Emery is the recipient of an R.W. Kern Center Student Grant, which funds research or projects relating to an aspect of the RWKC’s systems, mission, or to the Living Building Challenge.

Emery is creating their own off-grid home from an old RV. Below is their DIY guide to calculating energy demand and photovoltaic solar system requirements.

What will I be able to power with my solar system?

I calculated my energy usage by first listing every single appliance that I want to power. Then I estimated how long I’d want to power each appliance every day. I determined the wattage of each appliance (usually listed on the appliance somewhere, or on the internet), then I multiplied the wattage by the number of hours desired. This gave me the watt-hours of each appliance which I added up to get the total watt-hours my system will need to accommodate:

Appliance Hours/day Watts Watt hours/day
Mini fridge/freezer 24 36.7 881
Coffee maker .1 900 90
Microwave .05 700 35
4 ceiling fans 8 20 watts x 4 = 80 640
Compost toilet vent fan 24 1.8 43.2
8 LED lights 8 3 watts x 8 = 24 192
2 water pumps for the heated sump pump shower system 1 96 watts x 2 = 192 192
Alarm clock 24 2 48
1 computer charger 8 85 680
1 phone charger 8 6 48
2,027.5 total watts if all appliances are powered at once 2,850 watt-hours OR 3 kWh

Once I knew my total watt hours, I needed to figure out how many solar panels I’d need (based on the average sunlight hours during the least sunny month, which is 2.8 hours/day for where I am near Worchester, MA). I also needed to figure out the size of my battery bank (based on how much power I want to store throughout the day and how long I want that power to last during cloudy days), and the size of my inverter, which manages the amount of power being drawn from the system at any given time (for example, if you want to power your 40 watt fridge at the same time as your 700 watt microwave and your 170 watt computer charger, you would need a inverter that can handle at least 40+700+170=910 watts, which means you would need to purchase a 1kW, or, to be on the safe side, a 1.5kW inverter). Phew! So many parts!

To power my 3KW system with 2.8 hours of sun every day, I will need:

• PANELS: Five 180-watt solar panels (or nine 100-watt panels, which would be harder to fit on my roof!)
5 x 180W = 900W x 2.8 sun hours = 2520W, which is less than 3kW but still enough to power everything except for the ceiling vent fans (2,850-640 = 2210W), which will not be used during the winter or on rainy days anyway.
When calculated using the year-round sun average of 4.42 hours, this system is closer to 4kW!

• INVERTER: A 2,000 or 3,000-watt inverter
A 2kW inverter would mean I could never power all of my appliances at the same time, but doing so would never be absolutely necessary; a 3kW inverter is more expensive, but would mean I would never need to be conscious of how many appliances are running together)

If I want the solar system to run for 3 cloudy days in a row (without powering the ceiling vent fans, so 2210 watts total), I need to multiply 2210 x 3 = 6630 watt-hours. You can build your own lithium iron phosphate batteries very easily, and they can provide 2400 watt hours for around $500 each, so 3 of those would be more than enough.

This whole system could easily cost over $3000. The tricky thing is that the battery bank will cost about half the price of the entire system because of where we are in the Northeast! With my solar budget at $1500-$2000 max, I am in the process of doing research on energy efficient appliances for off-grid purposes and DIY options so that I can 1) lower the cost of the system and 2) lower my needs (and therefore lower my costs). Stay tuned!

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