Portable Solar Trackers

Build Plans For Active Solar Heater


Build Plans for

Active Solar Window Heater



Before you attempt this build I urge you to consider a few things.  First, verify that you have a clear view of the southern sky, and that you will have at least 4-5 solar hours on a good day.  Second, consider how you will use your power pack.  What do you want to power and when do you want to power it?  When selecting a power pack consider the size of the storage battery, and the power rating of the Inverter.

I propose that you use your power pack as a back-up power source, keep it plugged into the grid and fully charged.  In other words, use the active portion of this hybrid system to recharge the battery when you lose power and the solar furnace portion to supplement your heating system. Then, when you lose main power you will have heat and power.

General Concept

We would not want to try and power too much with such a small system, but it is conceivable that you could provide enough electrical power for a small room at night.  

Primarily, the solar furnace would operate during the winter and on cold days as a space heater.  The active component would charge your power pack when you have a power failure.   

Pictured below is how this furnace mounts in a typical window, the absorber (the solar panel) is beneath a layer of polycarbonate.

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(not all images zoom)

passive-active furnace



inside view solar furnace


Pictured above is how it would appear inside the room; shown with a small power pack.  The visible portion of the furnace, shown above, has an adjustable louver on the top and bottom.  These are used to control the flow of air during the day and to shut the system down at night to prevent a reverse siphon effect.

Parts List

The furnace dimensions will be based on the size of the solar panel you use.  This build is for this solar panel:

(1) HQST 100 Watt 12 Volt Monocrystalline Solar Panel

(4) 1” x 6” x 60” (The lighter you can make the frame the better)

(1) 4’ x 8’ x ¼” plywood

(1) Box – 1” wood screws

(1) Tube outdoor liquid nails/ caulking

(1) 4’ x 8’ x ¾ sheathing foam

(2) ¼” x 4” x  8’ poplar thin stock

(2) Heats Registers  2.75” x 10”

(2) Keyhole Fasteners

(1) Sheet Clear Polycarbonate¼” x 24” x 48”

(1) Poplar 1” x 8” x 96” board (top of inside unit)

Step 1:  Cut the following items to size.

left & right rail dimensions


Left & Right Side Frame

49” x 13/64” x 5” Long Cut

44 13/64” x 5” Short Cut


plywood bottom


Plywood Bottom

49 7/8” x 22 39/64” x ¼”

solar panel dimensions


Solar Panel Dimensions 

42 13/64” x 19 19/32” x 1 13/32”

Step 2:  Position the left and right rail as shown below. The two rails should be glued and screwed to the plywood backing.  Attach the bottom rail temporarily with a couple screws, we will need to remove it to install the polycarbonate later.

left & right rail install


left & right rail view2


Step 3:  Cut the foam insulator. Refer to the dimensions below.

Left & Right Rail Foam

47 19/32 x 3 13/32” Long Cut

44 13/64” Short Cut

Bottom Rail Foam

19 7/64” x 3 13/32”

left & right foam dimensions


Step 4:  Cut the panel holder. Refer to the dimensions below.

Left & Right Panel Holer

47 19/32 x 3 13/32” Long Cut

44 13/64” Short Cut

Bottom Rail Foam

19 7/64” x 3 13/32”

panel holder dimensions


Step 5:  Using panel adhesive glue the foam strips to the left, right and bottom rails.

install foam panels


Step 6: Continue mounting the foam and panel mounts to the rails as shown.  Remember, do not glue the bottom rail, it needs to be removed to install the polycarbonate panel.

install foam & panel holder


Step 7:  Install the solar panel it should drop straight down onto the panel mount.  The panel mounts are the ¼” thick poplar strips of wood.


install solar panel


Step 8:  Cut the panel retainer strips to the following dimensions.

Left & Right Retainer

44” x 1 5/32” x 1” (both)

Bottom Retainer

19 3/32” x 31/32” x 1 7/32” x 29/64”


panel retainer dimensions


Once the bottom retainer is installed the panel is locked in place. It is best to not make it to snug, we must allow for expansion and contraction of the frame.


installin panel retainers


Step 9:  Remove the bottom rail and slide the polycarbonate panel in place.  The poly should have been cut to: 

44 13/32” x 21 33/64”.  Be sure to use clear poly.

installing poly panel


Step 10:  Attach the bottom rail with wood screws, do not glue.  You may need to gain access to the solar panel at a later date.


reinstall bottom panel


Step 11:  The front of your build should look like this.

solar & poly installed



Step 12:  The next step is to cut the material to construct the window mount.  The size of your window mount may change depending on the width of your window opening and the thickness of your walls with siding.

window mount side rails



You can make the rail longer as needed.  The keyhole fastener position would be based on the actual dimensions of the keyhole fastener you use.

window mount side rail dimensions


The outside rails help create a framework for the rest of the window mount. The window mount is part of the supply and exhaust ducting for the furnace.  It directs the air above and below the absorber (the solar panel).

window mount dimensions 2


Step 13:  To construct the window mount you will need to prop the assembled frame at an angle similar to the photo below.  Using the Kreg system to drill pocket holes works well to mount the rails to the frame, I recommend screwing and glueing both.  There will be a lot of weight out there.  I also recommend a brace outside once 


install window rails
Install the plywood backer beneath the rails to fortify the piece. Be sure to bevel your cut on the plywood to provide a tight joint.


install plywood backing

Step 14:  Construct the inside ducting using the dimensions below.  The drawing calls for heat registers that are only 2.75″ x 10″ due to space constraints.
inside-ducting dimensions
Start constructing the ducting by attaching the top and bottom to the two side rails (show below).  Cut openings for the heat registers, approximately 2″ x 9″.
assemble sides top & bottom
Mount the wall side panel and then the room side panel.
Cut the guide rails to the dimensions below and attach them to the room side backing panel.
guide rail dimensions
attach guide rails
Cut the extra panel to the dimensions described below.
slide extra panel
The extra panel is used to create more useable heat.  This panel is an afterthought.  Ideally it should be a steel plate painted flat black. If you attach a wood turn knob at the bottom of the panel to hold on to, you can slide the panel up into position.  Turning the knob clockwise into a matching hole will keep the panel in position.  The front of the panel will increase the heated surface area and will promote air flow even without a fan.
This extra panel would greatly improve the performance of the heater.
Pictured below is the final version as it would appear in a window with a power pack on the side.  The solar panel cable will need to be snaked through the ducting to connect to the power pack.  They can also be run through bulkhead MC-4 connectors for a cleaner look.
Extra absorber
bulkhead mc4