SPF used in moisturisers are tested the same way as sunscreens, so an SPF 15 moisturiser should provide an SPF of 15. However, these formulas are less likely to be rub-resistant and water resistant, and most importantly are likely to be applied more sparingly than sunscreen. A thinner layer of protection amounts to a lower level of protection. A moisturiser with an SPF will help protect you against small amounts of UV exposure, such as when you walk to the car or pop outside to hang out the washing, but sunscreen is better suited for longer, more deliberate UV exposure, such as spending your lunch hour outside. It is also worth noting that moisturisers containing an SPF may not contain any UVA protection and as a result will not protect against UV ageing.