Are solar panels useless on cloudy/rainy days? 

Solar panels will work nearly all day every day between sun rise and sunset. It will take a small amount of time for your solar system to have enough solar production to make the inverter register the output but when up and running solar panels will still produce power even if it’s raining all day. It will not be the best production but a 250-watt solar panel in rainy conditions will still output around 10-20 watts. If this was a 5kw solar system then the production would still be around 200 to 400 watts of power in the most unsuitable conditions. Ideally panels do work there best when in full sun and this will always be preferable. I wouldn’t be turning on the Spa and hopping that the solar production would cover the energy usage on a cloudy day but it would cover most of your idle standby power in the house and reasonable amount of lighting in non-ideal conditions.