CA taxes non-resident independent contractors on income earned from services provided to CA-based clients. So you'll have to file a non-resident CA tax return (CA Form 540NR) in addition to your home state WI tax return. You'll be able to attach Schedule CA (540NR) to the CA tax return in order to separate out your CA business income and expenses.
ALL your income regardless of where earned is taxable by your home state of WI. You'll be able to claim a credit on your WI return for the taxes paid to CA on the income taxed by both states, so you won't be double-taxed.
**Answers are correct to the best of my ability but do not constitute tax or legal advice.
