How large of off grid solar system do I need?
When we think about “going solar”, this is the first question that comes to mind, especially when considering being off the grid. That, as well as “what does it cost?”, we remember the days when these were our own biggest questions, and even though it’s old hat for us now and we understand it all, we still deal with the same two questions every day, on behalf of our customers.
The thing is, that our lifestyles determine how much electricity we use. A small difference in lifestyle can result in a large difference in electricity usage. Compare two families, each with the same house and the same number of children. From an outside perspective, they appear to be the same, but small changes in their lifestyles at home, as well as the different type of appliances they have, might make one use 2 times as much energy as the other, significantly affecting the size of solar system they need.
Before we get into the charts and graphs that summarize this in a simple way, let’s go through a few things that make a big difference in solar system sizing, especially as it relates to off-grid solar systems:
Irrigation – A water well takes quite a bit of power. This doesn’t matter very much, though, if all a household is doing is washing dishes and showering. Where it becomes a point to consider is if the household has irrigation. Trees, lawn, flowers and animals all need water. They can all have water, as much of it as they will ever need, but if the water is coming from a well powered by solar, the solar system will need to be upsized considerably. It is not uncommon for a water well to use over 20% or even 40% of the total annual energy usage, if heavy irrigation is happening. The one good thing is that irrigation is generally higher in the summer, and most solar systems are oversized in the summer and produce extra energy.
Heat Loads – When heat is the end goal in a certain appliance, it is more efficient to burn propane or use a thermal solar system then it is to use photovoltaic solar for those loads. Why convert the sun’s energy to electricity and then right away convert it back to heat? Sometimes the control that electrical appliances give us are worth it, but it takes a larger solar system then.
A few common heat loads in a household are:
- Boiler, or HVAC Furnace
- Water Heater
- Kitchen Oven/Range
- Kitchen Stove
- Clothes Dryer
- Air Conditioning Unit (I know it blows cold air on you, but it’s the same as a heat load)
If sizing an off-grid solar system, consider moving these loads, or at least most of them, to propane. In some cases, like the boiler and water heater, solar thermal can be an excellent option.
Smart Controls – Basic, smart home controls can make a big difference in the size of a solar system. We’re not talking about fancy controls, just normal, common sense ones that everyone should do, even if they are using grid power. Things like motion activated yard lights instead of keeping the yard light on all night, timers on ceiling fans and bathroom fans, soft starters on the well pump to limit start-up amp draw, etc. Since off-grid solar systems are sized off of the winter months in most cases, a few basic controls built into the appliances and loads can make a substantial difference.
Keep in mind that technologically, there is simply no issue with high loads and large systems. They’re expensive, though, and often they are not necessary.
In summary, below is a cheat sheet which outlines the size ranges of solar systems, and what applications they are suitable for.
| Watt Rating | Size Category | Recommended Application |
|---|---|---|
| 1000 – 2500 Watts (1.0 – 2.5 KW) | Very Small | Mountain cabin or secondary home. A few essential loads only, like lights, a small refrigerator and occasional, light duty well pump use. |
| 2500 – 5000 Watts (2.5 – 5.0 KW) | Small | Small primary home. Essential loads like lights, refrigerator, freezer, and small well pump. Other occasional loads like a microwave, clothes washer, coffee pot, etc. are also possible. All heat loads must be propane. No livestock or lawn irrigation. |
| 5000 – 8000 Watts (5.0 – 8.0 KW) | Medium | Medium sized primary home. Essential loads like lights, refrigerator, freezer, and well pump. Other loads like a microwave, clothes washer, coffee pot, etc. can be used liberally. Most heat loads must be propane, but one or two can be electric, such as an electric oven or an electric dryer, if usage is light. A small amount of livestock or lawn irrigation is possible. |
| 8000 – 12000 Watts (8.0 – 12.0 KW) | Large | Large primary home. All essential loads plus moderate usage of non-essential loads. Most heat loads must be propane, but one or two can be electric, such as an electric oven or an electric dryer. Livestock or lawn irrigation is possible. |
| 12000 – 18000 Watts (12.0 – 18.0 KW) | Very Large | Large primary home. All essential loads plus moderate usage of non-essential loads. Most heat can be electric, such as an electric oven, electric dryer and even electric water heater. Livestock or lawn irrigation is possible. System large enough to support a hobby shop or a small business as well. |
