Why specifications?
I'll start by saying that I'm an engineer and specifications are bread and butter to engineers. However I believe specifications are really important for everyone. Specifications enable you to make decisions about hardware and design choices that are relevant to you. We are the ones that are building it and living in it so it's our specifications that we are designing for. It's wise to benchmark others' specifications but unwise to blindly follow them. A simple process, excluding iterations should go something like this:
Requirements > Specifications > Limitations > Design > Build
Requirements > Specifications > Limitations > Design > Build
Our requirements:
- On board toilet
- On board shower
- Ability to use electronic devices with no access to mains hookups.
- Permanent, full size bed
- Deployable guest bed (double)
- 500 mile range minimum
- 2 weeks drinking water supply (without showers)
- High ceiling (no crouching for me at 6'1")
- Decent cooking facilities (gas hobs, oven, and space for 2 people to prep at the same time
- Space to wash clothes
- A fridge
Specifications from requirements:
- 500 miles range translates to (at a conservative 10mpg) approximately 200L fuel carrying capacity. This can be a combination of fuel tank and secure jerry cans but fuel tank is preferred.
- At 10 litres of water per day (excluding showering), 2 weeks of supply requires 280 litre tank. With showering will require much more. Reverse osmosis uses approximately 4L of water to make 1L of drinkable water. We have decided to go for a 350 litre tank.
- An on board shower requires at least 700x700mm of space. For privacy we decided to have a separate room for the toilet and shower which requires at least 1200mm by 700m of space.
- The electrical demands we anticipate call for about 2.5kWh if storage and 400W solar. Lead acid batteries cannot be fully discharged so 2.5kWh actually required 5kWh, or approximately 400Ah at 12v.