Here's the EU statistics for average solar radiation falling on one square metre per year in UK and Ireland.
A square metre of unshaded ground in Belfast for example receives in excess on 900KwH per annum. That's the energy equivalent to 7500-8500 average kettle boils in one year, or just under 500 hot showers.
Now, this is only if you have a 100% efficient solar water heating panel, which doesn't exist, but, depending on the panel type and the weather conditions, efficiencies of 40-60% could be achievable. So, a 2 square metre evacuated tube solar panel, angled correctly (at 35-55 degrees) and facing somewhere between SE and SW, working at say an average 40% efficiency, could capture usable heat from the sun equivalent to say 6000 kettle boils, or 375 hot showers. I.e., equiavlent to one hot shower per day. Now that's pretty impressive.
Whilst this is a realistic energy capture figure for one correctly installed 2sq metre panel, unfortunately the sun doesn't have equal strength all through the year and you'll get a far greater percentage of that available annual energy in the months March-October than you'll see in the coldest months of the year.
This is why solar panels tend not to make economic sense for space heating, the heat energy just isn't there in the winter sunlight in the first place, and that's when you need space heating the most. It's also why panels make great sense for washing & showers in the warmer months - This is when people tend to need more showers.
The bar chart below will give you an idea of split of solar energy falling by month at these climes. The Y axis shows the percentage of the annual total sunshine energy, so in June you're getting 14% of the total of the sun's annual energy just in one month. If the sun shone equally brightly in all 12 months you'd only expect 8.3% (i.e., 1/12) in June. In December however we're only getting 2% of the annual total energy, and well below the average.
So, seven times as much solar energy available in peak summer compared with the depths of winter.
This is why you need an alternative method of heating your water, such as a wood burning stove or a gas or oil boiler, for say October- February.