From pub 529

Employee use of a computer or cellular telephone. If you are an employee and you use your computer or cellular telephone in connection with your work, its use generally must be:

1. For the convenience of your employer, and
2. Required as a condition of your employment.

For the convenience of your employer. This means that your use of the computer or cellular telephone is for a substantial business reason of your employer. You must consider all facts in making this determination. Use of your computer or cellular phone during your regular working hours to carry on your employer's business is generally for the convenience of your employer.

Required as a condition of your employment. This means that you cannot properly perform your duties without it. Whether or not you can properly perform your duties without the computer or cellular telephone depends on all the facts and circumstances. It is not necessary that your employer explicitly requires you to use your computer or cellular telephone. But neither is it enough that your employer merely states that your use of these items is a condition of your employment.

Example. You are an engineer with an engineering firm. You occasionally take work home at night rather than work late at the office. You own and use a computer that is similar to the one you use at the office to complete your work at home. Since your use of the computer is not for the convenience of your employer and is not required as a condition of your employment, you cannot claim a deduction.

Which depreciation method you can use. If you meet the two tests above, and also predominantly use (more than 50%) your home computer or cellular telephone in your work, you can claim an accelerated depreciation deduction using the General Depreciation System (GDS) and a Section 179 deduction.

If you meet the above two tests, but do not predominantly use your home computer or cellular telephone in your work, you must depreciate these items using the straight line method over the Alternative Depreciation System (ADS) recovery period and you cannot take a Section 179 deduction. Depreciation deductions using GDS and ADS and the Section 179 deduction are explained in Publication 946.

Caution. The restrictions in the preceding two paragraphs do not apply to a computer you own or lease and which you use only in your qualified home office.