Solo 401(k), SEP IRA, or Cash Balance Plan: How Business Owners Actually Choose

Somewhere in year two or three of a profitable business, every owner asks some version of the same question: “Should I open a SEP or a solo 401(k)?” It’s a reasonable question, and it’s also the wrong first one, because the plan isn’t really the decision. The plan is the output of four other numbers: your profit, your W-2 salary (if you’re an S corporation), your position relative to the QBI thresholds, and how many dollars you actually want to shelter this year and for the next decade.

Get those four numbers on the table and the right plan usually picks itself. Skip them and you can easily end up with the popular answer instead of the correct one, funding a SEP that costs you thousands in unnecessary salary, or missing the window where a cash balance plan would have sheltered six figures a year through your fifties.

This piece walks through the machinery of the three main owner plans, the wage-efficiency math that separates them, the QBI interaction most comparisons skip entirely, and how we model the decision for clients. Owner-focused throughout: businesses with non-owner employees face additional coverage and contribution rules that change the analysis, and that’s its own conversation.

The three machines

The SEP IRA: simple, employer-only. A SEP is the minimalist option: the business contributes up to 25% of compensation to the owner’s account, and that’s the entire feature list. No employee deferrals, which also means no catch-up contributions and no Roth option in practice for most owners. Setup is nearly instant, administration is nearly zero, and contribution deadlines are forgiving. Every dollar in is an employer dollar: a business deduction.

The solo 401(k): two engines instead of one. A solo 401(k) (for an owner, or an owner and spouse, with no other employees) combines two contribution types. The employee deferral lets you contribute up to $24,500 in 2026 from your own compensation, plus an $8,000 catch-up at age 50 or older or an enhanced $11,250 catch-up at ages 60 through 63, and the deferral can be traditional or Roth. On top of that, the business can add an employer contribution of up to 25% of compensation, same as the SEP. The two engines together can reach the overall defined-contribution limit of $72,000 for 2026, with catch-up contributions stacking above that ceiling. More paperwork than a SEP (a plan document, and an annual filing once assets pass a threshold), but for an owner-only business the ongoing burden is modest.

The cash balance plan: the heavy machinery. Cash balance plans are defined benefit plans wearing a 401(k)-style costume: instead of a contribution limit, the law limits the eventual benefit, and an actuary calculates each year’s required contribution to fund it. For an owner in their fifties with strong, stable profits, the allowable annual contribution routinely runs well into six figures, several times anything a defined-contribution plan permits, and it can be stacked on top of a 401(k) (with coordination limits an actuary manages). The law caps the eventual annual benefit the plan can fund, at $290,000 for 2026, and the actuary works backward from that ceiling, your age, and your compensation to this year’s allowable contribution. The trade: contributions are largely required, not optional, funding commitments span years, and the plan carries real actuarial and administrative cost. This is a commitment, not an account, and it rewards exactly one profile: the older owner with dependable cash flow, a large tax problem, and a compressed runway to retirement.

The math that separates the SEP from the solo 401(k)

For an owner-only business, the honest comparison between the first two plans comes down to one question: how much salary does it take to reach your savings target? The answer differs sharply, and for S corporation owners, salary is expensive (every dollar carries payroll tax), so wage efficiency is the whole game.

Because a SEP is employer-only at 25% of compensation, reaching the full $72,000 requires about $288,000 of W-2 compensation (25% of $288,000 = $72,000). The solo 401(k) gets there far cheaper: the $24,500 deferral needs only enough salary to fund itself, and the employer piece covers the remaining $47,500 at 25% of roughly $190,000 of compensation. Same $72,000 sheltered, nearly $100,000 less required salary, and every dollar of avoided salary above the Social Security wage base of $184,500 still saves the Medicare layers, while avoided salary below it saves much more.

That asymmetry is why, for owner-only S corporations with meaningful savings goals, the solo 401(k) usually dominates the SEP on pure economics. The SEP’s genuine advantages sit elsewhere: radical simplicity, and flexibility for businesses whose profits are lumpy or modest, where the full two-engine machinery isn’t earning its paperwork. There are also timing and setup nuances between the two that occasionally decide close cases, which is exactly the kind of detail we confirm against current rules when we run the analysis, rather than quoting from memory.

And at lower incomes the comparison inverts in the solo 401(k)’s favor even harder: an S-corp owner with $60,000 of W-2 compensation can defer $24,500 into a solo 401(k), over 40% of pay, while a SEP caps out at 25% of that same salary. The deferral engine is what makes the solo 401(k) the stronger tool at almost every income level; the SEP survives on simplicity.

The layer most comparisons skip: QBI

Here’s where plan choice quietly becomes tax strategy, and where we see even well-advised owners leave money on the table.

For S corporation owners, the two contribution types behave differently against the 20% QBI deduction, a distinction we covered in depth in our QBI piece. (Sole proprietors and partners follow different mechanics: their self-employed retirement deductions generally reduce QBI either way, which makes the S corporation’s split below one more advantage of the structure.)

Employer contributions (SEP contributions, solo 401(k) profit sharing, cash balance funding) are business deductions. They reduce your taxable income and they reduce your qualified business income dollar for dollar, which shrinks the 20% deduction’s base.

Your employee deferral reduces taxable income but does not reduce QBI. For an owner sitting near the QBI thresholds ($403,500 married filing jointly, $201,750 single for 2026), that’s a remarkable property: the deferral can pull taxable income below the threshold, restoring QBI deduction the phaseout was taking away, without shrinking the QBI base itself.

The practical consequence: a deferral dollar and an employer dollar are not interchangeable, and the optimal mix shifts with where the household’s income sits. A SEP, being 100% employer dollars, has no way to exploit this. A solo 401(k) lets you tune the blend. And a cash balance plan’s large employer contributions cut QBI hard, which is sometimes exactly what a high-income owner wants (a huge deduction) and sometimes costs more QBI than the owner realizes. This interaction alone can swing the plan decision, and it’s invisible to any comparison that only looks at contribution limits.

When the cash balance conversation starts

The defined-contribution plans top out at $72,000 plus catch-ups. For an owner who wants to shelter meaningfully more, and there’s a specific profile where that’s rational, the cash balance plan is the only door.

The profile: an owner in their fifties or early sixties, with consistently strong profits, a top-bracket tax rate, behind on retirement assets relative to the lifestyle the business funds, and a five-to-fifteen-year runway. For that owner, the plan converts the business’s peak earning years into six-figure annual deductions and a rapidly compounding retirement balance, then typically terminates and rolls into an IRA at or before the exit. It pairs naturally with a solo or small-group 401(k) running alongside, and it belongs in the same conversation as the exit plan itself, because the funding commitment and the sale timeline have to agree with each other.

The honest caveats carry real weight here. The contributions are commitments the business must meet in down years too. The actuarial and administration costs are material and recur annually. The benefit formula and combined-plan rules are genuinely technical, which is why these plans are built with a third-party actuary and administrator, and why we treat the specific contribution capacity as a number the actuary computes for your facts rather than one anyone should quote from a table. And an owner who might sell or wind down inside a couple of years is usually better served finishing strong in the defined-contribution world than opening heavy machinery they’ll immediately have to shut down.

How we actually run the decision

Because the retirement plan, the salary, and the QBI position move together, we model them together. Our analysis sweeps the owner’s salary across the defensible range and, at each level, jointly optimizes the deferral and the employer contribution, computing payroll tax, income tax, the QBI deduction, and retirement capacity as one system. Then it projects the picture forward several years, because the right plan at $150,000 of profit is often the wrong one at $400,000, and because the cash balance question is fundamentally a multi-year question.

A few patterns from running this repeatedly:

The solo 401(k) is the default winner for owner-only businesses with real savings goals, on wage efficiency and QBI flexibility. The SEP earns its keep in genuinely simple situations and modest-profit years.

The salary decision and the plan decision are the same decision. An S-corp owner’s compensation sets the ceiling on every plan; choosing a “low-salary” strategy and a “max the retirement plan” strategy simultaneously is a contradiction, and the modeling finds the salary where the combined result is actually best. (The salary must always clear the reasonable-compensation floor first; that constraint is never a variable.)

The Roth question rides along. The solo 401(k)’s deferral can be Roth, and owners with mega backdoor Roth ambitions or low-bracket years have another layer to optimize that the SEP simply doesn’t offer.

Revisit at every profit shift. The plan that fit the $120,000 year doesn’t fit the $500,000 year, and the cash balance window opens and closes with age and cash flow. This is an annual conversation, not a one-time setup.

The owners who get this right rarely picked a plan from a comparison chart. They set a savings target, found the salary and contribution mix that hit it at the lowest total tax cost, and let the plan fall out of the math.

Common Questions About Business Owner Retirement Plans

Should I choose a SEP IRA or a solo 401(k)?

For most owner-only businesses with meaningful savings goals, the solo 401(k) wins on the math: its employee deferral ($24,500 in 2026, plus catch-ups) reaches savings targets with far less W-2 salary than a SEP’s employer-only 25% of compensation, which matters enormously for S corporation owners paying payroll tax on every salary dollar. It also offers Roth deferrals and QBI flexibility a SEP can’t. The SEP’s advantages are simplicity and near-zero administration, which can win in modest-profit or lumpy-income situations.

How much can I contribute to a solo 401(k) in 2026?

Up to $24,500 as an employee deferral (traditional or Roth), plus an $8,000 catch-up at age 50 or older, or an enhanced $11,250 catch-up at ages 60 through 63, plus an employer contribution of up to 25% of compensation, up to a combined defined-contribution limit of $72,000 (catch-ups stack above that). Reaching the full $72,000 takes roughly $190,000 of W-2 compensation; the same target through a SEP alone requires about $288,000.

Do retirement contributions reduce my QBI deduction?

For S corporation owners, the two contribution types split. Employer dollars (SEP contributions, profit sharing, cash balance funding) are business deductions that shrink qualified business income dollar for dollar. The owner’s employee deferral leaves QBI untouched even as it lowers taxable income, which near the thresholds can restore deduction the phaseout was removing. Sole proprietors and partners follow different mechanics, where self-employed retirement deductions generally reduce QBI either way. The two dollar types aren’t interchangeable, so the best blend shifts with the household’s income level, one core reason plan choice is a modeling question rather than a chart lookup.

What is a cash balance plan and who should consider one?

A cash balance plan is a defined benefit plan that allows contributions far beyond the $72,000 defined-contribution ceiling, routinely well into six figures annually for owners in their fifties with strong profits. An actuary computes the required contribution each year based on age, compensation, and the plan’s benefit formula. The fit profile: older owners with dependable cash flow, high tax rates, a need to compress decades of saving into a shorter runway, and the discipline for a multi-year funding commitment. It stacks alongside a 401(k) and typically wraps up at or before the business exit.

Can I have both a solo 401(k) and a cash balance plan?

Yes, and the combination is the standard structure for owners maximizing late-career savings: the 401(k) provides the deferral (including Roth) and a portion of employer contribution, while the cash balance plan carries the large deductible funding. Combined-plan rules coordinate the limits, which is one of several reasons cash balance plans are built and maintained with a third-party actuary rather than assembled from a brochure.

Does my S-corp salary limit my retirement contributions?

Directly. Every plan’s employer contribution keys off W-2 compensation (25% of it for SEP and 401(k) employer pieces), and the deferral requires salary to defer from. A salary set low to minimize payroll tax simultaneously caps retirement capacity, which is why the salary decision and the plan decision have to be solved together, above the reasonable-compensation floor, rather than optimized separately and added up.