# How to figure hrs/day of sun?

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Solar Expert Posts: 136 ✭✭✭✭✭
Aloha, how are rated hours per day of sunlight calculated? Is there a formula? I thought that the total number of hours that the voltage is in excess of battery voltage as well as an arbitrary number of say over 50% of the panel rated wattage output??

frank

• Solar Expert Posts: 10,300 ✭✭✭✭
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Re: How to figure hrs/day of sun?

a full sun hour is an hour that consists of 1000w/m^2 of solar intensity. let's say if you get 2 hours of sunlight at 500w/m^2 that equals 1 hour of 1000w/^2 or 1 full sun hour. the same applies if it exceeds that 1000w/m^2 point, and it is rare, but you could have more than a full sun hour in that hour that it is exceeded.
w/m^2 is watts per meter squared for the record.
• Solar Expert Posts: 136 ✭✭✭✭✭
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Re: How to figure hrs/day of sun?
niel wrote: »
a full sun hour is an hour that consists of 1000w/m^2 of solar intensity. let's say if you get 2 hours of sunlight at 500w/m^2 that equals 1 hour of 1000w/^2 or 1 full sun hour. the same applies if it exceeds that 1000w/m^2 point, and it is rare, but you could have more than a full sun hour in that hour that it is exceeded.
w/m^2 is watts per meter squared for the record.

Aloha, not knowing exactly how to calculate the above, would this make allowances for a less efficient vs the most efficient panel design/brand?

frank
• Solar Expert Posts: 10,300 ✭✭✭✭
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Re: How to figure hrs/day of sun?

that standard is known as stc and you will find most pvs are measured using that intensity of sunshine at 25 degrees c. the trouble is most pvs when exposed to sunshine of that intensity are usually much higher in temperature than 25 degrees c even during some nonsummer times.