The Ultimate Guide to designing a solar system
How much power will be used?
If a village, business, farm, or school is getting electricity for the first time, it may be difficult to know what the load consumption will be. Sizing the system correctly will mean never running out of power, being efficient with your equipment, and decreasing costs. Counting how many outlets per house will not give you the info you need because each appliance uses different working wattages per hour. For example, a phone uses just 5 watts, where a kettle may use 1,500 watts. Planning for the right amount of working wattage or working power is critical.
How much of the daily power should be in the battery?
Typically, if the system is completely off-grid, without back up grid or diesel generators, we suggest having about 60-70% of the total 24 hour consumption put into batteries. This means if you use 32kWh of energy per day, you would have between 19 and 22kWh of usable (nominal) battery usage. If the system is used for periodic blackouts, then plan for the wattage used during the blackouts.
Batteries are not all made the same. NMC batteries can be fully discharged all the way down to zero percent, while LiFePO generally should be discharged down to 20%, meaning there is still 20% of energy left and lead Acid class batteries, (including deep discharge, gel, opzv, agm, etc) can only be discharged 50%, or they risk being permanently damaged. Below is a breakdown of usable power per battery type.
Example: Village requires 80kWh usable energy
You would need one of the following sized batteries to meet this load:
160kWh Lead Acid for 80kWh usable
100kWh LiFePO for 80kWh usable
80kWh NMC for 80kWh usable
How to size a system in a developed country with grid access?
Look at your utility bills for 12 months. Determine which is your peak and low months. Generally people spend the most on the coldest and hottest months for heating and air conditioning, with fall and spring months that have moderate weather, with lower electricity bills. You can take the average of all your months, or plan for the peak load so that you never run out of power.
How to size a village solar system that is getting electricity for the first time?
OPTION 1: Give each house an electricity budget.
If you are sizing a village for the first time, there are several strategies. You can be proscriptive, in the sense that you may have a fixed budget for a system, so the village will need to keep their usage within a finite amount of energy. For example, you may find that your budget allows for just basic electricity for 25 homes, and no more. This could allow for basic lighting, cell phone charging, and small appliances like a tv or small fridge. This would come to about 600 watts per family in a 24-hour period. If your budget allows for a little bit more electricity per house, you may add a little capacity for each home. For every appliance a family adds, you can calculate the impact on total load. If you want to limit the total watts consumption, you can put a meter on each house to limit what each family uses. For example
OPTION 2: Calculate current energy usage of each house.
If budget isn't a constraint, you may just start with what appliances that are being used, for how long, and determine what is the current total energy consumption for each household. From there, you get a total number and can design the system around this consumption. This is perhaps one of the most time consuming ways of sizing a system. You need to take each appliance, determine the wattage of this appliance (each manufacturer is different), multiply this times the hours it will be used each day.
A desktop computer with monitor, depending on the brand, will use between 200-500 watts per hour.
A laptop computer, depending on the brand, will use between 25-75 watts per hour.
A desktop computer actively being used for 10 hours (not in sleep mode) will consume between 2,000 and 5,000 watts per day.
A laptop computer actively being used for 10 hours will use between 250-750 watts per day.
As you can see, the desktop computer, while doing essentially the same thing as a laptop, uses exponentially more power.
To calculate the load of an entire house, you would need to calculate the number of hours that each and every appliance is plugged in.
Here is a sample house:
4 phones charged two hours each per day = 5 watts per hour x times two hours = 40 watts
1 small tv used for 5 hours each day x 50 watts per hour = 250 watts
1 small energy star (efficient) fridge plugged in 24 hours each day at 35 watts per hour x 24 hours = 840 watts
5 lights used for 5 hours at 5 watts each = 125 watts
Total watts used in 24 hours = 1,255 Wh.
When you plug in certain appliances there is a surge
Please note that when you plug in appliances like a fan, desktop computer, refrigerator or deep freeze, water pump, furnace, kettle, hair dryer, oven, hot plate or range, or microwave, the working wattage surges to two and sometimes three X. So if a kettle is operating at 1,000 watts at working power, right when it is plugged in, it may surge all the way to 2,000 watts or more. This means that you need surge protections in place in your fuses/ breakers, but also your battery capacity needs to accommodate this high spike in electricity load, and your inverter needs to give the extra capacity to handle the spike.