There are three ways by which heat can be transferred:
1.Conduction
2.Convection
3.Radiation
The first two ways require a medium while the last one doesn't. The energy from the sun reaches our earth through radiation i.e in the form of electromagnetic radiation(in simple words light). The photons which constitute light has momentum and energy. When these photons collide with particles the particles acquire energy and thus their speed(kinetic energy) increases.
This causes the thermal energy of the particles to increase and not the heat energy. Heat is a term used for energy in transfer.
How does heat pass through glass?
It is not heat that passes through the glass,but electromagnetic radiation in the infrared range, in the same way as the visible light passes. Glass is transparent to visible light and to most of the spectrum before and after visible light frequencies , depending on the optical properties of the specific glass.
So infra-red and ultra-violet waves from the sun heat up the glass by conduction then it then radiates this heat to heat up a room or a car? Is that right?
No. You are thinking of the metal roof of the car that either reflects or mostly absorbs all electromagnetic energy impinging on it, and transforms the energy to heat that radiates inside the car according to black body radiation. Glass lets a large part of the electromagnetic radiation go through unscathed, which radiation entering the room will heat up the air etc by absorption.
How would one calculate how much heat a type of glass keeps out??
People have measured and tabulates into charts the optical properties of glasses. One uses the absorption coefficients to see how much energy goes through unscathed.
Also how does glass coatings like tinting 'reflect' heat?
It reflects the infrared radiation part of the electromagnetic spectrum by increasing the glass' reflection coefficient for infrared radiation. It changes the refractive index of the material for infrared electromagnetic radiation.
Best Answer
You are looking for the rate of heat transfer $\dot q$ (joules per second).
Conduction
Fourier's law states the amount of heat conducted a certain distance/depth into a material per second, when there is a temperature difference:
$$\dot q=A \kappa \frac{dT}{dx}$$
The term $\frac{dT}{dx}$ is the so-called temperature gradiant (often written as $\nabla T$). For a symmetrical object, this is simplified to $\frac{\Delta T}{\Delta x}$.
Convection
Newton's law of cooling states the amount of heat transfered between the surface of a submerged object and the fluid every second:
$$\dot q=Ah(T_{s}-T_{\infty})$$
Convection is not simple. Convection can be forced (as by pumping) or natural (as by buoyancy), and the relative velocity $v$ of an object moving through a fluid, viscosity $\mu$ etc. play big roles. The equation appears simple at first sight, but includes the parameter $h$, which takes into account the process-specific details and may be very complicatedly tied upon other factors. This source shows some rough examples of values of $h$ in different situations.
In specific engineering cases, you either estimate (or numerically model) $h$ or apply a correlation of $h$ with other parameters that someone have found, if such exists for that specific case. Or you experiment and measure your way through it.
Radiation
The Stefan-Boltzmann law states that any surface with a temperature above $0\;\mathrm K$ radiates heat constantly:
$$\dot q=\varepsilon A\sigma T^4$$
An emissivity of $\varepsilon=1$ gives an ideally radiating body, a so-called blackbody.
Note: Literature might often omit the $A$ in the three laws above. The laws are then stated as heat fluxes instead of heat rates. When dividing through with $A$ the left-hand-side becomes $\frac{\dot q}A$, called heat flux (sometimes given the symbol $\Phi$ or $\Phi_q$ or $\Phi_h$), which is simply heat transfered per square meter every second.
This is not an either-or case. Heat moves at a specific rate and that depends on the materials involved. See the few parameters in the description above, which depend on material.
Heat only flows if there is a temperature difference, yes. The fact that you have two object is irrelevant - same is the case within one object. A temperature difference across two points will cause heat conduction, if they are in physical contact.