Calculating Solar Power Needs - How to Figure Out Your Energy Needs
Watts, Amps, Voltages? What does it all mean?
Calculating solar power needs
is not hard once you understand the basics. First you need to know how much
energy you require, and from there you can figure out the rest of the system.
In this example, we’ll use a 16 W compact florescent light bulb running on
AC current. Our calculations will start at the light bulb and will work
backwards to the solar panels.
How much energy do your appliances use?
Power consumption is recorded in Watts. For instance,
there are 16 W, 60 W and 100 W light bulbs. The higher the rating,
the more energy it consumes. If you have a 100 W light bulb turned
on for 1 hour, it will consume 0.1 kWH:
100 W x 1 Hour / (1000 W / 1 kW) = 0.1 kWH
In our example, a 16 W bulb running for 1 hour will use considerably
16 W x 1 Hour / (1000 W / 1 kW) = 0.016 kWH
For a larger load, add up all of the electrical needs for your
lights and appliances and multiply them by the number of hours that
they will be turned on in a day. Household energy usage varies depending
on the time of year, the number of people in your home and your lifestyle.
How much energy needs to be stored in a battery?
For our 16 W light bulb, the solar energy needs to be stored in a
battery for use when the sun goes down. Battery capacity is measured
in Amp Hours (i.e. 20 AH) and most batteries run at 12 V (48 V batteries
are just four 12 V batteries connected in a row).
A 20 AH battery will output the following power:
20 AH x 12 V = 240 WH
[current (A) x Voltage (V) = Watts (W)]
This 240 WH capacity can be used in different ways that always add
up to 240 WH such as:
240 W for 1 hour or
120 W for 2 hours or
60 W for 3 hours.
How long will this be able to run the 16 W light bulb? Taking into
account the loss of energy through the DC/AC inverter (96% efficient),
this battery could run a 16 W light bulb for:
[ 240 WH / (0.016 kWH * (1000 W / 1 kWH)) ] * 96% = 14.4 hours
How much energy will a solar panel generate?
The power rating on solar panels is listed in Watts and gives
the maximum amount of power a solar panel will generate in bright
sunlight. To calculate the amount of power it can supply to the
battery, we multiply:
(the rated power of the solar panel)
(the number of hours of direct sunlight)
(an estimated percentage of time it will receive direct sunlight (no clouds))
(the efficiency of the charge controller)
In our example, if we use a 80 W solar panel, it will generate 238 WH
of energy, almost enough to fully charge a battery:
(5 hours of sunlight)
(70% direct sunlight)
(85% eff. of the charge controller)
= 238 WH
Notice that the following conservative estimates were used in this calculation:
- The panel will have 5 hours of direct sunlight but also another 4 hours
of indirect sunlight (more in summer). This indirect sunlight is ignored for
the purpose of this calculation, however in reality, the panel will still
produce electricity with indirect light - just not at its peak rate.
- Within the 5 hours of direct sunlight, clouds may obscure the sunlight.
We assumed that 30% of the time, the panel will be in shadow (70% in full
- 85% efficiency is a typical value for a charge controller. Look for
a model that will electrically disconnect the battery once it is fully
charged to avoid damaging the electrolytes or posts.
Calculating solar power needs may
seem complicated at first but it really isn’t. Once you determine how
much power you need and then apply some conservative assumptions about
your location, you can then put together a solar power system
that will fit your lifestyle and budget.