We have been asked a few times recently how we calculate the estimated daily yield from each solar panel. You will find this info on each of the panel product pages and also the bundles in our shop. For example the one below is from the Maxeon 415W-R page:
How much power will I get? With a 12V system an estimated daily yield for a single 415W-R panel is potentially around 135 amp hours.
We use a bit of a cheat to quickly get to that number. You take the panel wattage and divide by 3 for Maxeon/SunPower cells to get an average daily yield in amp hours at 12V. For non-Maxeon/SunPower you would divide by 4 as they are less efficient and so produce less power. Other factors come into play such as Maxeon/SunPower cells are very good at handling shade & capturing power in low light conditions.
So 415W / 3 = 138Ah (at 12V)
(*Note on our product pages we rounded down the Ah a touch in our answers.)
You could look at it a different way and really dial in the number of sun hours based on your latitude and time of year etc. A good average to use would be 4.5 sun hours. So you take the panel wattage 415W x 4.5 sun hours = 1,867Wh and divide that by the charge voltage of the battery bank to get amp hours. 1,867Wh / 14V = 133Ah (pretty close to 138Ah)
We feel like most boaters talk Amp hours and understand Amp hours (over Watts and Watt hours) more than likely because their battery bank is in Amp hours. They likely know they have for example a 400Ah capacity AGM battery bank and after 2 days they need to fire up the engine because they are at 50% and so have used 100Ah per day.
As more boaters are switching to 24V and 48V banks we are noticing that Watts and Watt hours is becoming used a little more. It all gets you to the same number, work with the system you understand best. Click the button below to read our 'Solar Money' analogy for better understanding power aboard your boat.
What are sun hours?
Sun hours are the number of hours in a day that your solar panel can be expected to perform at it's rated output. For example a 415W panel is not going to produce 415W as soon as the sun comes up! It's going to give you say 100W then slowly ramp up production as the morning progresses, 200W, 300W, then by midday give you a few hours at full rating of 415W (maybe more if you're lucky!) and then slow down again during the afternoon. This equates to the same as 4.5 sun hours at full rating.
What if I have a 24V battery bank?
We use the same quick cheat as we do for 12V but for 24V battery banks divide the panel wattage by 6 and for 48V battery banks divide the panel wattage by 12. For example:
415W / 3 = 138Ah at 12V
415W / 6 = 69Ah at 24V
415W / 12 = 34.5Ah at 48V
Summary
Of course there are lots of variables to consider, for example: shading, panel temperature, are the panels clean, age of the panels and can the battery accept a full charge from the panels, but it gives you a good idea of the solar panel potential.
With this daily yield number you can compare it against your power usage aboard - hopefully the solar will come close to offsetting all of your power usage. If not it will certainly come close and help reduce the number of engine/generator hours needed for battery charging. It can also be compared against the capacity of your battery bank.
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