I live in a part of New England that frequently gets week long power outages, usually in the middle of winter. This is part and parcel of living here, and we have a generator that can keep the lights on and run things. I’ve always found them noisy, and expensive to run, and complete overkill for 80% of the time when all we want is some light, ability to charge a phone and power a radio.

After finding a couple of discarded small commercial Solar panels I decided to build a small off-grid battery backup system. This is designed to run some lights, charge cell phones, and run the blower in our gas fire, providing heat in the dead of winter during a power outage. Building this small scale system taught me a lot, and also a few surprises so I wanted to build a thread here where people can share what they’ve built and how it works, or just how cool it is. Share ideas, existing systems, planned systems, ask questions on ‘what if I want to power X’.

The few major discoveries so far:

• Solar Panels are cheap relatively compared to the high annual cost of batteries.
• Solar Panels last 30 years. Batteries don’t.
• If you use the batteries a lot (cycle them daily or weekly), they will need replacing every year or so.
• Everything has to be sized based on your expected consumpion and the amount of solar energy you get each day. You can’t live normal power consumpion lives from a solar battery system, even for a short time.
• Flooded Lead Acid batteries are best kept above 50% charge
• Shadows and clouds suck
• Purkets Law
• Don’t have unrealistic expectations of what devices you can run. Microwaves, fridge freezers, well pumps are pretty much right out.
• Do you want to run your PS4 for 2 hours tonight or have light and heat tomorrow night. Keep an eye on the upcoming weather and plan ahead.
• High current low volage outdoor cabling is expensive.
• MC4 connectors are complicated but also awesome
• Own a kill-a-watt meter for sizing loads
• Cheap inverters are horrible. Pure Sine Wave is the way to go.
• Correct sized 12V car fuses everywhere for safety. You don’t want a shorted battery connected to anything.
• Solar Charge Controllers behave oddly. Ours has *positive* lines tied together and adjusts the negative to change voltage - don’t assume common ground for panels, battery and load.
• Being able to monitor your solar system over the internet is addictive.
• It’s a lot of fun.

Know your power budget
Have a list of what you want to charge and expected AmpHour cost per day. Kill-a-watt meters are great for figuring this out per device. Run the device for an hour and look at peak and average power consumption. Don’t rely on the sticker on the product.
Charging a laptop - 7Ah battery, so would need around 10Ah to charge it.
Charging a cell phone - 3Ah battery
Running a blower in a gas fire for 4 hours - 5 amps per hour - 20 Ah
Running two uplighters with LED bulbs for 4 hours each evening - 1 amp per hours, 4 Ah
Running the inverter 0.5ah

So for an evening of two lights, charging a laptop, charging a cell phone and 4 hours of blown heat from a gas fire, and running the inverter we are around 45Ah of power.

This expected load defines both the size of the battery pack and the size of the solar panels.

To use 45 Ah of power every day, the battery should be at least 100Ah. I use Group 29 Deep Cycle batteries which are around 100Ah under a 5 amp load.

To put 45 Ah of power into the battery on a sunny day, you could assume 4-6 hours of good sunlight (there are online calculator that will give you an estimate based on your location of power from the sun) and divde that by the power you need. Around 7 amps per hour in the example above. 7 amps per hour in the middle of the day needs approximately 150W of 12V solar panels. This does not include any conversion or cabling losses.

To convert all that stored battery energy to 120V AC, you need an inverter. There are cheap ones that are modified sine wave. Very poor quality AC and will cause some devices to not work or fail. Does not work at all with motors or other devices expecting a proper AC sine wave. Even just charging a laptop, the power brick buzzes and gets hotter than expected running on a modified sine wave inverter. Worth investing in a pure sine wave inverter at the start. To run our blower we had to use a pure sine wave inverter.

Don’t oversize the inverter. A 1000W inverter is no use if it can drain the battery in an hour from full. It all comes back to your power budget and knowing the devices to run. On the other hand, if you are starting small and planning on growing from 150W to 500W of panels, you don’t want to have to buy a second inverter.

Our first (trial run) off the grid Solar Backup system

• 150W 12V panels
• LMS2430 solar charge controller - cheap PWM controller, but OK for 12V.
• Marine Deep Cycle Group 29 battery (~100Ah at 4amps output)
• 600W Pure Sine Wave inverter (planning on growing to 500W solar and 300Ah Battery)
• 12V automotive fuses everywhere. Couple of hundred amps from a battery is a fire and safety hazard. Fuse or breaker everything that can generate large uncontolled current.
• Raspberry Pi with USB microscope reading the LCD display on the Solar Charge Controller to monitor and log

Battery Voltage
Panel Voltage
Charging Current
Load Current
every 5 minutes and then plot them online.

When we are not expecting outages, I use the solar system to run a Mac Mini iTunes music server - just keeps it ticking along without deep cycling the battery and wearing it out.

The first battery I bought had an interesting problem where it charged too quicky and also discharged too quicky. It did get up to full charge voltage (12.7V). It was supposed to be 100Ah, but behaved like a 25Ah battery. Using a battery hydrometer, I determined it was faulty and got it replaced without issue. Without all the data logs from the Raspberry Pi it would have been difficult to know there was a problem.

Panels in their temporary home in the yard.



Solar Charge Converter



Good level of charging current



Free panels that started this project - the small panel has a fault, so is running a fan in my friends chicken coop



While I was modifying the Solar charge controller to be driven by the raspberry pi. Note how the + terminals are wired together




Inverter that’s now in use



Data log of voltage, load, charge





Live output, OCR’d then plotted


References that have been great help:
http://roadslesstraveled.us/rv-marine-battery-charging-basics/
http://www.solar-electric.com/deep-cycle-battery-information-faq.html

That's a pretty awesome project. The data mapping fixes my main concern about solar storage and consumption (accurate tracking that I wouldn't forget if I did go off the grid). Kudos on the Raspberry Pi implementation, those things are awesome.

Subbed. Always wished I had disposable income for something like this.

is speculawyer banned again?
He knows his solar stuff.

Awesome thread. Will be needing this info in the near future.

Will it power my flux capacitor?

Subbed, i'm thinking about making a solar based power plant for a farm in the near future. So any info about it would be good as my reference.

Great thread. I've been reading on solar power this past week. Would like to implement a solution but I'm dead broke atm

I know a bit about solar panels. Wouldn't call myself an expert but at my old job I frequently dealt with them. There is some sort of misconception where people feel as if installing them is out of their budget, but if you check with your state you may find that there are tax incentives to using them, and from the technicians I have spoken to it looks like half the time you don't pay a cent for the actual panels, but instead sign a contract to use them for x amount of years.

Another thing to consider is that some municipalities only allow partial implementation of panels, so I really advise that you do your homework before you start a project that may get shut down.

Nice thread. I will be building an off-grid system in a few years and those the same problems I've come across. It's easier to just do without power-hungry things instead of buying more panels and batteries. No microwave, no washing machine, no toaster, no tv, no blender, no hot water showers, no coffee maker. Just some lights, an Intel NUC, an usb-fan and Whynter freezer. I think I could do this on four 100W solar panels and a couple 100AH batteries. I'm thinking of buying one small panel and a small battery to test things out first. I hope more people share their experiences.

We have 5000 Watt Solarpanels on the roof. And are planing to add the offgrid option in the near future. We just need a 10kWh batterypack. If Tesla is successsful with ther Powerpack line this could be an option in the future...

self-sustainability is the future!

plus the freshwaterwell in our Garden and enough wood to warm the house without the support of the the main heatingsystem.

Awesome, will read with attention later, great job!

cool thread, thanks OP.

I have been studying this from the cabin/camping angle. slightly different parameters. the BioLite systems are interesting, if a bit expensive.

Maedre said:

We have 5000 Watt Solarpanels on the roof. And are planing to add the offgrid option in the near future. We just need a 10kWh batterypack. If Tesla is successsful with ther Powerpack line this could be an option in the future.

Click to expand...

Pretty much where I'm at except I have 7020 Watt of panels. 18 east and 9 west (no north facing roof). I don't think I would go completely off grid at the moment however.

I'm just starting my research into buying panels.

I'd love to have something like this, but it'd have to be camouflaged somehow due to living in a community with an HOA. I highly doubt that solar panels that are visible would be approved.

As well as backup for home, I'm using it as a trial run for an RV or cabin sometime in the future. Want to get comfortable and understand the naunces of off-grid power before building something bigger. With solar PV coming down in price so quickly, it's a good time to jump in.

I was also looking for a simple use for a Raspberry pi. I've had it for ages, but didn't have a good idea of what to use it for. The internet monitoring was a bonus, and the fact that the data logs helped me quickly debug a battery that wasn't running at it's stated capacity was a bonus. Being able to see bulk, absorb and float stages and see the difference between direct sunlight and cloud cover, and also between direct sunlight at 9am and at 12 oclock on a graph is very cool indeed. Now all we need is a snow storm so I can test it for real.

It also make sense why grid-tied is so useful but you then use the power company as the 'battery' by selling back. Storage is really only something for specific low power livestyle.

The roadslesstravelled website/blog is inspiring. A couple who have lived for 9 years off the grid in their RV and sail boat.

99hertz said:

Nice thread. I will be building an off-grid system in a few years and those the same problems I've come across. It's easier to just do without power-hungry things instead of buying more panels and batteries. No microwave, no washing machine, no toaster, no tv, no blender, no hot water showers, no coffee maker. Just some lights, an Intel NUC, an usb-fan and Whynter freezer. I think I could do this on four 100W solar panels and a couple 100AH batteries. I'm thinking of buying one small panel and a small battery to test things out first. I hope more people share their experiences.

Click to expand...

Yes, true off grid living means compromises and changes to your lifestyle. But that's kind of part of the fun.

I've been slowly building my own backup solar system on the cheap.
Almost all my parts so far have come from amazon warehouse deals and I get batteries free from work.
For a backup system aircraft batteries are perfect if you can get them somewhere.
We use concorde agm 24v 48ah batteries that are designed to keep the entire electrical system of a jet operating in case the charging system fails.
For the specific batteries we use we need to test them yearly using a special tester by taking the fully charged battery and subjecting it to a 40ah discharge for an hour, if the voltage drops below 20 volts before the hour is up the battery is written off.
These "junk" batteries are perfect for solar backup systems.

Got two batteries so far, now I just need to find some time to begin assembling my system.

This is a topic that's always tickled me and made me think. Definitely enjoyed the OP.

MJPIA said:

I've been slowly building my own backup solar system on the cheap.
Almost all my parts so far have come from amazon warehouse deals and I get batteries free from work.
For a backup system aircraft batteries are perfect if you can get them somewhere.
We use concorde agm 24v 48ah batteries that are designed to keep the entire electrical system of a jet operating in case the charging system fails.
For the specific batteries we use we need to test them yearly using a special tester by taking the fully charged battery and subjecting it to a 40ah discharge for an hour, if the voltage drops below 20 volts before the hour is up the battery is written off.
These "junk" batteries are perfect for solar backup systems.

Got two batteries so far, now I just need to find some time to begin assembling my system.

Click to expand...

If you can keep the panels at 24V too (so a bigger 250W panel for example) and a 24V inverter, then you are made. Batteries are the expensive bit. Make sure it's an AGM specfic solar controller and for the kind of system you are looking at, go MPPT and high quality on the controller not cheap.

None of this is of course 'cheaper' than the local power company, it's more about the independence and self-reliance. It's also a fun project, and by convincing my wife I can keep us warm (and have hot water) with 500 gal of propane in the yard for our gas fire, cooking and water heater and solar backup, it's a win win.

Generator for just the high power devices when needed (freezer once a day perhaps).

great thread.

we're looking at putting some of these on our new house, they are solar panels with wind turbines.
https://www.windstream-inc.com/my-solarmill

we get plenty of wind here in Houston. and our new house will be out in the country where the breezes can really operate without a bunch of houses in the way.

I've got an 11.7KW grid tied system that provides about 50% of my power needs . My inverters go offline in the event of a grid power loss (which happens almost daily this time of year) so I was thinking of either adding a battery system or a Generator to keep the inverters active .

Thanks for the thread. Very interesting

andylsun said:

If you can keep the panels at 24V too (so a bigger 250W panel for example) and a 24V inverter, then you are made. Batteries are the expensive bit. Make sure it's an AGM specfic solar controller and for the kind of system you are looking at, go MPPT and high quality on the controller not cheap.

None of this is of course 'cheaper' than the local power company, it's more about the independence and self-reliance. It's also a fun project, and by convincing my wife I can keep us warm (and have hot water) with 500 gal of propane in the yard for our gas fire, cooking and water heater and solar backup, it's a win win.

Generator for just the high power devices when needed (freezer once a day perhaps).

Click to expand...

Got a 24v 150w panel I picked up for $60 and two 24v 60w panels that I also picked up for $60 each so I'm pretty good on that front.
Controller is pwm rather than mppt but it's a fairly high end one with agm settings that can be customized to whatever I want so I can set charging parameters to exactly what the battery manual says.
Inverter is a massive 24v unit that can handle 1500wh loads continuously with peaks up to 3000.
It's modified sine wave but should do what I want for now at least.
Have a 24-12 converter as well for a led light system I'm writing on as well.

All I need at this point is figuring out where to mount the panels since the roof can't be used at this point and how to run the wires to the basement.

Here's the monitoring system

Raspberry Pi 1 Model B with Raspbin linux.



NTE opto coupler, with two wires going to the solar controller to 'push the button' to advance the controller to the next data. The button press goes through the various modes (voltage, current, charge, load). Due to the whole floating ground on the controller, it must be isolated from the Pi GPIO's.



USB microscope



It works OK, but if the display is changing when the picture is taken it comes up as a mis-read. Eventually I'll either swap the microscope out for a camera with a higher shutter speed, or see if the LCD controller has a hold pin. The other option would be to connect to the data stream from the controller microcontroller to the LCD controller and read it out directly. No idea if it's possible as I don't know the part number of either the microcontroller or the LCD driver controller.

Renogy 100w solar panels are a lightning deal on amazon now, 2 hours to go and just bought one for $115

Edit: It arrived! And I tiedied up the whole system to mount on a wall and keep safe.







Should give me around 12 amps charging current.