Grid Power or Solar? What an SSVEC Line Extension Actually Costs

This gets argued as a philosophy and it should be decided as arithmetic. Some parcels here should be on the grid. Some should not. The difference is usually distance to the nearest line, and the way to find out is to get both numbers.

One of those numbers costs $150 and most people never ask for it.

The grid side: how SSVEC actually quotes

Sulphur Springs Valley Electric Cooperative does not publish a per-mile or per-foot construction price, and it is worth understanding why rather than being annoyed by it. Every line extension is individually engineered — the route, the permitting, the easements that have to be acquired across whoever's land it crosses, the material, and the trip charges. Two parcels the same distance from the same transformer can price very differently because one needs an easement across three properties and the other does not.

So the process is: you pay a design fee, they send an Engineering Field Technician, and you get a real cost estimate.

After the deposit, the technician prepares a detailed cost estimate covering permitting, easement acquisition, required material and trip charges. You are then invoiced for what the co-op calls "Aid-to-Construction," with the possibility of further invoices for unforeseen costs.

Two practical notes. You will likely be asked for a service address, and most vacant parcels here do not have one — see our piece on rural addressing, and start that earlier than feels necessary. And easement acquisition is a genuine schedule risk: if the route crosses land you do not own, somebody has to agree, and that is not on your timetable.

The solar side: what the resource actually is

This is a good place for solar and the numbers are better than most people assume.

Modelled at Sunsites — 31.94° N, 109.84° W, about 1,323 m elevation — using PVGIS with ERA5 data for 2005–2023:

• Global horizontal irradiation: 5.93 kWh/m²/day (2,166 kWh/m²/yr)
• Fixed array, 32° tilt, due south: 6.78 kWh/m²/day annual average — call it 6.8 peak sun hours
• Estimated output: about 1,912 kWh per year per 1 kWp installed, at 14% system losses

Willcox comes out effectively identical at 6.77 kWh/m²/day and 1,903 kWh/yr per kWp.

The counterintuitive bit

Monthly output at 32° tilt runs from a low of 5.66 in December to a high of 7.91 in April. July is 6.16.

So the sunniest month here is April, not July — July output sits about 22 percent below April because monsoon cloud cover cuts into it. If you have been sizing a system on the assumption that midsummer is your peak, it is not. Your worst month is still December, but your second-worst stretch is the monsoon, and it arrives exactly when air conditioning load is highest.

That is the design constraint here in one sentence: your lowest generation and your highest demand overlap in July and August. Size storage accordingly.

How to actually compare them

Put both on the same footing before deciding.

Grid side: the Aid-to-Construction invoice, plus the meter and service equipment, plus your monthly bill for as long as you live there. Plus the schedule risk on easements.

Off-grid side: panels, racking or ground-mount footings, charge controller, inverter, batteries, wiring, disconnects and enclosure, plus installation, plus a battery replacement somewhere out in the future, plus a generator if you want one for the December stretch.

Two things people leave out of the off-grid column and should not: the ground-mount footings are earthwork, and in caliche that is a real line item; and battery replacement is a future cost, not a never cost, even with LiFePO4 cycle life being what it is.

Two things people leave out of the grid column: the monthly bill compounds over twenty years, and the line brings a permanent easement across your property.

Where the decision usually lands

Roughly, and with the enormous caveat that the engineered estimate is the only number that matters:

• Very close to an existing line — a few hundred feet, easy route, no third-party easements — the grid frequently wins on capital cost, and it removes the storage sizing problem entirely.
• A long way out, or with a route that crosses land you do not control, and off-grid usually wins. This is the common case on the parcels this article is written for.
• Somewhere in between and it comes down to how you value the monthly bill, the easement, and having a system nobody else controls.

There is also a hybrid worth naming: grid service to a small load with solar covering the rest, or grid service with battery backup. Not every parcel has to pick a side.

Sizing notes that save money

If you go off-grid, three things determine whether the system is right-sized or expensive.

Reduce the load before sizing the system. Every kWh you do not use is generation and storage you do not buy, and load reduction is far cheaper per kWh than capacity. Propane for water heating and cooking, an efficient mini-split rather than resistance heat, and a serious look at what actually runs all night.

Pick the system voltage before buying anything. A 3,000 W inverter on a 12 V bank pulls roughly 300 amps DC, which needs very short 4/0 runs and a serious fuse. That is the practical ceiling of a 12 V architecture. Past it, go to 24 V or 48 V — and decide before you buy batteries and a controller, because retrofitting a voltage change means replacing components.

Check the BMS limit, not just the amp-hours. A 100 Ah LiFePO4 battery with a 100 A BMS caps continuous discharge at about 1.28 kW. Three batteries is 300 Ah of capacity and about 3.8 kW of deliverable power — and it is the second number that decides whether your inverter can actually run at rating.

Size the string for a cold morning. Panel voltage rises as temperature falls, and a controller with a 100 V input ceiling can be fine in August and over-voltage in January. Design to the coldest day, not the average.

Where we come into it

We are not an electrical contractor and this is not a design. What we do is the ground work either route needs: trenching for a service run, footings and pads for a ground-mount array, the access a line crew or a delivery needs, and the site layout that keeps the array clear of shade and the equipment out of a drainage path.

The advice we would give anyone at this stage is short: pay the $150. Get the engineered estimate from SSVEC before you commit to an off-grid design, even if you are fairly sure you know the answer. It is the cheapest way to find out you were wrong, and on the occasional parcel it saves a great deal of money.