# How to calculate the voltage used to light up the emergency light from the given details of manual..?

I have an emergency light device which is given by a small manual back side of it with some electrical specification. They are:

Voltage: AC90-240V 50-60Hz

Current: 0.1A

Power: 18W

Can I calculate the actual voltage rating of this device?

The device has a rechargeable battery inside it and there is no other informations available in its manual.

When I used the formula:

$$P = V \cdot I$$ $$V=\frac{P}{I}=\frac{18}{0.1}=180~V$$

Is it correct that the device's output is 180V? (I think it is very high and my calculating approach is failed).

• Primary average voltage used may be >=180V but secondary depends on battery V like 3.6V * 5A or some variation Jun 13, 2018 at 16:49
• Ok. I think that the current rating( Current: 0.1A ) given in the manual indicates the current pulled by the device to recharge its battery but not the emergency light . Isn't..? Jun 13, 2018 at 16:59
• The emergency light would run from the battery inside. Traditionally a sealed 12v "gel cell" but perhaps something else today. What that voltage is cannot be determined from the nameplate specs for the mains input used to charge it. Jun 13, 2018 at 17:39
• Can I believe that the battery (can) deliver a maximum power of 18 Watts for powering the emergency lights..? Jun 13, 2018 at 17:56

Since the device has a wide operating voltage it is likely that it has a switched-mode universal power-supply.

We can do some maths and work out the current at 18 W for both 110 V (North America) and 230 V (Europe) power.

For 110 V, $I = \frac {P}{V} = \frac {18}{110} = 0.163 \ \text A$.

For 230 V, $I = \frac {P}{V} = \frac {18}{230} = 0.078 \ \text A$.

The 0.1 A rating on the PSU is normally the worst case at 90 V in your case. We can see that there is something wrong here so it may be that the manufacturer has erred or has given some sort of average current since once charged it will trickle-charge the battery and the power consumed will be very low.

Voltage: AC90-240V 50-60Hz

This describes a line voltage input, probably to a switching power supply.

There isn't any "actual" voltage rating -- the device can accept an input anywhere in the range from 90 to 240 volts. The amperage it draws from the line input will vary based on the voltage, probably to keep the total power around 18 watts. (The rating of "0.1 A" seems a little off, but it's approximate anyway.)

The emergency light lights up only when the AC power goes out.

The electrical specification that you included in your question is for the charging circuit which is active only when the AC power is on.

To find out the voltage that is used to light up the emergency light, check the replacement battery voltage, if it is stated in the manual.

Otherwise, open the device and check the label on the battery.