1. How much power do I need?
Residential homes use 900 – 5000kWh a month (30-166kWh per day). To generate this much power with solar / wind and to store it, you will need a lot of space and a large budget. It is unrealistic to generate that much power with portable power or when space is at a premium. Instead, power conservation and realistic expectations should drop your power needs by prioritizing what equipment and appliances are essential.
Something we rarely do when we are on the grid, but must be much more aware of when off-grid, is adding up the power usage from all sources.
Typical equipment and power that you may want to run are: *
- LED Lights 6W each x the number of hours you will have the lights on (6Wx6 lights x 7 hours=252Wh)
- TV (unplug when not in use, as it will siphon power even when turned off) 150-200W / hour 4 hours of viewing is 800W
- 12V fridge 24-48A x 12V 576Wh
- Larger AC Fridge 800Wh
- Satellite Receivers / Direct TV, usually have to run 24-7, 30-40W per hour 720W
- TV Antenna .1A 4W
- Radio (3.3A) 2 hours 40W
- Cell phone charge 10W
- Laptop computer 75W
- CPAP 30-60W
- Fantastic Fan (1.15/1.6/2A) 260-461W on medium (cycles on and off as needed)
- Bathroom Vent Fan (1.25A) 2 hours/ day 30W
- RV 12V Water pump (2.8-6.1A) 30 min / day 35W
- Fireplace fan (1.2A) 172.8 for 12 hours
- Heated blanket 40-50W per hour 7 hour 315W
- Furnace Blower for Propane Furnace (4.6A) x 8 hours 441.6W
- Radiant heater 1300W (not usually an option for off-grid)
- RV Roof Air conditioner with soft starter (to help with inverter overload)– newer type 1PS 13500 BTU 150A 1800W/hour
- Window AC unit 500-1440W/hour
- Floor Fan 100W / hour
- RV Compact Washer and Spin Dryer (not heat dryer) 300W washing, 150W drying/hour – 2-3 very small loads per hour.
- Full size washer, energy star rated 500W
- Full size dryer 3300W (this is just not an option for typical off-grid use)
- Blender 350W – 30W for 5 min
- Hot plate – induction, 1800W, 450W for 15 min
- Microwave, 1000W, 166W for 10 min
- Instapot for 30 minutes (pressure cook for an hour) 900W
- Keurig Coffee Maker – 12 minutes run time 36W
- Inverter power loss: 2 hours of AC output, min 50W **
*Note: the numbers above are average estimates only. For more exact numbers, look at the Amp / Wh rating on your appliances. Use DC appliances wherever possible as you lose power when you run it thru an inverter. Some items can be replaced by hand-crank appliances (blender, washer/dryer).
**Some, less expensive or older, inverters are highly inefficient and may double the power consumption for AC appliances. Check the specs.
Go thru this list, estimate your hours and add up your numbers in priority order of must haves to nice to haves. This total will give you how much power you need to have access to every day to live like you want to. This is your starting number. You may find that some items are not possible to use off-grid with your budget or space limitations. Keep running the calculations taking non-essentials off the list until you reach a balance of power that you can both afford and have room to install.
Take the Total Watt Hours from Step One, for example, 3000Wh. This is your one-day storage requirement but this leaves little room for variations in use and supply. One days storage is your bare minimum power. The recommendation for off-grid use is to multiply this bare minimum times three for 3 days Wh of storage needed.
3000Wh x 3 = 9000Wh
This gives you the range of Wh of storage that you will need.
Batteries are usually purchased in Ah. We are basing all our calculations on 12V batteries. You will need to adjust the math if the batteries are a different Voltage (for example 4V or 6V)
Take the Wh and divide by 12V for the amount of usable storage power in Ah that you will need.
3000Wh/12V = 250Ah
9000Wh/12V = 750Ah
- Space for storage of batteries
- Ability to recharge your batteries from solar and likelihood of bad weather
- Ability to recharge your batteries from a fuel generator, if needed
- Type of battery chemistry, lead acid cannot be depleted more than 50% but carbon foam and silicon dioxide can be depleted to 100%. You will need fewer days of reserve power if you are able to access the full power of the battery bank if needed.
- Ability to adjust consumption (reduce your energy use) if solar conditions are not good
- Number of days you will need power before recharging from shore power (i.e. totally off-grid all the time, only off-grid for a day or three at a time (weekend and back to home to recharge)
If you need 250Ah of available power at minimum, then that would look like this:
- 500Ah of lead acid batteries, can only use 50%
- 350-500Ah of carbon foam batteries
- 350-500Ah of silicon dioxide batteries
- 350-500Ah of li-ion batteries
A better storage number would be at least 500Ah of usable power, to avoid depleting your lead acid below the 50% threshold and to avoid running out of power.
That would look like this:
- 1000Ah of lead acid batteries
- 700Ah of carbon foam batteries
- 700Ah of silicon dioxide batteries
- 700Ah of li-ion batteries
If this number is too high, then go back to step one, and reduce your power consumption.
Solar panels are measured in Watts. This number represents the maximum power that the solar panel can produce under ideal conditions. It is not the actual power that you will get from your solar panels. For this reason, when figuring out how much solar power your will need , we always reduce the max power in by 25% to account for more typical power in (multiply Watts needed by 1.25).
Solar panels also only collect power during usable daylight hours. Much of the day, even when it is clear, the sun is at too steep of an angle to generate much power. The number of hours per day that the solar panels will collect significant power varies by season and by geography. Luckily, there are charts that will give you an estimate of how much sun insolation you can expect in your area. insolation chart for Canada
For example, Vancouver, BC
- Peak daylight in summertime 7.4 hours
- Low daylight in winter (without snowpack blocking the panels) 2.3 hours
- Average daylight 4.9 hours
To charge 250Ah of storage, you need to figure out your Watts in.
250Ah x 12V = 3000W of input at 12V
Take the total Watts needed / number of daylight hours
3000W /4.9 hours of light * 1.25 for efficiency loss = 765W needed.
So to generate enough solar input to recharge 250Ah of storage every day, this person would need approx. 765W of solar panels.
- 3 each 60 Cell 245W residential panels (for a cabin or tiny home) 735W
- 7-8 100W glass panels (for cabin, large RV or tiny home) 700-800W
- 3 210W folding panels for portable power (RV or seasonal cabin where you don’t want to keep the solar panels there off-season) 630W
In December & January, in Canada, the days are short and the cloud cover can be substantial, which makes it challenging to depend 100% on solar to replenish your battery storage banks. Expect that you may need to top up with generator charging or substantially restrict your energy consumption.
If you are only using your cabin in the summer, you may need fewer solar panels because you will have more hours of daylight or if you are willing to run a generator when you have bad weather to supplement your solar power.
Summer only use:
7.4 hours of sunlight
3000W / 7.4 x 1.25 = 506W of solar needed
- 2 245W glass 60cell residential panel for cabin or tiny home 490W
- 5 100W glass panels (for cabin, large RV, or tiny home) 500W
- 5 100W flexible panels (for RV, Marine, tiny home, curved roof trailer) 500W
- 2 x 210W folding panels for portable power 420W
- 3 x 150W folding panels for portable power 450W
1000 / 12 = 83Ah
- 170Ah at 12V of lead acid batteries
- 100Ah of carbon foam, silicon dioxide or li-ion batteries
** double or triple these batteries for 2-3 days of power without recharge.
Solar input needed:
Average Daylight hours 4.4 Quebec City, Quebec
1000W / 4.4 x 1.25 = 284W
- 3 100W panels in glass or flexible panels
- 2 150W folding panels
- 1 210W folding panel * some supplement with AC or generator charging may be needed on heavy use times.
Other equipment needed:
- Charge controller to control the charge from your solar panels to your battery. These are measured in Amps and the size of controller is based on the total Amps of your panels. These come in MPPT and PWM. MPPT is more efficient and is usually programmable to different types of batteries for more efficient charging. PWM is fine for smaller systems.
- Inverter – this converts the DC current from your batteries to AC current for your AC appliances. You will need a pure sine wave inverter if you are running any electronics other than lights. The size of the inverter is based on the maximum number of Watts of AC power that you need at one time. There are two ratings for inverters: continuous and peak/surge power. Continuous is what power is needed to run the appliances ongoing. Peak power is how much oomph you need to start up compressor motors, like in your fridge or air conditioner. It can be a significant number and affect what inverter will work for you.
- Cables and connectors and mounting hardware – the equipment all needs to be wired together and the panels may need to be mounted to a temporary or permanent structure.
- Monitoring hardware, circuit breakers, fuses
What if I can’t afford everything I need right away?
If you can’t afford to buy all the batteries and all the solar you need ideally, start with the correct number of batteries and a slightly undersized solar bank. It may mean that you need to run a generator more to recharge the batteries or go somewhere to plug in your batteries to recharge them, but it is more expandable.
It is hard to add more batteries once you start using them as battery banks need to be balanced… all the same sizes and ages to charge correctly. It is easy to add more solar panels if you find that you do not have enough solar in. Just make sure that you buy a slightly more powerful charge controller than you need for future expansion or assume that you may need to replace the charge controller if you purchase more solar panels.
Worksheet for solar and battery needs to help you calculate how many deep-cycle batteries and how many solar panels you need to power your off-grid tiny home, cabin or cottage or to boondock with your RV, tent trailer, camper van or vintage trailer.