Hi Jane,
To give you an answer to your specific question.
If your husband received Carer’s Allowance (£58.45 per week) he would become exempt from the main UKBA Financial Requirement of £18,600 annual minimum .
In which case he would then need to clearly show that you (the visa applicant) can be adequately maintained.
In basic principle, being adequately maintained means showing that when total housing costs are deducted from the sponsors net income then the balance is equivalent to or more than current Income Support rate. (£112.55 per week)
Sponsors income from benefits can be included as income for the above calculation.
Hope that clarifies for you