Standalone winter house for a pet in moderate and cold climates.

Features insulation, floor heating powered by a 12v solar charged battery and fan (optional) in a circuit triggered by a cat presence (PIR) sensor.

Total cost: About 200-250 USD.

Parts:

- Either a wooden box with lid or at least 30 planks of at least 800x100mm to make one. I won't go into details on making the wooden construction, there are already very nicely designed ones on instructables.com such as this one.

- Insulation (I recommend 20-30mm (1 inch) XPS or polystyrene panels due to their water resistance and easiness to cut)

- Cat flap/door, with tunnel. (e.g. Trixie Pet Products 4-Way Locking Cat Door with Tunnel)

- 12v battery (see step 1 for sizing, e.g. Tempest TR24-12B 12V 26Ah Sealed Lead Acid Battery)

- Charge controller (see step 1, e.g. XCSOURCE® 10A 12V/24V Solar Charge Controller)

- Solar panel (see step 1, e.g. DC House 100 Watt)

- Solar panel connection cable, as long as needed (MC4 connector on one end, bare on other e.g. Grape Solar 6 inch)

- 12v PIR sensor (e.g. Generic DC 12V-24V 8A Automatic LED PIR Motion Sensor)

- 12v Car seat heating kit (e.g. 12V Carbon fiber heating pad)

- Electrical wires (see wire gauge selection table here , 18AWG should be fine e.g. this)

- Fuse holder with fuse (e.g. this)

Optional parts:

- Water proof voltage indicator (e.g. Waterproof Monitor DC 4.5-150V 12/24/36/48V Volt Battery Meter)

- 12v Fan for active ventilation (e.g. 120mm x 25mm 12V 2Pin Sleeve Bearing Cooling Fan for Computer Case, I used a CPU fan from an old PC)

- Switch for 12v fan (SPST is sufficient, e.g. JR Products 12595 Black SPST Labeled On/Off Switch)

- Terminal block if you're going full option :) e.g. this to easily connect fan, heating pad and voltage indicator in parallel

- Window (I used 2mm plexiglass to make double glazing)

Step 1: Choosing Parts

1. Non-Electrical
The easiest I think would be to start from a wooden storage box with a top lid. I made the entire house from scratch however, using some leftover planks from another project.
Insulating the house is best done with water-resistant insulation like XPS or polystyrene. About 1 inch should be good for bottom and sides, a double layer in the top lid is good as most warmth escapes upward. This is even sufficient to keep the house somewhat warm just by the cat's own body heat. Tape pieces together with duct or aluminum tape, and seal off edges with silicone kit.
The top of the cat house is best slightly tilted for water to run off, with some water-proof (e.g. plastic) layer to protect the inside of the cat house against the elements (rain, snow, ...).
A window can be added, to make the cat feel more comfortable. I went with 2mm plexiglass.
2. Electrical
We want:
- A battery of sufficient capacity to power the heating pad
- Current flowing from solar panel to battery not to exceed battery limit
- No damage in case the circuit goes awry
Battery size
I live in not so sunny central Europe, and went with a 24Ah 12v battery from an old golf cart. 24 Ah basically means that the fully charged 12v battery can keep a 1 amp load going during 24 hours.
For the floor heating I recommend a motorcycle heating pad package as the link in the parts list. Then, the cat house heating then consumes 22 Watts, or 11 Watts in the low setting (which basically cuts the voltage in half). In
This means theoretically the fully charged battery can provide a day of heating on the low setting (11 Watts divided by 12v = 0.91Amps, 24Ah / 0.91A = 26 hours) or half of it in the 22 Watt high setting.
In practice it will be fewer hours because the charge controller will cut the load off when the battery runs low (see later), but our first concern (enough battery capacity) is addressed.
Solar Panel size
Ideally the panel should keep the battery charged in the darkest winter months, at least assuming the cat isn't in her house all the time :)
I went with a 100Wp panel (cost about 100 USD), and in my region I can count on about 0.5 kWh/kWp per day in the worst solar month December, or about 2-3 hours worth of heating in the low setting.
Important in terms of choosing a panel is to look at the maximum current the solar panel supplies (Imp), for my solar panel 5.56 Amps. The Imp value should always be below your battery's maximum charging current (for my battery this is 6 Amps), to avoid our 2nd concern (battery could be damaged)!
Charge Controller size
A cheap PWM charge controller (10-20 USD) is all that is needed. Solar charge controllers have a current rating, which should always be higher than both the maximum current of your solar panel (Imp) and the total current of your load (in our case 22 Watts / 12v = 1.8 Amps). I went with an oversize 30Amp charge controller, to allow connecting a second battery and solar panel in parallel should I ever want to. Again, make sure battery, solar panel and charge controller are matched!
I would recommend adding a fuse holder (with fuse 10 or 20 Amps) between charge controller and load, even though most charge controllers have short circuit protection in them) for extra safety.

Step 2: Building the House

A few pointers:

- Holes drilled to the outside (ventilation, solar cable, ...) should be directed at a down angle to avoid water getting in.

- Plan ahead where the switches and electric wires will go. It's convenient to have the switches near the cat flap to avoid having to lift off the top to switch heating, fan, light, ... off.

- Double glass can be made by carving out a few millimeters on each side of the wood and using silicone to cover the wood in the space in between the two plexiglass windows (otherwise condensation might occur).

- Think wet and dirty cat: cleanable panels on the inside, sealed with silicone. Wires tucked away behind them. Heating pad is just underneath the bottom panel for maximum heat transfer to the cat.

- There needs to be enough fresh air inside, if the cat flap is too air tight consider drilling some additional holes so air moves through the house when the fan is on (cat is inside).

Step 3: The Electrical Stuff

The charge controller has 3 connections: one to the solar panel, one to the battery and one to our load.

My battery is a deep cycle battery, so I could connect the load directly to it but I still think it's better to have the charge controller cut off the load if the battery voltage drops too low.

I chose to put everything behind the PIR sensor (which has 12v in and 12v out connectors), in other words connect the charge controller "load" connectors to the PIR sensor 12v "in". Then, connect the voltage monitor, heating pad switch and fan in parallel on the PIR sensor 12v out. (with switch in between + connector of fan and + out connector PIR sensor) This means the battery voltage is only displayed when the cat is in the house, conveniently doubling that way as a cat in the house indicator :)

To cleanly connect fan, voltage indicator and heating pad to the 12v PIR out connector, I recommend using the terminal block in the parts list.

I used one 10 amp fuse, in between the load connector on the charge controller and the PIR sensor.

One thing I noticed was the PC Fan I used was going quite hard/loud at 12v, lowered voltage to the fan to 5v with a car USB charger's circuit. Now it is quiet. If you experience the fan going too hard, you could use this in between the PIR sensor out and PC fan connectors to lower the fan speed the way I did.

Wires are all tucked away behind inner walls so the cat can't accidentally damage anything. For convenience, switches are mounted right above where the cat flap is located, so I don't need to lift the roof to flip them. A big drill bit should be enough to do this.

Step 4: Finally: the Test

Got the cat in with some food. The PIR sensor works very well: heating is only enabled when the cat is in and stops 10 seconds after the cat is out. She loves her cat house!