The force required $F_{\text{r}}$ for driving a wheeled vehicle with mass $m$ is given the following formula (I will neglect slip):
$$F_{\text{r}}=F_{\text{tire}}+F_{\text{aero}}+F_{\text{acc.}}+F_{\text{slope}}.$$
The frictional force of the tires is given by $F_{\text{tire}}=c_{\text{tire}}mg.$ The dimensionless tire friction coefficient $c_{\text{tire}}$ is in general between $0.005$ and $0.010$.
Aerodynamic resistance is given by $F_{\text{aero}}=\frac{1}{2}\rho_{\text{air}}c_{\text{D}}A_{\text{ref}}v^2_{\text{rel}}.$ The dimensionless drag coefficient is between $0.28$ (good aerodynamics) and $0.80$ (bad aerodynamics e.g. for trucks). The reference area $A_{\text{ref}}$ is the projected area of the front face of the vehicle. For very small relative velocities $v_{\text{rel}}=v+v_{\text{wind}}$ aerodynamical resistance can be neglected.
The force needed for a given translative acceleration $\ddot{x}$ is given by $F_{\text{acc.}}=e_{\text{m}}m\ddot{x}$. The dimensionless coefficient $e_{\text{m}}$ is there to account that it is also necessary to accelerate the components of the motor, gearbox and so forth. In most cases it is between $1.05$ and $1.40$.
The force $F_{\text{slope}}=mg\sin{\alpha}$ is necessary to overcome a slope of $\alpha$ (in degrees, so remember to calculate $\sin{\alpha}$ in degrees and not in rad).
So in total we get:
$$F_{\text{r}}=c_{\text{tire}}mg+\frac{1}{2}\rho_{\text{air}}c_{\text{D}}A_{\text{ref}}v^2_{\text{rel}}+e_{\text{m}}m\ddot{x}+mg\sin{\alpha}.$$
in order to get the required power $P_{\text{r}}$ you simply multiply $F_{\text{r}}$ with the velocity $v$ of the vehicle. Note that in general, when considering the wind this velocity $v$ will not be equal to the relative velocity $v_{\text{rel}}$.
