What this means is to heat a room overnight, a 400 Watt system would easily charge a 1 kWh battery pack and that would heat a room for 10 hours overnight and have energy to spare to heat during the day. Presently I am working on a battery pack made from surplus cells costing $50 for a kWh and more to come on that.