Allocation method
Multiply total annual utility costs by your home office business-use percentage (sq ft of office ÷ total sq ft of home). Most utilities are indirect — they serve the entire home — so the percentage method is appropriate. Direct utilities (a separately metered office) are 100% deductible.
What counts
Electricity, natural gas, propane, heating oil, water, sewer, garbage collection, and homeowners association fees that include utility services. The first telephone landline to your home is not deductible (Section 262), but a second line dedicated to business is fully deductible.
Internet and phone
Internet service is deductible for the business-use portion. A reasonable allocation (often 50%-80% if the office is the primary internet user) is acceptable when you cannot precisely measure usage. Cell phone service is similarly deductible for the business-use portion — keep records of business calls if business use is less than 100%.
Recordkeeping
Save monthly bills (12 per year for each utility) and keep them with your tax records. Most modern utility companies provide annual usage summaries downloadable from their websites — a single download replaces twelve PDFs.
Simplified method effect
Under the simplified home-office method, utilities are subsumed in the $5/sq ft rate — you do not separately deduct them. Switch to the actual method in years when high utility costs make the actual deduction larger than $1,500.
Worked example with numbers
Consider a sole prop with $100,000 in gross receipts and $30,000 in legitimate Schedule C deductions, including this category. Each additional $1,000 of qualifying expense reduces Schedule C net profit by $1,000, which reduces self-employment tax by approximately $1,000 × 92.35% × 15.3% ≈ $141, and reduces income tax by $1,000 × marginal rate. At a 22% federal marginal rate, the combined federal tax savings on each additional $1,000 of legitimate deduction is roughly $361, and state savings sit on top of that. The math is why disciplined categorization throughout the year pays for itself.
Common mistakes that disallow the deduction
The recurring ways this deduction gets disallowed in examination cluster in four categories: (1) personal-use expenses bundled with business (the deduction is disallowed entirely or apportioned downward); (2) inadequate substantiation (no receipt, no invoice, no business-purpose note); (3) the wrong line on Schedule C (not fatal, but it weakens audit defense); and (4) double-counting with another line (for example, deducting an expense on Schedule C and also on Form 8829, or as a personal itemized deduction on Schedule A). The fix in every case is contemporaneous bookkeeping and a clean chart of accounts.