the electricity is all generated during the working day, mostly between about 9.30 and 4pm if they face South.
If you get in the habit of using the tumbledrier (especially), oven, washer and dishwasher during those hours, you will save some electricity and thus money. Sometime it will catch a sunny spell, sometimes it won't. If you have a gas boiler, the cost of hot water is trivial, so don't waste your money on an immersion heater electricity diverter. Once the novelty wears off you should stop fretting about it.
They would save you money if you had a plug-in electric vehicle and were going to use electricity anyway.
Today and yesterday we had spells of very cold, with bright sun, and mine were making between 1 and 2kW for some of the time, so contributed to the cost of the Sunday roast and the washing. On a cloudy day the generation is about nothing. In sunny summers it is much more.
The panels on the house you are looking at may be fairly new, and the subsidy (FITS payment) is very low, so they are not a worthwhile financal investment. You can work it out if you know the commission date, the compass orientation, and the latitude. FITS payments are calculated on the assumption that 50% of the energy will be exported to the grid. If you actually export more, you get paid no extra. If you use it all yourself, you get paid no less.
Quite handy if you have them, but not a big deal either way. Don't think they add much to the value of the house. If the vendor tries to tell you otherwise, ask to see the remittance advices for (at least) the last two years.