The best way to figure out how big your solar thermal hot water installation must be is to calculate your hot water load and work backwards from there. Remember that it is extremely rare to size a solar thermal system to provide 100% of your year-round heating requirement, as such a setup will overheat in hot months. Typically, you'll get about 50% of your winter hot water and about 75% of your total annual load from your renewable source and top it up with grid-tied heat.

The quickest and easiest way to work out how much hot water you use is to multiply the number of people in the house by 20 gallons per day. Of course, this is only a very rough guide. If you know you're very water-conservative, reduce that amount; if you enjoy a relaxing soak in a 60-gallon hot tub every Friday night after work, you'll have to increase your estimate. More people in a house use less water as well: each additional person after the first uses increasingly less water (shorter showers so there's no queue, more full laundry loads, the dishwasher always runs with a full load and so on) so you can count 10%-20% less water per person in a family of five than in a family of three, for example.

As a rule of thumb, this estimate is a great starting point in residential situations. If you're planning for a commercial setup or are uncomfortable working on such a vague assumption, you'll need to look deeper.

At the other extreme of water load measurement is the option to actually meter your daily usage for a period (ideally a year, though a month would work fine). This can be a very expensive process, as you would need to purchase and install a water meter for your hot water line, monitor it closely for the entire period, then work out your average consumption figures. While it may be practical for large companies looking to switch to renewable energy with a long-term return on a very big initial installation investment, it's likely to be beyond the reach of an average householder.

Between the two extremes, there is a third option that works very well – for once, your utility bills will serve a greater purpose than emptying your bank account! Dig out your bill for July and, depending on how you heat your water, work out your needs.

• If you use natural gas, your only usage in July will be for water heating, unless you have a gas cooking range or pool heater. It will also be the cheapest month with the least use. Take the July figure and increase it by between 5% and 10% to get a decent estimate of your monthly heating need.
• If you use propane tanks for your heating, the only way to get an estimate is to fill the tanks just after the heating season. Refill them again just before the next heating season and work out average usage from the amount used over the low-heating period.
• If you use an