The Effective Radiated Power (ERP) is the percieved power of an antenna along the direction of its gain. The Effective Isotropic Radiated Power (EIRP) is the same as the ERP, except that the gain is measured relative to isotropic antennas (dBi), while ERP measures gain relative to half-wave dipole antennas (dBd).

EIRP(dB) = ERP(dB) + 2.15 dB

Law

The limits are generally known as link budget.

India allows 4W ERP (36 dBm) in 2.4GHz and in 865-868 MHz.
The tranmission power is 1W (30 dBm).

FCC allows 36 dBm EIRP for Point to Point (PtP) and 30 dB EIRP for Point to Multipoint (PtMP) connections.

In general, for every 1 dB lowered at the transmitter, the antenna gain can be increased by 3 dB.

Measurement of ERP

ERP(dbW) = \(P_{TX}\) (dBW) - \(P_{loss}\) (dB) + G(dBi) - 2.15 dB

EIRP(dbW) = \(P_{TX}\) (dBW) - \(P_{loss}\) (dB) + G(dBi)

Note: Antenna losses are included in the gain

How to convert dBm to Watts

Conversion formula:

dBm = 10*log10(mW) mW = 10^(dBm/10)

Typical values:

dBm	Watts	dBm	Watts	dBm	Watts
0	1.0 mW	16	40 mW	32	1.6 W
1	1.3 mW	17	50 mW	33	2.0 W
2	1.6 mW	18	63 mW	34	2.5 W
3	2.0 mW	19	79 mW	35	3.2 W
4	2.5 mW	20	100 mW	36	4.0 W
5	3.2 mW	21	126 mW	37	5.0 W
6	4 mW	22	158 mW	38	6.3 W
7	5 mW	23	200 mW	39	8.0 W
8	6 mW	24	250 mW	40	10 W
9	8 mW	25	316 mW	41	13 W
10	10 mW	26	398 mW	42	16 W
11	13 mW	27	500 mW	43	20 W
12	16 mW	28	630 mW	44	25 W
13	20 mW	29	800 mW	45	32 W
14	25 mW	30	1.0 W	46	40 W
15	32 mW	31	1.3 W	47	50 W

Additional Link factors

Radiation angle (direction in degrees)
Polarity (horizontal, vertical, circular)
Beam width (In degrees)
Fresnel Zone (Propagation path, an ellipsoidal region around the LoS; Important to check to decide if the channel will have a proper link)
Link Budget = Power_TX + Gain_TX + Gain_RX + RX_Sensitivity – Path Loss
Calculators
Fresnel Zone: https://www.everythingrf.com/rf-calculators/fresnel-zone-calculator

Left-click: follow link, Right-click: select node, Scroll: zoom